title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Call a web API from an ASP.NET Core Blazor app	guardrex	Learn how to call a web API from Blazor apps.	>= aspnetcore-3.1	riande	mvc	03/08/2024	blazor/call-web-api

Call a web API from ASP.NET Core Blazor

[!INCLUDE]

This article describes how to call a web API from a Blazor app.

Package

The System.Net.Http.Json package provides extension methods for xref:System.Net.Http.HttpClient?displayProperty=fullName and xref:System.Net.Http.HttpContent?displayProperty=fullName that perform automatic serialization and deserialization using System.Text.Json. The System.Net.Http.Json package is provided by the .NET shared framework and doesn't require adding a package reference to the app.

:::moniker range=">= aspnetcore-8.0"

Sample apps

See the sample apps in the dotnet/blazor-samples GitHub repository.

`BlazorWebAppCallWebApi`

Call an external (not in the Blazor Web App) todo list web API from a Blazor Web App:

Backend: A web API app for maintaining a todo list, based on Minimal APIs. The web API app is a separate app from the Blazor Web App, possibly hosted on a different server.
BlazorApp/BlazorApp.Client: A Blazor Web App that calls the web API app with an xref:System.Net.Http.HttpClient for todo list operations, such as creating, reading, updating, and deleting (CRUD) items from the todo list.

For client-side rendering (CSR), which includes Interactive WebAssembly components and Auto components that have adopted CSR, calls are made with a preconfigured xref:System.Net.Http.HttpClient registered in the Program file of the client project (BlazorApp.Client):

builder.Services.AddScoped(sp =>
    new HttpClient
    {
        BaseAddress = new Uri(builder.Configuration["FrontendUrl"] ?? "https://localhost:5002")
    });

For server-side rendering (SSR), which includes prerendered and interactive Server components, prerendered WebAssembly components, and Auto components that are prerendered or have adopted SSR, calls are made with an xref:System.Net.Http.HttpClient registered in the Program file of the server project (BlazorApp):

builder.Services.AddHttpClient();

Call an internal (inside the Blazor Web App) movie list API, where the API resides in the server project of the Blazor Web App:

BlazorApp: A Blazor Web App that maintains a movie list:
- When operations are performed on the movie list within the app on the server, ordinary API calls are used.
- When API calls are made by a web-based client, a web API is used for movie list operations, based on Minimal APIs.
BlazorApp.Client: The client project of the Blazor Web App, which contains Interactive WebAssembly and Auto components for user management of the movie list.

For CSR, which includes Interactive WebAssembly components and Auto components that have adopted CSR, calls to the API are made via a client-based service (ClientMovieService) that uses a preconfigured xref:System.Net.Http.HttpClient registered in the Program file of the client project (BlazorApp.Client). Because these calls are made over a public or private web, the movie list API is a web API.

The following example obtains a list of movies from the /movies endpoint:

public class ClientMovieService(HttpClient http) : IMovieService
{
    public async Task<Movie[]> GetMoviesAsync(bool watchedMovies)
    {
        return await http.GetFromJsonAsync<Movie[]>("movies") ?? [];
    }
}

For SSR, which includes prerendered and interactive Server components, prerendered WebAssembly components, and Auto components that are prerendered or have adopted SSR, calls are made directly via a server-based service (ServerMovieService). The API doesn't rely on a network, so it's a standard API for movie list CRUD operations.

The following example obtains a list of movies:

public class ServerMovieService(MovieContext db) : IMovieService
{
    public async Task<Movie[]> GetMoviesAsync(bool watchedMovies)
    {
        return watchedMovies ? 
            await db.Movies.Where(t => t.IsWatched).ToArrayAsync() : 
            await db.Movies.ToArrayAsync();
    }
}

`BlazorWebAppCallWebApi_Weather`

A weather data sample app that uses streaming rendering for weather data.

`BlazorWebAssemblyCallWebApi`

Calls a todo list web API from a Blazor WebAssembly app:

Backend: A web API app for maintaining a todo list, based on Minimal APIs.
BlazorTodo: A Blazor WebAssembly app that calls the web API with a preconfigured xref:System.Net.Http.HttpClient for todo list CRUD operations.

:::moniker-end

Server-side scenarios for calling external web APIs

Server-based components call external web APIs using xref:System.Net.Http.HttpClient instances, typically created using xref:System.Net.Http.IHttpClientFactory. For guidance that applies to server-side apps, see xref:fundamentals/http-requests.

A server-side app doesn't include an xref:System.Net.Http.HttpClient service. Provide an xref:System.Net.Http.HttpClient to the app using the HttpClient factory infrastructure.

In the Program file:

builder.Services.AddHttpClient();

The following Razor component makes a request to a web API for GitHub branches similar to the Basic Usage example in the xref:fundamentals/http-requests article.

