title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Make HTTP requests using IHttpClientFactory in ASP.NET Core	stevejgordon	Learn about using the IHttpClientFactory interface to manage logical HttpClient instances in ASP.NET Core.	>= aspnetcore-2.1	tdykstra	mvc	11/09/2021	fundamentals/http-requests

Make HTTP requests using IHttpClientFactory in ASP.NET Core

:::moniker range=">= aspnetcore-6.0"

By Kirk Larkin, Steve Gordon, Glenn Condron, and Ryan Nowak.

An xref:System.Net.Http.IHttpClientFactory can be registered and used to configure and create xref:System.Net.Http.HttpClient instances in an app. IHttpClientFactory offers the following benefits:

Provides a central location for naming and configuring logical HttpClient instances. For example, a client named github could be registered and configured to access GitHub. A default client can be registered for general access.
Codifies the concept of outgoing middleware via delegating handlers in HttpClient. Provides extensions for Polly-based middleware to take advantage of delegating handlers in HttpClient.
Manages the pooling and lifetime of underlying HttpClientMessageHandler instances. Automatic management avoids common DNS (Domain Name System) problems that occur when manually managing HttpClient lifetimes.
Adds a configurable logging experience (via ILogger) for all requests sent through clients created by the factory.

The sample code in this topic version uses xref:System.Text.Json to deserialize JSON content returned in HTTP responses. For samples that use Json.NET and ReadAsAsync<T>, use the version selector to select a 2.x version of this topic.

Consumption patterns

There are several ways IHttpClientFactory can be used in an app:

Basic usage
Named clients
Typed clients
Generated clients

The best approach depends upon the app's requirements.

Basic usage

Register IHttpClientFactory by calling AddHttpClient in Program.cs:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientBasic" highlight="4":::

An IHttpClientFactory can be requested using dependency injection (DI). The following code uses IHttpClientFactory to create an HttpClient instance:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Pages/Consumption/Basic.cshtml.cs" id="snippet_Class" highlight="5-6,23":::

Using IHttpClientFactory like in the preceding example is a good way to refactor an existing app. It has no impact on how HttpClient is used. In places where HttpClient instances are created in an existing app, replace those occurrences with calls to xref:System.Net.Http.IHttpClientFactory.CreateClient%2A.

Named clients

Named clients are a good choice when:

The app requires many distinct uses of HttpClient.
Many HttpClients have different configuration.

Specify configuration for a named HttpClient during its registration in Program.cs:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientNamed":::

In the preceding code the client is configured with:

The base address https://api.github.com/.
Two headers required to work with the GitHub API.

CreateClient

Each time xref:System.Net.Http.IHttpClientFactory.CreateClient%2A is called:

A new instance of HttpClient is created.
The configuration action is called.

To create a named client, pass its name into CreateClient:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Pages/Consumption/NamedClient.cshtml.cs" id="snippet_Class" highlight="12":::

In the preceding code, the request doesn't need to specify a hostname. The code can pass just the path, since the base address configured for the client is used.

Typed clients

Typed clients:

Provide the same capabilities as named clients without the need to use strings as keys.
Provides IntelliSense and compiler help when consuming clients.
Provide a single location to configure and interact with a particular HttpClient. For example, a single typed client might be used:
- For a single backend endpoint.
- To encapsulate all logic dealing with the endpoint.
Work with DI and can be injected where required in the app.

A typed client accepts an HttpClient parameter in its constructor:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/GitHub/GitHubService.cs" id="snippet_Class" highlight="5":::

In the preceding code:

The configuration is moved into the typed client.
The provided HttpClient instance is stored as a private field.

API-specific methods can be created that expose HttpClient functionality. For example, the GetAspNetCoreDocsBranches method encapsulates code to retrieve docs GitHub branches.

The following code calls xref:Microsoft.Extensions.DependencyInjection.HttpClientFactoryServiceCollectionExtensions.AddHttpClient%2A in Program.cs to register the GitHubService typed client class:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientTyped":::

The typed client is registered as transient with DI. In the preceding code, AddHttpClient registers GitHubService as a transient service. This registration uses a factory method to:

Create an instance of HttpClient.
Create an instance of GitHubService, passing in the instance of HttpClient to its constructor.

The typed client can be injected and consumed directly:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Pages/Consumption/TypedClient.cshtml.cs" id="snippet_Class" highlight="5-6,14":::

The configuration for a typed client can also be specified during its registration in Program.cs, rather than in the typed client's constructor:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/Program.cs" id="snippet_AddHttpClientTypedInline":::

Generated clients

IHttpClientFactory can be used in combination with third-party libraries such as Refit. Refit is a REST library for .NET. It converts REST APIs into live interfaces. Call AddRefitClient to generate a dynamic implementation of an interface, which uses HttpClient to make the external HTTP calls.

A custom interface represents the external API:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/GitHub/IGitHubClient.cs" id="snippet_Interface":::

Call AddRefitClient to generate the dynamic implementation and then call ConfigureHttpClient to configure the underlying HttpClient:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddRefitClient" highlight="1-2":::

Use DI to access the dynamic implementation of IGitHubClient:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Pages/Refit.cshtml.cs" id="snippet_Class" highlight="5-6,14":::

Make POST, PUT, and DELETE requests

In the preceding examples, all HTTP requests use the GET HTTP verb. HttpClient also supports other HTTP verbs, including:

POST
PUT
DELETE
PATCH

For a complete list of supported HTTP verbs, see xref:System.Net.Http.HttpMethod.

The following example shows how to make an HTTP POST request:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/TodoClient.cs" id="snippet_POST":::

In the preceding code, the CreateItemAsync method:

Serializes the TodoItem parameter to JSON using System.Text.Json.
Creates an instance of xref:System.Net.Http.StringContent to package the serialized JSON for sending in the HTTP request's body.
Calls xref:System.Net.Http.HttpClient.PostAsync%2A to send the JSON content to the specified URL. This is a relative URL that gets added to the HttpClient.BaseAddress.
Calls xref:System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode%2A to throw an exception if the response status code doesn't indicate success.

HttpClient also supports other types of content. For example, xref:System.Net.Http.MultipartContent and xref:System.Net.Http.StreamContent. For a complete list of supported content, see xref:System.Net.Http.HttpContent.

The following example shows an HTTP PUT request:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/TodoClient.cs" id="snippet_PUT":::

The preceding code is similar to the POST example. The SaveItemAsync method calls xref:System.Net.Http.HttpClient.PutAsync%2A instead of PostAsync.

The following example shows an HTTP DELETE request:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/TodoClient.cs" id="snippet_DELETE":::

In the preceding code, the DeleteItemAsync method calls xref:System.Net.Http.HttpClient.DeleteAsync%2A. Because HTTP DELETE requests typically contain no body, the DeleteAsync method doesn't provide an overload that accepts an instance of HttpContent.

To learn more about using different HTTP verbs with HttpClient, see xref:System.Net.Http.HttpClient.

Outgoing request middleware

HttpClient has the concept of delegating handlers that can be linked together for outgoing HTTP requests. IHttpClientFactory:

Simplifies defining the handlers to apply for each named client.
Supports registration and chaining of multiple handlers to build an outgoing request middleware pipeline. Each of these handlers is able to perform work before and after the outgoing request. This pattern:
- Is similar to the inbound middleware pipeline in ASP.NET Core.
- Provides a mechanism to manage cross-cutting concerns around HTTP requests, such as:
  - caching
  - error handling
  - serialization
  - logging

To create a delegating handler:

Derive from xref:System.Net.Http.DelegatingHandler.
Override xref:System.Net.Http.DelegatingHandler.SendAsync%2A. Execute code before passing the request to the next handler in the pipeline:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Handlers/ValidateHeaderHandler.cs" id="snippet_Class":::

The preceding code checks if the X-API-KEY header is in the request. If X-API-KEY is missing, xref:System.Net.HttpStatusCode.BadRequest is returned.

More than one handler can be added to the configuration for an HttpClient with xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.AddHttpMessageHandler%2A?displayProperty=fullName:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpMessageHandler":::

In the preceding code, the ValidateHeaderHandler is registered with DI. Once registered, xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.AddHttpMessageHandler%2A can be called, passing in the type for the handler.

Multiple handlers can be registered in the order that they should execute. Each handler wraps the next handler until the final HttpClientHandler executes the request:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpMessageHandlerMultiple":::

In the preceding code, SampleHandler1 runs first, before SampleHandler2.

Use DI in outgoing request middleware

When IHttpClientFactory creates a new delegating handler, it uses DI to fulfill the handler's constructor parameters. IHttpClientFactory creates a separate DI scope for each handler, which can lead to surprising behavior when a handler consumes a scoped service.

For example, consider the following interface and its implementation, which represents a task as an operation with an identifier, OperationId:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Models/OperationScoped.cs" id="snippet_Types":::

As its name suggests, IOperationScoped is registered with DI using a scoped lifetime:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_OperationScoped":::

The following delegating handler consumes and uses IOperationScoped to set the X-OPERATION-ID header for the outgoing request:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Handlers/OperationHandler.cs" id="snippet_Class" highlight="5-6,11":::

In the HttpRequestsSample download, navigate to /Operation and refresh the page. The request scope value changes for each request, but the handler scope value only changes every 5 seconds.

Handlers can depend upon services of any scope. Services that handlers depend upon are disposed when the handler is disposed.

Use one of the following approaches to share per-request state with message handlers:

Pass data into the handler using xref:System.Net.Http.HttpRequestMessage.Options%2A?displayProperty=nameWithType.
Use xref:Microsoft.AspNetCore.Http.IHttpContextAccessor to access the current request.
Create a custom xref:System.Threading.AsyncLocal%601 storage object to pass the data.

Use Polly-based handlers

IHttpClientFactory integrates with the third-party library Polly. Polly is a comprehensive resilience and transient fault-handling library for .NET. It allows developers to express policies such as Retry, Circuit Breaker, Timeout, Bulkhead Isolation, and Fallback in a fluent and thread-safe manner.

Extension methods are provided to enable the use of Polly policies with configured HttpClient instances. The Polly extensions support adding Polly-based handlers to clients. Polly requires the Microsoft.Extensions.Http.Polly NuGet package.

Handle transient faults

Faults typically occur when external HTTP calls are transient. xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddTransientHttpErrorPolicy%2A allows a policy to be defined to handle transient errors. Policies configured with AddTransientHttpErrorPolicy handle the following responses:

xref:System.Net.Http.HttpRequestException
HTTP 5xx
HTTP 408

AddTransientHttpErrorPolicy provides access to a PolicyBuilder object configured to handle errors representing a possible transient fault:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientPollyWaitAndRetry":::

In the preceding code, a WaitAndRetryAsync policy is defined. Failed requests are retried up to three times with a delay of 600 ms between attempts.