CallWebAPI.razor:

@page "/call-web-api"
@using System.Text.Json
@using System.Text.Json.Serialization
@inject IHttpClientFactory ClientFactory

<h1>Call web API from a Blazor Server Razor component</h1>

@if (getBranchesError || branches is null)
{
    <p>Unable to get branches from GitHub. Please try again later.</p>
}
else
{
    <ul>
        @foreach (var branch in branches)
        {
            <li>@branch.Name</li>
        }
    </ul>
}

@code {
    private IEnumerable<GitHubBranch>? branches = [];
    private bool getBranchesError;
    private bool shouldRender;

    protected override bool ShouldRender() => shouldRender;

    protected override async Task OnInitializedAsync()
    {
        var request = new HttpRequestMessage(HttpMethod.Get,
            "https://api.github.com/repos/dotnet/AspNetCore.Docs/branches");
        request.Headers.Add("Accept", "application/vnd.github.v3+json");
        request.Headers.Add("User-Agent", "HttpClientFactory-Sample");

        var client = ClientFactory.CreateClient();

        var response = await client.SendAsync(request);

        if (response.IsSuccessStatusCode)
        {
            using var responseStream = await response.Content.ReadAsStreamAsync();
            branches = await JsonSerializer.DeserializeAsync
                <IEnumerable<GitHubBranch>>(responseStream);
        }
        else
        {
            getBranchesError = true;
        }

        shouldRender = true;
    }

    public class GitHubBranch
    {
        [JsonPropertyName("name")]
        public string? Name { get; set; }
    }
}

In the preceding example for C# 12 or later, an empty array ([]) is created for the branches variable. For earlier versions of C#, create an empty array (Array.Empty<GitHubBranch>()).

For an additional working example, see the server-side file upload example that uploads files to a web API controller in the xref:blazor/file-uploads#upload-files-to-a-server-with-server-side-rendering article.

:::moniker range=">= aspnetcore-8.0"

Service abstractions for web API calls

This section applies to Blazor Web Apps that maintain a web API in the server project or transform web API calls to an external web API.

When using the interactive WebAssembly and Auto render modes, components are prerendered by default. Auto components are also initially rendered interactively from the server before the Blazor bundle downloads to the client and the client-side runtime activates. This means that components using these render modes should be designed so that they run successfully from both the client and the server. If the component must call a server project-based API or transform a request to an external web API (one that's outside of the Blazor Web App) when running on the client, the recommended approach is to abstract that API call behind a service interface and implement client and server versions of the service:

The client version calls the web API with a preconfigured xref:System.Net.Http.HttpClient.
The server version can typically access the server-side resources directly. Injecting an xref:System.Net.Http.HttpClient on the server that makes calls back to the server isn't recommended, as the network request is typically unnecessary. Alternatively, the API might be external to the server project, but a service abstraction for the server is required to transform the request in some way, for example to add an access token to a proxied request.

When using the WebAssembly render mode, you also have the option of disabling prerendering, so the components only render from the client. For more information, see xref:blazor/components/render-modes#prerendering.

Examples (sample apps):

Movie list web API in the BlazorWebAppCallWebApi sample app.
Streaming rendering weather data web API in the BlazorWebAppCallWebApi_Weather sample app.
Weather data returned to the client in either the BlazorWebAppOidc (non-BFF pattern) or BlazorWebAppOidcBff (BFF pattern) sample apps. These apps demonstrate secure (web) API calls. For more information, see xref:blazor/security/blazor-web-app-oidc.

Blazor Web App external web APIs

This section applies to Blazor Web Apps that call a web API maintained by a separate (external) project, possibly hosted on a different server.

Blazor Web Apps normally prerender client-side WebAssembly components, and Auto components render on the server during static or interactive server-side rendering (SSR). xref:System.Net.Http.HttpClient services aren't registered by default in a Blazor Web App's main project. If the app is run with only the xref:System.Net.Http.HttpClient services registered in the .Client project, as described in the Add the HttpClient service section, executing the app results in a runtime error:

:::no-loc text="InvalidOperationException: Cannot provide a value for property 'Http' on type '...{COMPONENT}'. There is no registered service of type 'System.Net.Http.HttpClient'.":::

Use either of the following approaches:

Add the xref:System.Net.Http.HttpClient services to the server project to make the xref:System.Net.Http.HttpClient available during SSR. Use the following service registration in the server project's Program file:
```
builder.Services.AddHttpClient();
```
xref:System.Net.Http.HttpClient services are provided by the shared framework, so a package reference in the app's project file isn't required.

Example: Todo list web API in the BlazorWebAppCallWebApi sample app
If prerendering isn't required for a WebAssembly component that calls the web API, disable prerendering by following the guidance in xref:blazor/components/render-modes#prerendering. If you adopt this approach, you don't need to add xref:System.Net.Http.HttpClient services to the main project of the Blazor Web App because the component isn't prerendered on the server.

For more information, see Client-side services fail to resolve during prerendering.

Prerendered data

When prerendering, components render twice: first statically, then interactively. State doesn't automatically flow from the prerendered component to the interactive one. If a component performs asynchronous initialization operations and renders different content for different states during initialization, such as a "Loading..." progress indicator, you may see a flicker when the component renders twice.