Dynamically select policies

Extension methods are provided to add Polly-based handlers, for example, xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddPolicyHandler%2A. The following AddPolicyHandler overload inspects the request to decide which policy to apply:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientPollyDynamic":::

In the preceding code, if the outgoing request is an HTTP GET, a 10-second timeout is applied. For any other HTTP method, a 30-second timeout is used.

Add multiple Polly handlers

It's common to nest Polly policies:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientPollyMultiple":::

In the preceding example:

Two handlers are added.
The first handler uses xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddTransientHttpErrorPolicy%2A to add a retry policy. Failed requests are retried up to three times.
The second AddTransientHttpErrorPolicy call adds a circuit breaker policy. Further external requests are blocked for 30 seconds if 5 failed attempts occur sequentially. Circuit breaker policies are stateful. All calls through this client share the same circuit state.

Add policies from the Polly registry

An approach to managing regularly used policies is to define them once and register them with a PolicyRegistry. For example:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/Program.cs" id="snippet_AddHttpClientPollyRegistry":::

In the preceding code:

Two policies, Regular and Long, are added to the Polly registry.
xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddPolicyHandlerFromRegistry%2A configures individual named clients to use these policies from the Polly registry.

For more information on IHttpClientFactory and Polly integrations, see the Polly wiki.

HttpClient and lifetime management

A new HttpClient instance is returned each time CreateClient is called on the IHttpClientFactory. An xref:System.Net.Http.HttpMessageHandler is created per named client. The factory manages the lifetimes of the HttpMessageHandler instances.

IHttpClientFactory pools the HttpMessageHandler instances created by the factory to reduce resource consumption. An HttpMessageHandler instance may be reused from the pool when creating a new HttpClient instance if its lifetime hasn't expired.

Pooling of handlers is desirable as each handler typically manages its own underlying HTTP connections. Creating more handlers than necessary can result in connection delays. Some handlers also keep connections open indefinitely, which can prevent the handler from reacting to DNS (Domain Name System) changes.

The default handler lifetime is two minutes. The default value can be overridden on a per named client basis:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientHandlerLifetime":::

HttpClient instances can generally be treated as .NET objects not requiring disposal. Disposal cancels outgoing requests and guarantees the given HttpClient instance can't be used after calling xref:System.IDisposable.Dispose%2A. IHttpClientFactory tracks and disposes resources used by HttpClient instances.

Keeping a single HttpClient instance alive for a long duration is a common pattern used before the inception of IHttpClientFactory. This pattern becomes unnecessary after migrating to IHttpClientFactory.

Alternatives to IHttpClientFactory

Using IHttpClientFactory in a DI-enabled app avoids:

Resource exhaustion problems by pooling HttpMessageHandler instances.
Stale DNS problems by cycling HttpMessageHandler instances at regular intervals.

There are alternative ways to solve the preceding problems using a long-lived xref:System.Net.Http.SocketsHttpHandler instance.

Create an instance of SocketsHttpHandler when the app starts and use it for the life of the app.
Configure xref:System.Net.Http.SocketsHttpHandler.PooledConnectionLifetime to an appropriate value based on DNS refresh times.
Create HttpClient instances using new HttpClient(handler, disposeHandler: false) as needed.

The preceding approaches solve the resource management problems that IHttpClientFactory solves in a similar way.

The SocketsHttpHandler shares connections across HttpClient instances. This sharing prevents socket exhaustion.
The SocketsHttpHandler cycles connections according to PooledConnectionLifetime to avoid stale DNS problems.

Logging

Clients created via IHttpClientFactory record log messages for all requests. Enable the appropriate information level in the logging configuration to see the default log messages. Additional logging, such as the logging of request headers, is only included at trace level.

The log category used for each client includes the name of the client. A client named MyNamedClient, for example, logs messages with a category of "System.Net.Http.HttpClient.MyNamedClient.LogicalHandler". Messages suffixed with LogicalHandler occur outside the request handler pipeline. On the request, messages are logged before any other handlers in the pipeline have processed it. On the response, messages are logged after any other pipeline handlers have received the response.

Logging also occurs inside the request handler pipeline. In the MyNamedClient example, those messages are logged with the log category "System.Net.Http.HttpClient.MyNamedClient.ClientHandler". For the request, this occurs after all other handlers have run and immediately before the request is sent. On the response, this logging includes the state of the response before it passes back through the handler pipeline.

Enabling logging outside and inside the pipeline enables inspection of the changes made by the other pipeline handlers. This may include changes to request headers or to the response status code.

Including the name of the client in the log category enables log filtering for specific named clients.

Configure the HttpMessageHandler

It may be necessary to control the configuration of the inner HttpMessageHandler used by a client.

An IHttpClientBuilder is returned when adding named or typed clients. The xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.ConfigurePrimaryHttpMessageHandler%2A extension method can be used to define a delegate. The delegate is used to create and configure the primary HttpMessageHandler used by that client:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientConfigureHttpMessageHandler":::

Cookies

The pooled HttpMessageHandler instances results in CookieContainer objects being shared. Unanticipated CookieContainer object sharing often results in incorrect code. For apps that require cookies, consider either:

Disabling automatic cookie handling
Avoiding IHttpClientFactory

Call xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.ConfigurePrimaryHttpMessageHandler%2A to disable automatic cookie handling:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Program.cs" id="snippet_AddHttpClientNoAutomaticCookies":::

Use IHttpClientFactory in a console app

In a console app, add the following package references to the project:

Microsoft.Extensions.Hosting
Microsoft.Extensions.Http

In the following example:

xref:System.Net.Http.IHttpClientFactory and GitHubService are registered in the Generic Host's service container.
GitHubService is requested from DI, which in-turn requests an instance of IHttpClientFactory.
GitHubService uses IHttpClientFactory to create an instance of HttpClient, which it uses to retrieve docs GitHub branches.

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsConsoleSample/Program.cs" highlight="10-11,17-18,40-41,56":::

Header propagation middleware

Header propagation is an ASP.NET Core middleware to propagate HTTP headers from the incoming request to the outgoing HttpClient requests. To use header propagation:

Install the Microsoft.AspNetCore.HeaderPropagation package.
Configure the HttpClient and middleware pipeline in Program.cs:

:::code language="csharp" source="http-requests/samples/6.x/HttpRequestsSample/Snippets/Program.cs" id="snippet_AddHttpClientHeaderPropagation" highlight="4-10,17":::
Make outbound requests using the configured HttpClient instance, which includes the added headers.

Additional resources

View or download sample code (how to download)
Use HttpClientFactory to implement resilient HTTP requests
Implement HTTP call retries with exponential backoff with HttpClientFactory and Polly policies
Implement the Circuit Breaker pattern
How to serialize and deserialize JSON in .NET

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

By Kirk Larkin, Steve Gordon, Glenn Condron, and Ryan Nowak.

An xref:System.Net.Http.IHttpClientFactory can be registered and used to configure and create xref:System.Net.Http.HttpClient instances in an app. IHttpClientFactory offers the following benefits:

Provides a central location for naming and configuring logical HttpClient instances. For example, a client named github could be registered and configured to access GitHub. A default client can be registered for general access.
Codifies the concept of outgoing middleware via delegating handlers in HttpClient. Provides extensions for Polly-based middleware to take advantage of delegating handlers in HttpClient.
Manages the pooling and lifetime of underlying HttpClientMessageHandler instances. Automatic management avoids common DNS (Domain Name System) problems that occur when manually managing HttpClient lifetimes.
Adds a configurable logging experience (via ILogger) for all requests sent through clients created by the factory.

View or download sample code (how to download).

The sample code in this topic version uses xref:System.Text.Json to deserialize JSON content returned in HTTP responses. For samples that use Json.NET and ReadAsAsync<T>, use the version selector to select a 2.x version of this topic.

Consumption patterns

There are several ways IHttpClientFactory can be used in an app:

Basic usage
Named clients
Typed clients
Generated clients

The best approach depends upon the app's requirements.

Basic usage

IHttpClientFactory can be registered by calling AddHttpClient:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet1" highlight="13":::

An IHttpClientFactory can be requested using dependency injection (DI). The following code uses IHttpClientFactory to create an HttpClient instance:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/BasicUsage.cshtml.cs" id="snippet1" highlight="9-12,21":::

Using IHttpClientFactory like in the preceding example is a good way to refactor an existing app. It has no impact on how HttpClient is used. In places where HttpClient instances are created in an existing app, replace those occurrences with calls to xref:System.Net.Http.IHttpClientFactory.CreateClient%2A.

Named clients

Named clients are a good choice when:

The app requires many distinct uses of HttpClient.
Many HttpClients have different configuration.

Configuration for a named HttpClient can be specified during registration in Startup.ConfigureServices:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet2":::

In the preceding code the client is configured with:

The base address https://api.github.com/.
Two headers required to work with the GitHub API.

CreateClient

Each time xref:System.Net.Http.IHttpClientFactory.CreateClient%2A is called:

A new instance of HttpClient is created.
The configuration action is called.

To create a named client, pass its name into CreateClient:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/NamedClient.cshtml.cs" id="snippet1" highlight="21":::

In the preceding code, the request doesn't need to specify a hostname. The code can pass just the path, since the base address configured for the client is used.

Typed clients

Typed clients:

Provide the same capabilities as named clients without the need to use strings as keys.
Provides IntelliSense and compiler help when consuming clients.
Provide a single location to configure and interact with a particular HttpClient. For example, a single typed client might be used:
- For a single backend endpoint.
- To encapsulate all logic dealing with the endpoint.
Work with DI and can be injected where required in the app.

A typed client accepts an HttpClient parameter in its constructor:

:::code language="csharp" source="http-requests/samples/5.x/HttpClientFactorySample/GitHub/GitHubService.cs" id="snippet1" highlight="5":::

In the preceding code:

The configuration is moved into the typed client.
The HttpClient object is exposed as a public property.

API-specific methods can be created that expose HttpClient functionality. For example, the GetAspNetDocsIssues method encapsulates code to retrieve open issues.

The following code calls xref:Microsoft.Extensions.DependencyInjection.HttpClientFactoryServiceCollectionExtensions.AddHttpClient%2A in Startup.ConfigureServices to register a typed client class:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet3":::

The typed client is registered as transient with DI. In the preceding code, AddHttpClient registers GitHubService as a transient service. This registration uses a factory method to:

Create an instance of HttpClient.
Create an instance of GitHubService, passing in the instance of HttpClient to its constructor.

The typed client can be injected and consumed directly:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/TypedClient.cshtml.cs" id="snippet1" highlight="11-14,20":::

The configuration for a typed client can be specified during registration in Startup.ConfigureServices, rather than in the typed client's constructor:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet4":::

The HttpClient can be encapsulated within a typed client. Rather than exposing it as a property, define a method which calls the HttpClient instance internally:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/GitHub/RepoService.cs" id="snippet1" highlight="4":::

In the preceding code, the HttpClient is stored in a private field. Access to the HttpClient is by the public GetRepos method.