You can address this by flowing prerendered state using the Persistent Component State API, which the BlazorWebAppCallWebApi and BlazorWebAppCallWebApi_Weather sample apps demonstrate. When the component renders interactively, it can render the same way using the same state. However, the API doesn't currently work with enhanced navigation, which you can work around by disabling enhanced navigation on links to the page (data-enhanced-nav=false). For more information, see the following resources:

xref:blazor/components/prerender#persist-prerendered-state
xref:blazor/fundamentals/routing#enhanced-navigation-and-form-handling
Support persistent component state across enhanced page navigations (dotnet/aspnetcore #51584)

:::moniker-end

Add the `HttpClient` service

The guidance in this section applies to client-side scenarios.

Client-side components call web APIs using a preconfigured xref:System.Net.Http.HttpClient service, which is focused on making requests back to the server of origin. Additional xref:System.Net.Http.HttpClient service configurations for other web APIs can be created in developer code. Requests are composed using Blazor JSON helpers or with xref:System.Net.Http.HttpRequestMessage. Requests can include Fetch API option configuration.

The configuration examples in this section are only useful when a single web API is called for a single xref:System.Net.Http.HttpClient instance in the app. When the app must call multiple web APIs, each with its own base address and configuration, you can adopt the following approaches, which are covered later in this article:

Named HttpClient with IHttpClientFactory: Each web API is provided a unique name. When app code or a Razor component calls a web API, it uses a named xref:System.Net.Http.HttpClient instance to make the call.
Typed HttpClient: Each web API is typed. When app code or a Razor component calls a web API, it uses a typed xref:System.Net.Http.HttpClient instance to make the call.

In the Program file, add an xref:System.Net.Http.HttpClient service if it isn't already present from a Blazor project template used to create the app:

builder.Services.AddScoped(sp => 
    new HttpClient
    {
        BaseAddress = new Uri(builder.HostEnvironment.BaseAddress)
    });

The preceding example sets the base address with builder.HostEnvironment.BaseAddress (xref:Microsoft.AspNetCore.Components.WebAssembly.Hosting.IWebAssemblyHostEnvironment.BaseAddress%2A?displayProperty=nameWithType), which gets the base address for the app and is typically derived from the <base> tag's href value in the host page.

The most common use cases for using the client's own base address are:

The client project (.Client) of a Blazor Web App (.NET 8 or later) makes web API calls from WebAssembly components or code that runs on the client in WebAssembly to APIs in the server app.
The client project (:::no-loc text="Client":::) of a hosted Blazor WebAssembly app makes web API calls to the server project (:::no-loc text="Server":::). Note that the hosted Blazor WebAssembly project template is no longer available in .NET 8 or later. However, hosted Blazor WebAssembly apps remain supported for .NET 8.

If you're calling an external web API (not in the same URL space as the client app), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddScoped(sp => 
    new HttpClient
    {
        BaseAddress = new Uri("https://localhost:5001")
    });

JSON helpers

xref:System.Net.Http.HttpClient is available as a preconfigured service for making requests back to the origin server.

xref:System.Net.Http.HttpClient and JSON helpers (xref:System.Net.Http.Json.HttpClientJsonExtensions?displayProperty=nameWithType) are also used to call third-party web API endpoints. xref:System.Net.Http.HttpClient is implemented using the browser's Fetch API and is subject to its limitations, including enforcement of the same-origin policy, which is discussed later in this article in the Cross-Origin Resource Sharing (CORS) section.

The client's base address is set to the originating server's address. Inject an xref:System.Net.Http.HttpClient instance into a component using the @inject directive:

@using System.Net.Http
@inject HttpClient Http

Use the xref:System.Net.Http.Json?displayProperty=fullName namespace for access to xref:System.Net.Http.Json.HttpClientJsonExtensions, including xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A, xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A, and xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A:

@using System.Net.Http.Json

The following sections cover JSON helpers:

GET
POST
PUT
PATCH

xref:System.Net.Http includes additional methods for sending HTTP requests and receiving HTTP responses, for example to send a DELETE request. For more information, see the DELETE and additional extension methods section.

GET from JSON (`GetFromJsonAsync`)

xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A sends an HTTP GET request and parses the JSON response body to create an object.

In the following component code, the todoItems are displayed by the component. xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A is called when the component is finished initializing (OnInitializedAsync).

todoItems = await Http.GetFromJsonAsync<TodoItem[]>("todoitems");

POST as JSON (`PostAsJsonAsync`)

xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A sends a POST request to the specified URI containing the value serialized as JSON in the request body.

In the following component code, newItemName is provided by a bound element of the component. The AddItem method is triggered by selecting a <button> element.

await Http.PostAsJsonAsync("todoitems", addItem);

xref:System.Net.Http.Json.HttpClientJsonExtensions.PostAsJsonAsync%2A returns an xref:System.Net.Http.HttpResponseMessage. To deserialize the JSON content from the response message, use the xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A extension method. The following example reads JSON weather data as an array:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ?? 
    Array.Empty<WeatherForecast>();

PUT as JSON (`PutAsJsonAsync`)

xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A sends an HTTP PUT request with JSON-encoded content.