Generated clients

IHttpClientFactory can be used in combination with third-party libraries such as Refit. Refit is a REST library for .NET. It converts REST APIs into live interfaces. An implementation of the interface is generated dynamically by the RestService, using HttpClient to make the external HTTP calls.

An interface and a reply are defined to represent the external API and its response:

public interface IHelloClient
{
    [Get("/helloworld")]
    Task<Reply> GetMessageAsync();
}

public class Reply
{
    public string Message { get; set; }
}

A typed client can be added, using Refit to generate the implementation:

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient("hello", c =>
    {
        c.BaseAddress = new Uri("http://localhost:5000");
    })
    .AddTypedClient(c => Refit.RestService.For<IHelloClient>(c));

    services.AddControllers();
}

The defined interface can be consumed where necessary, with the implementation provided by DI and Refit:

[ApiController]
public class ValuesController : ControllerBase
{
    private readonly IHelloClient _client;

    public ValuesController(IHelloClient client)
    {
        _client = client;
    }

    [HttpGet("/")]
    public async Task<ActionResult<Reply>> Index()
    {
        return await _client.GetMessageAsync();
    }
}

Make POST, PUT, and DELETE requests

In the preceding examples, all HTTP requests use the GET HTTP verb. HttpClient also supports other HTTP verbs, including:

POST
PUT
DELETE
PATCH

For a complete list of supported HTTP verbs, see xref:System.Net.Http.HttpMethod.

The following example shows how to make an HTTP POST request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_POST":::

In the preceding code, the CreateItemAsync method:

Serializes the TodoItem parameter to JSON using System.Text.Json. This uses an instance of xref:System.Text.Json.JsonSerializerOptions to configure the serialization process.
Creates an instance of xref:System.Net.Http.StringContent to package the serialized JSON for sending in the HTTP request's body.
Calls xref:System.Net.Http.HttpClient.PostAsync%2A to send the JSON content to the specified URL. This is a relative URL that gets added to the HttpClient.BaseAddress.
Calls xref:System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode%2A to throw an exception if the response status code does not indicate success.

HttpClient also supports other types of content. For example, xref:System.Net.Http.MultipartContent and xref:System.Net.Http.StreamContent. For a complete list of supported content, see xref:System.Net.Http.HttpContent.

The following example shows an HTTP PUT request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_PUT":::

The preceding code is very similar to the POST example. The SaveItemAsync method calls xref:System.Net.Http.HttpClient.PutAsync%2A instead of PostAsync.

The following example shows an HTTP DELETE request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_DELETE":::

In the preceding code, the DeleteItemAsync method calls xref:System.Net.Http.HttpClient.DeleteAsync%2A. Because HTTP DELETE requests typically contain no body, the DeleteAsync method doesn't provide an overload that accepts an instance of HttpContent.

To learn more about using different HTTP verbs with HttpClient, see xref:System.Net.Http.HttpClient.

Outgoing request middleware

HttpClient has the concept of delegating handlers that can be linked together for outgoing HTTP requests. IHttpClientFactory:

Simplifies defining the handlers to apply for each named client.
Supports registration and chaining of multiple handlers to build an outgoing request middleware pipeline. Each of these handlers is able to perform work before and after the outgoing request. This pattern:
- Is similar to the inbound middleware pipeline in ASP.NET Core.
- Provides a mechanism to manage cross-cutting concerns around HTTP requests, such as:
  - caching
  - error handling
  - serialization
  - logging

To create a delegating handler:

Derive from xref:System.Net.Http.DelegatingHandler.
Override xref:System.Net.Http.DelegatingHandler.SendAsync%2A. Execute code before passing the request to the next handler in the pipeline:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Handlers/ValidateHeaderHandler.cs" id="snippet1":::

The preceding code checks if the X-API-KEY header is in the request. If X-API-KEY is missing, xref:System.Net.HttpStatusCode.BadRequest is returned.

More than one handler can be added to the configuration for an HttpClient with xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.AddHttpMessageHandler%2A?displayProperty=fullName:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup2.cs" id="snippet1":::

In the preceding code, the ValidateHeaderHandler is registered with DI. Once registered, xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.AddHttpMessageHandler%2A can be called, passing in the type for the handler.

Multiple handlers can be registered in the order that they should execute. Each handler wraps the next handler until the final HttpClientHandler executes the request:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet6":::

Use DI in outgoing request middleware

When IHttpClientFactory creates a new delegating handler, it uses DI to fulfill the handler's constructor parameters. IHttpClientFactory creates a separate DI scope for each handler, which can lead to surprising behavior when a handler consumes a scoped service.

For example, consider the following interface and its implementation, which represents a task as an operation with an identifier, OperationId:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/OperationScoped.cs" id="snippet_Types":::

As its name suggests, IOperationScoped is registered with DI using a scoped lifetime:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Startup.cs" id="snippet_IOperationScoped" highlight="18,26":::

The following delegating handler consumes and uses IOperationScoped to set the X-OPERATION-ID header for the outgoing request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Handlers/OperationHandler.cs" id="snippet_Class" highlight="13":::

In the HttpRequestsSample download], navigate to /Operation and refresh the page. The request scope value changes for each request, but the handler scope value only changes every 5 seconds.

Handlers can depend upon services of any scope. Services that handlers depend upon are disposed when the handler is disposed.

Use one of the following approaches to share per-request state with message handlers:

Pass data into the handler using xref:System.Net.Http.HttpRequestMessage.Options%2A?displayProperty=nameWithType.
Use xref:Microsoft.AspNetCore.Http.IHttpContextAccessor to access the current request.
Create a custom xref:System.Threading.AsyncLocal%601 storage object to pass the data.

Use Polly-based handlers

IHttpClientFactory integrates with the third-party library Polly. Polly is a comprehensive resilience and transient fault-handling library for .NET. It allows developers to express policies such as Retry, Circuit Breaker, Timeout, Bulkhead Isolation, and Fallback in a fluent and thread-safe manner.

Extension methods are provided to enable the use of Polly policies with configured HttpClient instances. The Polly extensions support adding Polly-based handlers to clients. Polly requires the Microsoft.Extensions.Http.Polly NuGet package.

Handle transient faults

Faults typically occur when external HTTP calls are transient. xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddTransientHttpErrorPolicy%2A allows a policy to be defined to handle transient errors. Policies configured with AddTransientHttpErrorPolicy handle the following responses:

xref:System.Net.Http.HttpRequestException
HTTP 5xx
HTTP 408

AddTransientHttpErrorPolicy provides access to a PolicyBuilder object configured to handle errors representing a possible transient fault:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup3.cs" id="snippet1":::

In the preceding code, a WaitAndRetryAsync policy is defined. Failed requests are retried up to three times with a delay of 600 ms between attempts.

Dynamically select policies

Extension methods are provided to add Polly-based handlers, for example, xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddPolicyHandler%2A. The following AddPolicyHandler overload inspects the request to decide which policy to apply:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet8":::

In the preceding code, if the outgoing request is an HTTP GET, a 10-second timeout is applied. For any other HTTP method, a 30-second timeout is used.

Add multiple Polly handlers

It's common to nest Polly policies:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet9":::

In the preceding example:

Two handlers are added.
The first handler uses xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddTransientHttpErrorPolicy%2A to add a retry policy. Failed requests are retried up to three times.
The second AddTransientHttpErrorPolicy call adds a circuit breaker policy. Further external requests are blocked for 30 seconds if 5 failed attempts occur sequentially. Circuit breaker policies are stateful. All calls through this client share the same circuit state.

Add policies from the Polly registry

An approach to managing regularly used policies is to define them once and register them with a PolicyRegistry.

In the following code:

The "regular" and "long" policies are added.
xref:Microsoft.Extensions.DependencyInjection.PollyHttpClientBuilderExtensions.AddPolicyHandlerFromRegistry%2A adds the "regular" and "long" policies from the registry.

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup4.cs" id="snippet1":::

For more information on IHttpClientFactory and Polly integrations, see the Polly wiki.

HttpClient and lifetime management

A new HttpClient instance is returned each time CreateClient is called on the IHttpClientFactory. An xref:System.Net.Http.HttpMessageHandler is created per named client. The factory manages the lifetimes of the HttpMessageHandler instances.

IHttpClientFactory pools the HttpMessageHandler instances created by the factory to reduce resource consumption. An HttpMessageHandler instance may be reused from the pool when creating a new HttpClient instance if its lifetime hasn't expired.

Pooling of handlers is desirable as each handler typically manages its own underlying HTTP connections. Creating more handlers than necessary can result in connection delays. Some handlers also keep connections open indefinitely, which can prevent the handler from reacting to DNS (Domain Name System) changes.

The default handler lifetime is two minutes. The default value can be overridden on a per named client basis:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup5.cs" id="snippet1":::

HttpClient instances can generally be treated as .NET objects not requiring disposal. Disposal cancels outgoing requests and guarantees the given HttpClient instance can't be used after calling xref:System.IDisposable.Dispose%2A. IHttpClientFactory tracks and disposes resources used by HttpClient instances.

Keeping a single HttpClient instance alive for a long duration is a common pattern used before the inception of IHttpClientFactory. This pattern becomes unnecessary after migrating to IHttpClientFactory.

Alternatives to IHttpClientFactory

Using IHttpClientFactory in a DI-enabled app avoids:

Resource exhaustion problems by pooling HttpMessageHandler instances.
Stale DNS problems by cycling HttpMessageHandler instances at regular intervals.

There are alternative ways to solve the preceding problems using a long-lived xref:System.Net.Http.SocketsHttpHandler instance.

Create an instance of SocketsHttpHandler when the app starts and use it for the life of the app.
Configure xref:System.Net.Http.SocketsHttpHandler.PooledConnectionLifetime to an appropriate value based on DNS refresh times.
Create HttpClient instances using new HttpClient(handler, disposeHandler: false) as needed.

The preceding approaches solve the resource management problems that IHttpClientFactory solves in a similar way.

The SocketsHttpHandler shares connections across HttpClient instances. This sharing prevents socket exhaustion.
The SocketsHttpHandler cycles connections according to PooledConnectionLifetime to avoid stale DNS problems.

Cookies

The pooled HttpMessageHandler instances results in CookieContainer objects being shared. Unanticipated CookieContainer object sharing often results in incorrect code. For apps that require cookies, consider either:

Disabling automatic cookie handling
Avoiding IHttpClientFactory

Call xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.ConfigurePrimaryHttpMessageHandler%2A to disable automatic cookie handling:

:::code language="csharp" source="http-requests/samples/2.x/HttpClientFactorySample/Startup.cs" id="snippet13":::

Logging

Clients created via IHttpClientFactory record log messages for all requests. Enable the appropriate information level in the logging configuration to see the default log messages. Additional logging, such as the logging of request headers, is only included at trace level.