In the following component code, editItem values for Name and IsCompleted are provided by bound elements of the component. The item's Id is set when the item is selected in another part of the UI (not shown) and EditItem is called. The SaveItem method is triggered by selecting the <button> element. The following example doesn't show loading todoItems for brevity. See the GET from JSON (GetFromJsonAsync) section for an example of loading items.

await Http.PutAsJsonAsync($"todoitems/{editItem.Id}", editItem);

xref:System.Net.Http.Json.HttpClientJsonExtensions.PutAsJsonAsync%2A returns an xref:System.Net.Http.HttpResponseMessage. To deserialize the JSON content from the response message, use the xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A extension method. The following example reads JSON weather data as an array:

var content = await response.Content.ReadFromJsonAsync<WeatherForecast[]>() ?? 
    Array.Empty<WeatherForecast>();

:::moniker range=">= aspnetcore-7.0"

PATCH as JSON (`PatchAsJsonAsync`)

xref:System.Net.Http.Json.HttpClientJsonExtensions.PatchAsJsonAsync%2A sends an HTTP PATCH request with JSON-encoded content.

Note

For more information, see xref:web-api/jsonpatch.

In the following example, xref:System.Net.Http.Json.HttpClientJsonExtensions.PatchAsJsonAsync%2A receives a JSON PATCH document as a plain text string with escaped quotes:

await Http.PatchAsJsonAsync(
    $"todoitems/{id}", 
    "[{\"operationType\":2,\"path\":\"/IsComplete\",\"op\":\"replace\",\"value\":true}]");

xref:System.Net.Http.Json.HttpClientJsonExtensions.PatchAsJsonAsync%2A returns an xref:System.Net.Http.HttpResponseMessage. To deserialize the JSON content from the response message, use the xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A extension method. The following example reads JSON todo item data as an array. An empty array is created if no item data is returned by the method, so content isn't null after the statement executes:

var response = await Http.PatchAsJsonAsync(...);
var content = await response.Content.ReadFromJsonAsync<TodoItem[]>() ??
    Array.Empty<TodoItem>();

Laid out with indentation, spacing, and unescaped quotes, the unencoded PATCH document appears as the following JSON:

[
  {
    "operationType": 2,
    "path": "/IsComplete",
    "op": "replace",
    "value": true
  }
]

To simplify the creation of PATCH documents in the app issuing PATCH requests, an app can use .NET JSON PATCH support, as the following guidance demonstrates.

Install the Microsoft.AspNetCore.JsonPatch NuGet package and use the API features of the package to compose a xref:Microsoft.AspNetCore.JsonPatch.JsonPatchDocument for a PATCH request.

[!INCLUDE]

Add @using directives for the xref:System.Text.Json?displayProperty=fullName, xref:System.Text.Json.Serialization?displayProperty=fullName, and xref:Microsoft.AspNetCore.JsonPatch?displayProperty=fullName namespaces to the top of the Razor component:

@using System.Text.Json
@using System.Text.Json.Serialization
@using Microsoft.AspNetCore.JsonPatch

Compose the xref:Microsoft.AspNetCore.JsonPatch.JsonPatchDocument for a TodoItem with IsComplete set to true using the xref:Microsoft.AspNetCore.JsonPatch.JsonPatchDocument.Replace%2A method:

var patchDocument = new JsonPatchDocument<TodoItem>()
    .Replace(p => p.IsComplete, true);

Pass the document's operations (patchDocument.Operations) to the xref:System.Net.Http.Json.HttpClientJsonExtensions.PatchAsJsonAsync%2A call:

private async Task UpdateItem(long id)
{
    await Http.PatchAsJsonAsync(
        $"todoitems/{id}", 
        patchDocument.Operations, 
        new JsonSerializerOptions()
        {
            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingDefault
        });
}

xref:System.Text.Json.JsonSerializerOptions.DefaultIgnoreCondition?displayProperty=nameWithType is set to xref:System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault?displayProperty=nameWithType to ignore a property only if it equals the default value for its type.

Add xref:System.Text.Json.JsonSerializerOptions.WriteIndented?displayProperty=nameWithType set to true if you want to present the JSON payload in a pleasant format for display. Writing indented JSON has no bearing on processing PATCH requests and isn't typically performed in production apps for web API requests.

Follow the guidance in the xref:web-api/jsonpatch article to add a PATCH controller action to the web API. Alternatively, PATCH request processing can be implemented as a Minimal API with the following steps.

Add a package reference for the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package to the web API app.

Note

There's no need to add a package reference for the Microsoft.AspNetCore.JsonPatch package to the app because the reference to the Microsoft.AspNetCore.Mvc.NewtonsoftJson package automatically transitively adds a package reference for Microsoft.AspNetCore.JsonPatch.

In the Program file add an @using directive for the xref:Microsoft.AspNetCore.JsonPatch?displayProperty=fullName namespace:

using Microsoft.AspNetCore.JsonPatch;

Provide the endpoint to the request processing pipeline of the web API:

app.MapPatch("/todoitems/{id}", async (long id, TodoContext db) =>
{
    if (await db.TodoItems.FindAsync(id) is TodoItem todo)
    {
        var patchDocument = 
            new JsonPatchDocument<TodoItem>().Replace(p => p.IsComplete, true);
        patchDocument.ApplyTo(todo);
        await db.SaveChangesAsync();

        return TypedResults.Ok(todo);
    }

    return TypedResults.NoContent();
});

Warning

As with the other examples in the xref:web-api/jsonpatch article, the preceding PATCH API doesn't protect the web API from over-posting attacks. For more information, see xref:tutorials/first-web-api#prevent-over-posting.