The log category used for each client includes the name of the client. A client named MyNamedClient, for example, logs messages with a category of "System.Net.Http.HttpClient.MyNamedClient.LogicalHandler". Messages suffixed with LogicalHandler occur outside the request handler pipeline. On the request, messages are logged before any other handlers in the pipeline have processed it. On the response, messages are logged after any other pipeline handlers have received the response.

Logging also occurs inside the request handler pipeline. In the MyNamedClient example, those messages are logged with the log category "System.Net.Http.HttpClient.MyNamedClient.ClientHandler". For the request, this occurs after all other handlers have run and immediately before the request is sent. On the response, this logging includes the state of the response before it passes back through the handler pipeline.

Enabling logging outside and inside the pipeline enables inspection of the changes made by the other pipeline handlers. This may include changes to request headers or to the response status code.

Including the name of the client in the log category enables log filtering for specific named clients.

Configure the HttpMessageHandler

It may be necessary to control the configuration of the inner HttpMessageHandler used by a client.

An IHttpClientBuilder is returned when adding named or typed clients. The xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.ConfigurePrimaryHttpMessageHandler%2A extension method can be used to define a delegate. The delegate is used to create and configure the primary HttpMessageHandler used by that client:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup6.cs" id="snippet1":::

Use IHttpClientFactory in a console app

In a console app, add the following package references to the project:

Microsoft.Extensions.Hosting
Microsoft.Extensions.Http

In the following example:

xref:System.Net.Http.IHttpClientFactory is registered in the Generic Host's service container.
MyService creates a client factory instance from the service, which is used to create an HttpClient. HttpClient is used to retrieve a webpage.
Main creates a scope to execute the service's GetPage method and write the first 500 characters of the webpage content to the console.

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactoryConsoleSample/Program.cs" highlight="14-15,20,26-27,59-62":::

Header propagation middleware

Header propagation is an ASP.NET Core middleware to propagate HTTP headers from the incoming request to the outgoing HTTP Client requests. To use header propagation:

Reference the Microsoft.AspNetCore.HeaderPropagation package.
Configure the middleware and HttpClient in Startup:

:::code language="csharp" source="http-requests/samples/3.x/Startup.cs" id="snippet" highlight="5-9,21":::

The client includes the configured headers on outbound requests:

var client = clientFactory.CreateClient("MyForwardingClient");
var response = client.GetAsync(...);

Additional resources

Use HttpClientFactory to implement resilient HTTP requests
Implement HTTP call retries with exponential backoff with HttpClientFactory and Polly policies
Implement the Circuit Breaker pattern
How to serialize and deserialize JSON in .NET

:::moniker-end

:::moniker range=">= aspnetcore-3.0 < aspnetcore-5.0"

By Kirk Larkin, Steve Gordon, Glenn Condron, and Ryan Nowak.

An xref:System.Net.Http.IHttpClientFactory can be registered and used to configure and create xref:System.Net.Http.HttpClient instances in an app. IHttpClientFactory offers the following benefits:

Provides a central location for naming and configuring logical HttpClient instances. For example, a client named github could be registered and configured to access GitHub. A default client can be registered for general access.
Codifies the concept of outgoing middleware via delegating handlers in HttpClient. Provides extensions for Polly-based middleware to take advantage of delegating handlers in HttpClient.
Manages the pooling and lifetime of underlying HttpClientMessageHandler instances. Automatic management avoids common DNS (Domain Name System) problems that occur when manually managing HttpClient lifetimes.
Adds a configurable logging experience (via ILogger) for all requests sent through clients created by the factory.

View or download sample code (how to download).

The sample code in this topic version uses xref:System.Text.Json to deserialize JSON content returned in HTTP responses. For samples that use Json.NET and ReadAsAsync<T>, use the version selector to select a 2.x version of this topic.

Consumption patterns

There are several ways IHttpClientFactory can be used in an app:

Basic usage
Named clients
Typed clients
Generated clients

The best approach depends upon the app's requirements.

Basic usage

IHttpClientFactory can be registered by calling AddHttpClient:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet1" highlight="13":::

An IHttpClientFactory can be requested using dependency injection (DI). The following code uses IHttpClientFactory to create an HttpClient instance:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/BasicUsage.cshtml.cs" id="snippet1" highlight="9-12,21":::

Using IHttpClientFactory like in the preceding example is a good way to refactor an existing app. It has no impact on how HttpClient is used. In places where HttpClient instances are created in an existing app, replace those occurrences with calls to xref:System.Net.Http.IHttpClientFactory.CreateClient%2A.

Named clients

Named clients are a good choice when:

The app requires many distinct uses of HttpClient.
Many HttpClients have different configuration.

Configuration for a named HttpClient can be specified during registration in Startup.ConfigureServices:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet2":::

In the preceding code the client is configured with:

The base address https://api.github.com/.
Two headers required to work with the GitHub API.

CreateClient

Each time xref:System.Net.Http.IHttpClientFactory.CreateClient%2A is called:

A new instance of HttpClient is created.
The configuration action is called.

To create a named client, pass its name into CreateClient:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/NamedClient.cshtml.cs" id="snippet1" highlight="21":::

In the preceding code, the request doesn't need to specify a hostname. The code can pass just the path, since the base address configured for the client is used.

Typed clients

Typed clients:

Provide the same capabilities as named clients without the need to use strings as keys.
Provides IntelliSense and compiler help when consuming clients.
Provide a single location to configure and interact with a particular HttpClient. For example, a single typed client might be used:
- For a single backend endpoint.
- To encapsulate all logic dealing with the endpoint.
Work with DI and can be injected where required in the app.