For a fully working PATCH experience, see the BlazorWebAppCallWebApi sample app.

:::moniker-end

DELETE (`DeleteAsync`) and additional extension methods

xref:System.Net.Http includes additional extension methods for sending HTTP requests and receiving HTTP responses. xref:System.Net.Http.HttpClient.DeleteAsync%2A?displayProperty=nameWithType is used to send an HTTP DELETE request to a web API.

In the following component code, the <button> element calls the DeleteItem method. The bound <input> element supplies the id of the item to delete.

await Http.DeleteAsync($"todoitems/{id}");

Named `HttpClient` with `IHttpClientFactory`

xref:System.Net.Http.IHttpClientFactory services and the configuration of a named xref:System.Net.Http.HttpClient are supported.

Note

An alternative to using a named xref:System.Net.Http.HttpClient from an xref:System.Net.Http.IHttpClientFactory is to use a typed xref:System.Net.Http.HttpClient. For more information, see the Typed HttpClient section.

Add the Microsoft.Extensions.Http NuGet package to the app.

[!INCLUDE]

In the Program file of a client project:

builder.Services.AddHttpClient("WebAPI", client => 
    client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));

:::moniker range=">= aspnetcore-8.0"

If the named client is to be used by prerendered client-side components of a Blazor Web App, the preceding service registration should appear in both the server project and the .Client project. On the server, builder.HostEnvironment.BaseAddress is replaced by the web API's base address, which is described further below.

:::moniker-end

The preceding client-side example sets the base address with builder.HostEnvironment.BaseAddress (xref:Microsoft.AspNetCore.Components.WebAssembly.Hosting.IWebAssemblyHostEnvironment.BaseAddress%2A?displayProperty=nameWithType), which gets the base address for the client-side app and is typically derived from the <base> tag's href value in the host page.

:::moniker range=">= aspnetcore-8.0"

The most common use cases for using the client's own base address are:

The client project (.Client) of a Blazor Web App that makes web API calls from WebAssembly/Auto components or code that runs on the client in WebAssembly to APIs in the server app at the same host address.
The client project (:::no-loc text="Client":::) of a hosted Blazor WebAssembly app that makes web API calls to the server project (:::no-loc text="Server":::).

:::moniker-end

:::moniker range="< aspnetcore-8.0"

The most common use case for using the client's own base address is in the client project (:::no-loc text="Client":::) of a hosted Blazor WebAssembly app that makes web API calls to the server project (:::no-loc text="Server":::).

:::moniker-end

If you're calling an external web API (not in the same URL space as the client app) or you're configuring the services in a server-side app (for example to deal with prerendering of client-side components on the server), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddHttpClient("WebAPI", client => 
    client.BaseAddress = new Uri(https://localhost:5001));

In the following component code:

An instance of xref:System.Net.Http.IHttpClientFactory creates a named xref:System.Net.Http.HttpClient.
The named xref:System.Net.Http.HttpClient is used to issue a GET request for JSON weather forecast data from the web API at /forecast.

@inject IHttpClientFactory ClientFactory

...

@code {
    private Forecast[]? forecasts;

    protected override async Task OnInitializedAsync()
    {
        var client = ClientFactory.CreateClient("WebAPI");

        forecasts = await client.GetFromJsonAsync<Forecast[]>("forecast") ?? [];
    }
}

:::moniker range=">= aspnetcore-8.0"

The BlazorWebAppCallWebApi sample app demonstrates calling a web API with a named xref:System.Net.Http.HttpClient in its CallTodoWebApiCsrNamedClient component. For an additional working demonstration in a client app based on calling Microsoft Graph with a named xref:System.Net.Http.HttpClient, see xref:blazor/security/webassembly/graph-api?pivots=named-client-graph-api.

:::moniker-end

:::moniker range="< aspnetcore-8.0"

For a working demonstration in a client app based on calling Microsoft Graph with a named xref:System.Net.Http.HttpClient, see xref:blazor/security/webassembly/graph-api?pivots=named-client-graph-api.

:::moniker-end

Typed `HttpClient`

Typed xref:System.Net.Http.HttpClient uses one or more of the app's xref:System.Net.Http.HttpClient instances, default or named, to return data from one or more web API endpoints.

Note

An alternative to using a typed xref:System.Net.Http.HttpClient is to use a named xref:System.Net.Http.HttpClient from an xref:System.Net.Http.IHttpClientFactory. For more information, see the Named HttpClient with IHttpClientFactory section.

Add the Microsoft.Extensions.Http NuGet package to the app.

[!INCLUDE]

The following example issues a GET request for JSON weather forecast data from the web API at /forecast.

ForecastHttpClient.cs:

using System.Net.Http.Json;

namespace BlazorSample.Client;

public class ForecastHttpClient(HttpClient http)
{
    public async Task<Forecast[]> GetForecastAsync()
    {
        return await http.GetFromJsonAsync<Forecast[]>("forecast") ?? [];
    }
}

In the Program file of a client project:

builder.Services.AddHttpClient<ForecastHttpClient>(client => 
    client.BaseAddress = new Uri(builder.HostEnvironment.BaseAddress));

:::moniker range=">= aspnetcore-8.0"

If the typed client is to be used by prerendered client-side components of a Blazor Web App, the preceding service registration should appear in both the server project and the .Client project. On the server, builder.HostEnvironment.BaseAddress is replaced by the web API's base address, which is described further below.

:::moniker-end

The preceding example sets the base address with builder.HostEnvironment.BaseAddress (xref:Microsoft.AspNetCore.Components.WebAssembly.Hosting.IWebAssemblyHostEnvironment.BaseAddress%2A?displayProperty=nameWithType), which gets the base address for the client-side app and is typically derived from the <base> tag's href value in the host page.

:::moniker range=">= aspnetcore-8.0"

The most common use cases for using the client's own base address are:

The client project (.Client) of a Blazor Web App that makes web API calls from WebAssembly/Auto components or code that runs on the client in WebAssembly to APIs in the server app at the same host address.
The client project (:::no-loc text="Client":::) of a hosted Blazor WebAssembly app that makes web API calls to the server project (:::no-loc text="Server":::).

:::moniker-end

:::moniker range="< aspnetcore-8.0"

The most common use case for using the client's own base address is in the client project (:::no-loc text="Client":::) of a hosted Blazor WebAssembly app that makes web API calls to the server project (:::no-loc text="Server":::).

:::moniker-end

If you're calling an external web API (not in the same URL space as the client app) or you're configuring the services in a server-side app (for example to deal with prerendering of client-side components on the server), set the URI to the web API's base address. The following example sets the base address of the web API to https://localhost:5001, where a separate web API app is running and ready to respond to requests from the client app:

builder.Services.AddHttpClient<ForecastHttpClient>(client => 
    client.BaseAddress = new Uri(https://localhost:5001));

Components inject the typed xref:System.Net.Http.HttpClient to call the web API.

In the following component code:

An instance of the preceding ForecastHttpClient is injected, which creates a typed xref:System.Net.Http.HttpClient.
The typed xref:System.Net.Http.HttpClient is used to issue a GET request for JSON weather forecast data from the web API.

@inject ForecastHttpClient Http

...

@code {
    private Forecast[]? forecasts;

    protected override async Task OnInitializedAsync()
    {
        forecasts = await Http.GetForecastAsync();
    }
}

:::moniker range=">= aspnetcore-8.0"

The BlazorWebAppCallWebApi sample app demonstrates calling a web API with a typed xref:System.Net.Http.HttpClient in its CallTodoWebApiCsrTypedClient component. Note that the component adopts and client-side rendering (CSR) (InteractiveWebAssembly render mode) with prerendering, so the typed client service registration appears in the Program file of both the server project and the .Client project.

:::moniker-end

Cookie-based request credentials

The guidance in this section applies to client-side scenarios that rely upon an authentication cookie.

For cookie-based authentication, which is considered more secure than bearer token authentication, cookie credentials can be sent with each web API request by calling xref:Microsoft.Extensions.DependencyInjection.HttpClientBuilderExtensions.AddHttpMessageHandler%2A with a xref:System.Net.Http.DelegatingHandler on a preconfigured xref:System.Net.Http.HttpClient. The handler configures xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestCredentials%2A with xref:Microsoft.AspNetCore.Components.WebAssembly.Http.BrowserRequestCredentials.Include?displayProperty=nameWithType, which advises the browser to send credentials with each request, such as cookies or HTTP authentication headers, including for cross-origin requests.

CookieHandler.cs:

public class CookieHandler : DelegatingHandler
{
    protected override Task<HttpResponseMessage> SendAsync(
        HttpRequestMessage request, CancellationToken cancellationToken)
    {
        request.SetBrowserRequestCredentials(BrowserRequestCredentials.Include);
        request.Headers.Add("X-Requested-With", ["XMLHttpRequest"]);

        return base.SendAsync(request, cancellationToken);
    }
}

The CookieHandler is registered in the Program file:

builder.Services.AddTransient<CookieHandler>();

The message handler is added to any preconfigured xref:System.Net.Http.HttpClient that requires cookie authentication:

builder.Services.AddHttpClient(...)
    .AddHttpMessageHandler<CookieHandler>();

:::moniker range=">= aspnetcore-8.0"

For a demonstration, see xref:blazor/security/webassembly/standalone-with-identity.

:::moniker-end

When composing an xref:System.Net.Http.HttpRequestMessage, set the browser request credentials and header directly:

var requestMessage = new HttpRequestMessage() { ... };

requestMessage.SetBrowserRequestCredentials(BrowserRequestCredentials.Include);
requestMessage.Headers.Add("X-Requested-With", ["XMLHttpRequest"]);

`HttpClient` and `HttpRequestMessage` with Fetch API request options

The guidance in this section applies to client-side scenarios that rely upon bearer token authentication.

HttpClient (API documentation) and xref:System.Net.Http.HttpRequestMessage can be used to customize requests. For example, you can specify the HTTP method and request headers. The following component makes a POST request to a web API endpoint and shows the response body.

TodoRequest.razor:

@page "/todo-request"
@using System.Net.Http.Headers
@using Microsoft.AspNetCore.Components.WebAssembly.Authentication
@inject HttpClient Http
@inject IAccessTokenProvider TokenProvider

<h1>ToDo Request</h1>

<h1>ToDo Request Example</h1>

<button @onclick="PostRequest">Submit POST request</button>

<p>Response body returned by the server:</p>

<p>@responseBody</p>

@code {
    private string? responseBody;

    private async Task PostRequest()
    {
        var requestMessage = new HttpRequestMessage()
        {
            Method = new HttpMethod("POST"),
            RequestUri = new Uri("https://localhost:10000/todoitems"),
            Content =
                JsonContent.Create(new TodoItem
                {
                    Name = "My New Todo Item",
                    IsComplete = false
                })
        };

        var tokenResult = await TokenProvider.RequestAccessToken();

        if (tokenResult.TryGetToken(out var token))
        {
            requestMessage.Headers.Authorization =
                new AuthenticationHeaderValue("Bearer", token.Value);

            requestMessage.Content.Headers.TryAddWithoutValidation(
                "x-custom-header", "value");

            var response = await Http.SendAsync(requestMessage);
            var responseStatusCode = response.StatusCode;

            responseBody = await response.Content.ReadAsStringAsync();
        }
    }

    public class TodoItem
    {
        public long Id { get; set; }
        public string? Name { get; set; }
        public bool IsComplete { get; set; }
    }
}

Blazor's client-side implementation of xref:System.Net.Http.HttpClient uses Fetch API and configures the underlying request-specific Fetch API options via xref:System.Net.Http.HttpRequestMessage extension methods and xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions. Set additional options using the generic xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestOption%2A extension method. Blazor and the underlying Fetch API don't directly add or modify request headers. For more information on how user agents, such as browsers, interact with headers, consult external user agent documentation sets and other web resources.

The HTTP response is typically buffered to enable support for synchronous reads on the response content. To enable support for response streaming, use the xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserResponseStreamingEnabled%2A extension method on the request.

To include credentials in a cross-origin request, use the xref:Microsoft.AspNetCore.Components.WebAssembly.Http.WebAssemblyHttpRequestMessageExtensions.SetBrowserRequestCredentials%2A extension method:

requestMessage.SetBrowserRequestCredentials(BrowserRequestCredentials.Include);

For more information on Fetch API options, see MDN web docs: WindowOrWorkerGlobalScope.fetch(): Parameters.

Handle errors

Handle web API response errors in developer code when they occur. For example, xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A expects a JSON response from the web API with a Content-Type of application/json. If the response isn't in JSON format, content validation throws a xref:System.NotSupportedException.

In the following example, the URI endpoint for the weather forecast data request is misspelled. The URI should be to WeatherForecast but appears in the call as WeatherForcast, which is missing the letter e in Forecast.

The xref:System.Net.Http.Json.HttpClientJsonExtensions.GetFromJsonAsync%2A call expects JSON to be returned, but the web API returns HTML for an unhandled exception with a Content-Type of text/html. The unhandled exception occurs because the path to /WeatherForcast isn't found and middleware can't serve a page or view for the request.

In xref:Microsoft.AspNetCore.Components.ComponentBase.OnInitializedAsync%2A on the client, xref:System.NotSupportedException is thrown when the response content is validated as non-JSON. The exception is caught in the catch block, where custom logic could log the error or present a friendly error message to the user.

ReturnHTMLOnException.razor:

@page "/return-html-on-exception"
@using {PROJECT NAME}.Shared
@inject HttpClient Http

<h1>Fetch data but receive HTML on unhandled exception</h1>

@if (forecasts == null)
{
    <p><em>Loading...</em></p>
}
else
{
    <h2>Temperatures by Date</h2>

    <ul>
        @foreach (var forecast in forecasts)
        {
            <li>
                @forecast.Date.ToShortDateString():
                @forecast.TemperatureC &#8451;
                @forecast.TemperatureF &#8457;
            </li>
        }
    </ul>
}

<p>
    @exceptionMessage
</p>

@code {
    private WeatherForecast[]? forecasts;
    private string? exceptionMessage;

    protected override async Task OnInitializedAsync()
    {
        try
        {
            // The URI endpoint "WeatherForecast" is misspelled on purpose on the 
            // next line. See the preceding text for more information.
            forecasts = await Http.GetFromJsonAsync<WeatherForecast[]>("WeatherForcast");
        }
        catch (NotSupportedException exception)
        {
            exceptionMessage = exception.Message;
        }
    }
}

Note

The preceding example is for demonstration purposes. A web API can be configured to return JSON even when an endpoint doesn't exist or an unhandled exception occurs on the server.

For more information, see xref:blazor/fundamentals/handle-errors.

Cross-Origin Resource Sharing (CORS)

Browser security restricts a webpage from making requests to a different domain than the one that served the webpage. This restriction is called the same-origin policy. The same-origin policy restricts (but doesn't prevent) a malicious site from reading sensitive data from another site. To make requests from the browser to an endpoint with a different origin, the endpoint must enable Cross-Origin Resource Sharing (CORS).

For more information on server-side CORS, see xref:security/cors. The article's examples don't pertain directly to Razor component scenarios, but the article is useful for learning general CORS concepts.

For information on client-side CORS requests, see xref:blazor/security/webassembly/additional-scenarios#cross-origin-resource-sharing-cors.

:::moniker range=">= aspnetcore-8.0"

Antiforgery support

To add antiforgery support to an HTTP request, inject the AntiforgeryStateProvider and add a RequestToken to the headers collection as a RequestVerificationToken:

@inject AntiforgeryStateProvider Antiforgery

private async Task OnSubmit()
{
    var antiforgery = Antiforgery.GetAntiforgeryToken();
    var request = new HttpRequestMessage(HttpMethod.Post, "action");
    request.Headers.Add("RequestVerificationToken", antiforgery.RequestToken);
    var response = await client.SendAsync(request);
    ...
}

For more information, see xref:blazor/security/index#antiforgery-support.

:::moniker-end

Blazor framework component examples for testing web API access

Various network tools are publicly available for testing web API backend apps directly, such as Firefox Browser Developer. Blazor framework's reference source includes xref:System.Net.Http.HttpClient test assets that are useful for testing:

HttpClientTest assets in the dotnet/aspnetcore GitHub repository

[!INCLUDE]

Additional resources

General

Cross-Origin Resource Sharing (CORS) at W3C
xref:security/cors: Although the content applies to ASP.NET Core apps, not Razor components, the article covers general CORS concepts.

Mitigation of overposting attacks

Web APIs can be vulnerable to an overposting attack, also known as a mass assignment attack. An overposting attack occurs when a malicious user issues an HTML form POST to the server that processes data for properties that aren't part of the rendered form and that the developer doesn't wish to allow users to modify. The term "overposting" literally means that the malicious user has over-POSTed with the form.

For guidance on mitigating overposting attacks, see xref:tutorials/first-web-api#prevent-over-posting.

Server-side

xref:blazor/security/server/additional-scenarios: Includes coverage on using xref:System.Net.Http.HttpClient to make secure web API requests.
xref:fundamentals/http-requests
xref:security/enforcing-ssl
Kestrel HTTPS endpoint configuration

Client-side

xref:blazor/security/webassembly/additional-scenarios: Includes coverage on using xref:System.Net.Http.HttpClient to make secure web API requests.
xref:blazor/security/webassembly/graph-api
Fetch API

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call-web-api.md

call-web-api.md

Call a web API from ASP.NET Core Blazor

Package

Sample apps

`BlazorWebAppCallWebApi`

`BlazorWebAppCallWebApi_Weather`

`BlazorWebAssemblyCallWebApi`

Server-side scenarios for calling external web APIs

Service abstractions for web API calls

Blazor Web App external web APIs

Prerendered data

Add the `HttpClient` service

JSON helpers

GET from JSON (`GetFromJsonAsync`)

POST as JSON (`PostAsJsonAsync`)

PUT as JSON (`PutAsJsonAsync`)

PATCH as JSON (`PatchAsJsonAsync`)

DELETE (`DeleteAsync`) and additional extension methods

Named `HttpClient` with `IHttpClientFactory`

Typed `HttpClient`

Cookie-based request credentials

`HttpClient` and `HttpRequestMessage` with Fetch API request options

Handle errors

Cross-Origin Resource Sharing (CORS)

Antiforgery support

Blazor framework component examples for testing web API access

Additional resources

General

Mitigation of overposting attacks

Server-side

Client-side

Files

call-web-api.md

Latest commit

History

call-web-api.md

File metadata and controls

Call a web API from ASP.NET Core Blazor

Package

Sample apps

BlazorWebAppCallWebApi

BlazorWebAppCallWebApi_Weather

BlazorWebAssemblyCallWebApi

Server-side scenarios for calling external web APIs

Service abstractions for web API calls

Blazor Web App external web APIs

Prerendered data

Add the HttpClient service

JSON helpers

GET from JSON (GetFromJsonAsync)

POST as JSON (PostAsJsonAsync)

PUT as JSON (PutAsJsonAsync)

PATCH as JSON (PatchAsJsonAsync)

DELETE (DeleteAsync) and additional extension methods

Named HttpClient with IHttpClientFactory

Typed HttpClient

Cookie-based request credentials

HttpClient and HttpRequestMessage with Fetch API request options

Handle errors

Cross-Origin Resource Sharing (CORS)

Antiforgery support

Blazor framework component examples for testing web API access

Additional resources

General

Mitigation of overposting attacks

Server-side

Client-side

`BlazorWebAppCallWebApi`

`BlazorWebAppCallWebApi_Weather`

`BlazorWebAssemblyCallWebApi`

Add the `HttpClient` service

GET from JSON (`GetFromJsonAsync`)

POST as JSON (`PostAsJsonAsync`)

PUT as JSON (`PutAsJsonAsync`)

PATCH as JSON (`PatchAsJsonAsync`)

DELETE (`DeleteAsync`) and additional extension methods

Named `HttpClient` with `IHttpClientFactory`

Typed `HttpClient`

`HttpClient` and `HttpRequestMessage` with Fetch API request options