A typed client accepts an HttpClient parameter in its constructor:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/GitHub/GitHubService.cs" id="snippet1" highlight="5"::: [!INCLUDEabout the series]

In the preceding code:

The configuration is moved into the typed client.
The HttpClient object is exposed as a public property.

API-specific methods can be created that expose HttpClient functionality. For example, the GetAspNetDocsIssues method encapsulates code to retrieve open issues.

The following code calls xref:Microsoft.Extensions.DependencyInjection.HttpClientFactoryServiceCollectionExtensions.AddHttpClient%2A in Startup.ConfigureServices to register a typed client class:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet3":::

The typed client is registered as transient with DI. In the preceding code, AddHttpClient registers GitHubService as a transient service. This registration uses a factory method to:

Create an instance of HttpClient.
Create an instance of GitHubService, passing in the instance of HttpClient to its constructor.

The typed client can be injected and consumed directly:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Pages/TypedClient.cshtml.cs" id="snippet1" highlight="11-14,20":::

The configuration for a typed client can be specified during registration in Startup.ConfigureServices, rather than in the typed client's constructor:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/Startup.cs" id="snippet4":::

The HttpClient can be encapsulated within a typed client. Rather than exposing it as a property, define a method which calls the HttpClient instance internally:

:::code language="csharp" source="http-requests/samples/3.x/HttpClientFactorySample/GitHub/RepoService.cs" id="snippet1" highlight="4":::

In the preceding code, the HttpClient is stored in a private field. Access to the HttpClient is by the public GetRepos method.

Generated clients

IHttpClientFactory can be used in combination with third-party libraries such as Refit. Refit is a REST library for .NET. It converts REST APIs into live interfaces. An implementation of the interface is generated dynamically by the RestService, using HttpClient to make the external HTTP calls.

An interface and a reply are defined to represent the external API and its response:

public interface IHelloClient
{
    [Get("/helloworld")]
    Task<Reply> GetMessageAsync();
}

public class Reply
{
    public string Message { get; set; }
}

A typed client can be added, using Refit to generate the implementation:

public void ConfigureServices(IServiceCollection services)
{
    services.AddHttpClient("hello", c =>
    {
        c.BaseAddress = new Uri("http://localhost:5000");
    })
    .AddTypedClient(c => Refit.RestService.For<IHelloClient>(c));

    services.AddControllers();
}

The defined interface can be consumed where necessary, with the implementation provided by DI and Refit:

[ApiController]
public class ValuesController : ControllerBase
{
    private readonly IHelloClient _client;

    public ValuesController(IHelloClient client)
    {
        _client = client;
    }

    [HttpGet("/")]
    public async Task<ActionResult<Reply>> Index()
    {
        return await _client.GetMessageAsync();
    }
}

Make POST, PUT, and DELETE requests

In the preceding examples, all HTTP requests use the GET HTTP verb. HttpClient also supports other HTTP verbs, including:

POST
PUT
DELETE
PATCH

For a complete list of supported HTTP verbs, see xref:System.Net.Http.HttpMethod.

The following example shows how to make an HTTP POST request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_POST":::

In the preceding code, the CreateItemAsync method:

Serializes the TodoItem parameter to JSON using System.Text.Json. This uses an instance of xref:System.Text.Json.JsonSerializerOptions to configure the serialization process.
Creates an instance of xref:System.Net.Http.StringContent to package the serialized JSON for sending in the HTTP request's body.
Calls xref:System.Net.Http.HttpClient.PostAsync%2A to send the JSON content to the specified URL. This is a relative URL that gets added to the HttpClient.BaseAddress.
Calls xref:System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode%2A to throw an exception if the response status code does not indicate success.

HttpClient also supports other types of content. For example, xref:System.Net.Http.MultipartContent and xref:System.Net.Http.StreamContent. For a complete list of supported content, see xref:System.Net.Http.HttpContent.

The following example shows an HTTP PUT request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_PUT":::

The preceding code is very similar to the POST example. The SaveItemAsync method calls xref:System.Net.Http.HttpClient.PutAsync%2A instead of PostAsync.

The following example shows an HTTP DELETE request:

:::code language="csharp" source="http-requests/samples/3.x/HttpRequestsSample/Models/TodoClient.cs" id="snippet_DELETE":::

In the preceding code, the DeleteItemAsync method calls xref:System.Net.Http.HttpClient.DeleteAsync%2A. Because HTTP DELETE requests typically contain no body, the DeleteAsync method doesn't provide an overload that accepts an instance of HttpContent.

To learn more about using different HTTP verbs with HttpClient, see xref:System.Net.Http.HttpClient.

Outgoing request middleware

HttpClient has the concept of delegating handlers that can be linked together for outgoing HTTP requests. IHttpClientFactory:

Simplifies defining the handlers to apply for each named client.
Supports registration and chaining of multiple handlers to build an outgoing request middleware pipeline. Each of these handlers is able to perform work before and after the outgoing request. This pattern:
- Is similar to the inbound middleware pipeline in ASP.NET Core.
- Provides a mechanism to manage cross-cutting concerns around HTTP requests, such as:
  - caching
  - error handling
  - serialization
  - logging

To create a delegating handler:

Derive from xref:System.Net.Http.DelegatingHandler.
Override xref:System.Net.Http.DelegatingHandler.SendAsync%2A. Execute code before passing the request to the next handler in the pipeline: