title	description	ms.date	ms.topic
Create custom resource types for .NET Aspire	Learn how to create a custom resource for an existing containerized application.	08/12/2024	how-to

Create custom resource types for .NET Aspire

.NET Aspire improves the development experience by providing reusable building blocks that can be used to quickly arrange application dependencies and expose them to your own code. One of the key building blocks of an Aspire-based application is the resource. Consider the code below:

var builder = DistributedApplication.CreateBuilder(args);

var redis = builder.AddRedis("cache");

var db = builder.AddPostgres("pgserver")
                .AddDatabase("inventorydb");

builder.AddProject<Projects.InventoryService>("inventoryservice")
       .WithReference(redis)
       .WithReference(db);

In the preceding code there are four resources represented:

cache: A Redis container.
pgserver: A Postgres container.
inventorydb: A database hosted on pgserver.
inventoryservice: An ASP.NET Core application.

Most .NET Aspire-related code that the average developer will write, centers around adding resources to the app model and creating references between them.

Key elements of a .NET Aspire custom resource

Building a custom resource in .NET Aspire requires the following:

A custom resource type that implements xref:Aspire.Hosting.ApplicationModel.IResource
An extension method for xref:Aspire.Hosting.IDistributedApplicationBuilder named Add{CustomResource} where {CustomResource} is the name of the custom resource.

When custom resource requires optional configuration, developers may wish to implement With* suffixed extension methods to make these configuration options discoverable using the builder pattern.

A practical example: MailDev

To help understand how to develop custom resources, this article shows an example of how to build a custom resource for MailDev. MailDev is an open-source tool which provides a local mail server specifically designed to allow developers to test e-mail sending behaviors within their app. For more information, see the MailDev GitHub repository.

In this example you create a new .NET Aspire project as a test environment for the MailDev resource that you create. While you can create custom resources in existing .NET Aspire projects it's a good idea to consider whether the custom resource might be used across multiple .NET Aspire-based solutions and should be developed as a reusable integration.

Setup the starter project

Create a new .NET Aspire project that will be used to test out the new resource that we are developing.

dotnet new aspire -o MailDevResource
cd MailDevResource
dir

Once the project is created you should see a listing containing the following:

MailDevResource.AppHost: The app host used to test out the custom resource.
MailDevResource.ServiceDefaults: The service defaults project for use in service-related projects.
MailDevResource.sln: The solution file referencing both projects.

Verify that the project can build and run successfully by executing the following command:

dotnet run --project MailDevResource.AppHost/MailDevResource.AppHost.csproj

The console output should look similar to the following:

Building...
info: Aspire.Hosting.DistributedApplication[0]
      Aspire version: 8.0.0+d215c528c07c7919c3ac30b35d92f4e51a60523b
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application starting.
info: Aspire.Hosting.DistributedApplication[0]
      Application host directory is:
      ..\docs-aspire\docs\extensibility\snippets\MailDevResource\MailDevResource.AppHost
info: Aspire.Hosting.DistributedApplication[0]
      Now listening on: https://localhost:17251
info: Aspire.Hosting.DistributedApplication[0]
      Login to the dashboard at https://localhost:17251/login?t=928db244c720c5022a7a9bf5cf3a3526
info: Aspire.Hosting.DistributedApplication[0]
      Distributed application started. Press Ctrl+C to shut down.

Select the dashboard link in the browser to see the .NET Aspire dashboard:

:::image type="content" source="media/maildevresource-empty-dashboard.png" lightbox="media/maildevresource-empty-dashboard.png" alt-text="A screenshot of the empty .NET Aspire dashboard for test project.":::

Press Ctrl+C to shutdown the app (you can close the browser tab).

Create library for resource extension

.NET Aspire resources are just classes and methods contained within a class library that references the .NET Aspire Hosting library (Aspire.Hosting). By placing the resource in a separate project you can more easily share it between .NET Aspire-based apps and potentially package and share it on NuGet.

Create the class library project named MailDev.Hosting.
```
dotnet new classlib -o MailDev.Hosting
```
Add Aspire.Hosting to the class library as a package reference.
```
dotnet add ./MailDev.Hosting/MailDev.Hosting.csproj package Aspire.Hosting --version 8.0.0
```
[!IMPORTANT] The version you specify here should match the version of the .NET Aspire workload installed.

Add class library reference to the MailDevResource.AppHost project.

dotnet add ./MailDevResource.AppHost/MailDevResource.AppHost.csproj reference ./MailDev.Hosting/MailDev.Hosting.csproj

Add class library project to the solution file.

dotnet sln ./MailDevResource.sln add ./MailDev.Hosting/MailDev.Hosting.csproj

Once the following steps are performed you can launch the project:

dotnet run --project ./MailDevResource.AppHost/MailDevResource.AppHost.csproj

This will result in a warning being displayed to the console:

.\.nuget\packages\aspire.hosting.apphost\8.0.0\build\Aspire.Hosting.AppHost.targets(174,5): warning ASPIRE004: '..\MailDev.Hosting\MailDev.Hosting.csproj' is referenced by an A
spire Host project, but it is not an executable. Did you mean to set IsAspireProjectResource="false"? [D:\source\repos\docs-aspire\docs\extensibility\snippets\MailDevResource\MailDevResource.AppHost\MailDevRe
source.AppHost.csproj]

This is because .NET Aspire treats project references in the app host as if they are service projects. To tell .NET Aspire that the project reference should be treated as a non-service project modify the :::no-loc text="MailDevResource.AppHost\MailDevResource.AppHost.csproj"::: files reference to the MailDev.Hosting project to be the following:

<ItemGroup>
  <!-- The IsAspireProjectResource attribute tells .NET Aspire to treat this 
       reference as a standard project reference and not attempt to generate
       a metadata file -->
  <ProjectReference Include="..\MailDev.Hosting\MailDev.Hosting.csproj"
                    IsAspireProjectResource="false" />
</ItemGroup>

Now when you launch the app host, there's no warning displayed to the console.

Define the resource types

The MailDev.Hosting class library contains the resource type and extension methods for adding the resource to the app host. You should first think about the experience that you want to give developers when using your custom resource. In the case of this custom resource, you would want developers to be able to write code like the following:

var builder = DistributedApplication.CreateBuilder(args);

var maildev = builder.AddMailDev("maildev");

builder.AddProject<Projects.NewsletterService>("newsletterservice")
       .WithReference(maildev);

To achieve this, you need a custom resource named MailDevResource which implements xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString so that consumers can use it with xref:Aspire.Hosting.ResourceBuilderExtensions.WithReference%2A extension to inject the connection details for the MailDev server as a connection string.

MailDev is available as a container resource, so you'll also want to derive from xref:Aspire.Hosting.ApplicationModel.ContainerResource so that we can make use of various pre-existing container-focused extensions in .NET Aspire.

Replace the contents of the :::no-loc text="Class1.cs"::: file in the MailDev.Hosting project, and rename the file to :::no-loc text="MailDevResource.cs"::: with the following code:

:::code language="csharp" source="snippets/MailDevResource/MailDev.Hosting/MailDevResource.cs":::

In the preceding custom resource, the xref:Aspire.Hosting.ApplicationModel.EndpointReference and xref:Aspire.Hosting.ApplicationModel.ReferenceExpression are examples of several types which implement a collection of interfaces, such as xref:Aspire.Hosting.ApplicationModel.IManifestExpressionProvider, xref:Aspire.Hosting.ApplicationModel.IValueProvider, and xref:Aspire.Hosting.ApplicationModel.IValueWithReferences. For more information about these types and their role in .NET Aspire, see technical details.

Define the resource extensions

To make it easy for developers to use the custom resource an extension method named AddMailDev needs to be added to the :::no-loc text="MailDev.Hosting"::: project. The AddMailDev extension method is responsible for configuring the resource so it can start successfully as a container.

Add the following code to a new file named MailDevResourceBuilderExtensions.cs in the MailDev.Hosting project:

:::code language="csharp" source="snippets/MailDevResource/MailDev.Hosting/MailDevResourceBuilderExtensions.cs":::

Validate custom integration inside the app host

Now that the basic structure for the custom resource is complete it is time to test it in a real AppHost project. Open the :::no-loc text="Program.cs"::: file in the :::no-loc text="MailDevResource.AppHost"::: project and update it with the following code:

var builder = DistributedApplication.CreateBuilder(args);

var maildev = builder.AddMailDev("maildev");

builder.Build().Run();

After updating the :::no-loc text="Program.cs"::: file, launch the app host project and open the dashboard:

dotnet run --project ./MailDevResource.AppHost/MailDevResource.AppHost.csproj

After a few moments the dashboard will show that the maildev resource is running and a hyperlink will be available that navigates to the MailDev web app, which shows the content of each e-mail that your app sends.

The .NET Aspire dashboard should look similar to the following:

:::image type="content" source="media/maildev-in-aspire-dashboard.png" lightbox="media/maildev-in-aspire-dashboard.png" alt-text="MailDev resource visible in .NET Aspire dashboard.":::

The MailDev web app should look similar to the following:

:::image type="content" source="media/maildev-web-ui.png" lightbox="media/maildev-web-ui.png" alt-text="MailDev web-based user interface running as a container managed by .NET Aspire.":::

Add a .NET service project to the app host for testing

Once .NET Aspire can successfully launch the MailDev integration, it's time to consume the connection information for MailDev within a .NET project. In .NET Aspire it's common for there to be a hosting package and one or more component packages. For example consider:

Hosting package: Used to represent resources within the app model.
- Aspire.Hosting.Redis
Component packages: Used to configure and consume client libraries.
- Aspire.StackExchange.Redis
- Aspire.StackExchange.Redis.DistributedCaching
- Aspire.StackExchange.Redis.OutputCaching

In the case of the MailDev resource, the .NET platform already has an SMTP client in the form of xref:System.Net.Mail.SmtpClient. In this example you use this existing API for the sake of simplicity, although other resource types may benefit from custom integration libraries to assist developers.

In order to test the end-to-end scenario, you need a .NET project which we can inject the connection information into for the MailDev resource. Add a web api project:

Create a new .NET project named :::no-loc text="MailDevResource.NewsletterService":::.
```
dotnet new webapi --use-minimal-apis -o MailDevResource.NewsletterService
```

Add a reference to the :::no-loc text="MailDev.Hosting"::: project.

dotnet add ./MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj reference ./MailDev.Hosting/MailDev.Hosting.csproj

Add a referece to the :::no-loc text="MailDevResource.AppHost"::: project.

dotnet add ./MailDevResource.AppHost/MailDevResource.AppHost.csproj reference ./MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj

Add the new project to the solution file.

dotnet sln ./MailDevResource.sln add ./MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj

After the project has been added and references have been updated, open the :::no-loc text="Program.cs"::: of the :::no-loc text="MailDevResource.AppHost.csproj"::: project, and update the source file to look like the following:

:::code source="snippets/MailDevResource/MailDevResource.AppHost/Program.cs":::

After updating the :::no-loc text="Program.cs"::: file, launch the app host again. Then verify that the Newsletter Service started and that the environment variable ConnectionStrings__maildev was added to the process. From the Resources page, find the newsletterservice row, and select the View link on the Details column:

:::image type="content" source="media/maildev-envvar.png" lightbox="media/maildev-envvar.png" alt-text="Environment variables for Newsletter Service in .NET Aspire Dashboard.":::

The preceding screenshot shows the environment variables for the newsletterservice project. The ConnectionStrings__maildev environment variable is the connection string that was injected into the project by the maildev resource.

Use connection string to send messages

To use the SMTP connection details that were injected into the newsletter service project, you inject an instance of xref:System.Net.Mail.SmtpClient into the dependency injection container as a singleton. Add the following code to the :::no-loc text="Program.cs"::: file in the :::no-loc text="MailDevResource.NewsletterService"::: project to setup the singleton service. In the Program class, immediately following the // Add services to the container comment, add the following code:

:::code source="snippets/MailDevResource/MailDevResource.NewsletterService/Program.cs" id="smtp":::

Tip

This code snippet relies on the official SmtpClient, however; this type is obsolete on some platforms and not recommended on others. This is used here to demonstrate a non-componentized approach to using the MailDev resource. For a more modern approach using MailKit, see Create custom .NET Aspire integration.

To test the client, add two simple subscribe and unsubscribe POST methods to the newsletter service. Add the following code replacing the "weatherforecast" MapGet call in the :::no-loc text="Program.cs"::: file of the MailDevResource.NewsletterService project to setup the ASP.NET Core routes:

:::code source="snippets/MailDevResource/MailDevResource.NewsletterService/Program.cs" id="subs":::

Tip

Remember to reference the System.Net.Mail and Microsoft.AspNetCore.Mvc namespaces in :::no-loc text="Program.cs"::: if your code editor doesn't automatically add them.

Once the :::no-loc text="Program.cs"::: file is updated, launch the app host and use your browser, or curl to hit the following URLs (alternatively if you're using Visual Studio you can use .http files):

POST /subscribe?email=test@test.com HTTP/1.1
Host: localhost:7251
Content-Type: application/json

To use this API, you can use curl to send the request. The following curl command sends an HTTP POST request to the subscribe endpoint, and it expects an email query string value to subscribe to the newsletter. The Content-Type header is set to application/json to indicate that the request body is in JSON format.:

Unix

curl -H "Content-Type: application/json" --request POST https://localhost:7251/subscribe?email=test@test.com

Windows

curl -H @{ ContentType = "application/json" } -Method POST https://localhost:7251/subscribe?email=test@test.com

The next API is the unsubscribe endpoint. This endpoint is used to unsubscribe from the newsletter.

POST /unsubscribe?email=test@test.com HTTP/1.1
Host: localhost:7251
Content-Type: application/json

To unsubscribe from the newsletter, you can use the following curl command, passing an email parameter to the unsubscribe endpoint as a query string:

Unix

curl -H "Content-Type: application/json" --request POST https://localhost:7251/unsubscribe?email=test@test.com

Windows

curl -H @{ ContentType = "application/json" } -Method POST https://localhost:7251/unsubscribe?email=test@test.com

Tip

Make sure that you replace the https://localhost:7251 with the correct localhost port (the URL of the app host that you are running).

If those API calls return a successful response (HTTP 200, Ok) then you should be able to click on the maildev resource the dashboard and the :::no-loc text="MailDev UI"::: will show the emails that have been sent to the SMTP endpoint.

:::image type="content" source="media/maildev-emails.png" lightbox="media/maildev-emails.png" alt-text="E-mails visible in the MailDev UI":::

Technical details

In the following sections, various technical details are discussed which are important to understand when developing custom resources for .NET Aspire.

Secure networking

In this example, the MailDev resource is a container resource which is exposed to the host machine over HTTP and SMTP. The MailDev resource is a development tool and isn't intended for production use. To instead use HTTPS, see MailDev: Configure HTTPS.

When developing custom resources that expose network endpoints, it's important to consider the security implications of the resource. For example, if the resource is a database, it's important to ensure that the database is secure and that the connection string is not exposed to the public internet.

The `ReferenceExpression` and `EndpointReference` type

In the preceding code, the MailDevResource had two properties:

SmtpEndpoint: xref:Aspire.Hosting.ApplicationModel.EndpointReference type.
ConnectionStringExpression: xref:Aspire.Hosting.ApplicationModel.ReferenceExpression type.

These types are among several which are used throughout .NET Aspire to represent configuration data, which isn't finalized until the .NET Aspire project is either run or published to the cloud via a tool such as Azure Developer CLI (azd).

The fundamental problem that these types help to solve, is deferring resolution of concrete configuration information until all the information is available.

For example, the MailDevResource exposes a property called ConnectionStringExpression as required by the xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString interface. The type of the property is xref:Aspire.Hosting.ApplicationModel.ReferenceExpression and is created by passing in an interpolated string to the xref:Aspire.Hosting.ApplicationModel.ReferenceExpression.Create%2A method.

public ReferenceExpression ConnectionStringExpression =>
    ReferenceExpression.Create(
        $"smtp://{SmtpEndpoint.Property(EndpointProperty.Host)}:{SmtpEndpoint.Property(EndpointProperty.Port)}"
    );

The signature for the xref:Aspire.Hosting.ApplicationModel.ReferenceExpression.Create%2A method is as follows:

public static ReferenceExpression Create(
    in ExpressionInterpolatedStringHandler handler)

This isn't a regular xref:System.String argument. The method makes use of the interpolated string handler pattern, to capture the interpolated string template and the values referenced within it to allow for custom processing. In the case of .NET Aspire, these details are captured in a xref:Aspire.Hosting.ApplicationModel.ReferenceExpression which can be evaluated as each value referenced in the interpolated string becomes available.

Here's how the flow of execution works:

A resource which implements xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString is added to the model (for example, AddMailDev(...)).
The IResourceBuilder<MailDevResource> is passed to the xref:Aspire.Hosting.ResourceBuilderExtensions.WithReference%2A which has a special overload for handling xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString implementors.
The WithReference wraps the resource in a xref:Aspire.Hosting.ApplicationModel.ConnectionStringReference instance and the object is captured in a xref:Aspire.Hosting.ApplicationModel.EnvironmentCallbackAnnotation which is evaluated after the .NET Aspire project is built and starts running.
As the process that references the connection string starts .NET Aspire starts evaluating the expression. It first gets the xref:Aspire.Hosting.ApplicationModel.ConnectionStringReference and calls xref:Aspire.Hosting.ApplicationModel.ConnectionStringReference.Aspire%23Hosting%23ApplicationModel%23IValueProvider%23GetValueAsync%2A.
The GetValueAsync method gets the value of the xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString.ConnectionStringExpression property to get the xref:Aspire.Hosting.ApplicationModel.ReferenceExpression instance.
The xref:Aspire.Hosting.ApplicationModel.ConnectionStringReference.Aspire%23Hosting%23ApplicationModel%23IValueProvider%23GetValueAsync%2A method then calls xref:Aspire.Hosting.ApplicationModel.ReferenceExpression.GetValueAsync%2A to process the previously captured interpolated string.
Because the interpolated string contains references to other reference types such as xref:Aspire.Hosting.ApplicationModel.EndpointReference they are also evaluated and real value substituted (which at this time are now available).

Manifest publishing

The xref:Aspire.Hosting.ApplicationModel.IManifestExpressionProvider interface is designed to solve the problem of sharing connection information between resources at deployment. The solution for this particular problem is described in the .NET Aspire inner-loop networking overview. Similarly to local development, many of the values are necessary to configure the app, yet they cannot be determined until the app is being deployed via a tool, such as azd (Azure Developer CLI).

To solve this problem .NET Aspire produces a manifest file which azd and other deployment tools interpret. Rather than specifying concrete values for connection information between resources an expression syntax is used which deployment tools evaluate. Generally the manifest file isn't visible to developers but it's possible to generate one for manual inspection. The command below can be used on the app host to produce a manifest.

dotnet run --project MailDevResource.AppHost/MailDevResource.AppHost.csproj -- --publisher manifest --output-path aspire-manifest.json

This command produces a manifest file like the following:

{
  "resources": {
    "maildev": {
      "type": "container.v0",
      "connectionString": "smtp://{maildev.bindings.smtp.host}:{maildev.bindings.smtp.port}",
      "image": "docker.io/maildev/maildev:2.0.2",
      "bindings": {
        "http": {
          "scheme": "http",
          "protocol": "tcp",
          "transport": "http",
          "targetPort": 1080
        },
        "smtp": {
          "scheme": "tcp",
          "protocol": "tcp",
          "transport": "tcp",
          "targetPort": 1025
        }
      }
    },
    "newsletterservice": {
      "type": "project.v0",
      "path": "../MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj",
      "env": {
        "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EXCEPTION_LOG_ATTRIBUTES": "true",
        "OTEL_DOTNET_EXPERIMENTAL_OTLP_EMIT_EVENT_LOG_ATTRIBUTES": "true",
        "OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY": "in_memory",
        "ASPNETCORE_FORWARDEDHEADERS_ENABLED": "true",
        "ConnectionStrings__maildev": "{maildev.connectionString}"
      },
      "bindings": {
        "http": {
          "scheme": "http",
          "protocol": "tcp",
          "transport": "http"
        },
        "https": {
          "scheme": "https",
          "protocol": "tcp",
          "transport": "http"
        }
      }
    }
  }
}

Because the MailDevResource implements xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString the manifest publishing logic in .NET Aspire knows that even though MailDevResource is a container resource, it also needs a connectionString field. The connectionString field references other parts of the maildev resource in the manifest to produce the final string:

{
    // ... other content omitted.
    "connectionString": "smtp://{maildev.bindings.smtp.host}:{maildev.bindings.smtp.port}"
}

.NET Aspire knows how to form this string because it looks at xref:Aspire.Hosting.ApplicationModel.IResourceWithConnectionString.ConnectionStringExpression and builds up the final string via the xref:Aspire.Hosting.ApplicationModel.IManifestExpressionProvider interface (in much the same way as the xref:Aspire.Hosting.ApplicationModel.IValueProvider interface is used).

The MailDevResource automatically gets included in the manifest because it's derived from xref:Aspire.Hosting.ApplicationModel.ContainerResource. Resource authors can choose to suppress outputting content to the manifest by using the xref:Aspire.Hosting.ResourceBuilderExtensions.ExcludeFromManifest%2A extension method on the resource builder.

public static IResourceBuilder<MailDevResource> AddMailDev(
    this IDistributedApplicationBuilder builder, 
    string name,
    int? httpPort = null,
    int? smtpPort = null)
{
    var resource = new MailDevResource(name);

    return builder.AddResource(resource)
                  .WithImage(MailDevContainerImageTags.Image)
                  .WithImageRegistry(MailDevContainerImageTags.Registry)
                  .WithImageTag(MailDevContainerImageTags.Tag)
                  .WithHttpEndpoint(
                      targetPort: 1080,
                      port: httpPort,
                      name: MailDevResource.HttpEndpointName)
                  .WithEndpoint(
                      targetPort: 1025,
                      port: smtpPort,
                      name: MailDevResource.SmtpEndpointName)
                  .ExcludeFromManifest(); // This line was added!
}

Careful consideration should be given as to whether the resource should be present in the manifest, or whether it should be suppressed. If the resource is being added to the manifest it should be configured in such a way that it's safe and secure to use.

Summary

In the custom resource tutorial, you learned how to create a custom .NET Aspire resource which uses an existing containerized application (MailDev). You then used that to improve the local development experience by making it easy to test e-mail capabilities that might be used within an app. These learnings can be applied to building out other custom resources that can be used in .NET Aspire-based applications. This specific example didn't include any custom integrations, but it's possible to build out custom integrations to make it easier for developers to use the resource. In this scenario you were able to rely on the existing SmtpClient class in the .NET platform to send e-mails.

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custom-resources.md

custom-resources.md

Create custom resource types for .NET Aspire

Key elements of a .NET Aspire custom resource

A practical example: MailDev

Setup the starter project

Create library for resource extension

Define the resource types

Define the resource extensions

Validate custom integration inside the app host

Add a .NET service project to the app host for testing

Use connection string to send messages

Unix

Windows

Unix

Windows

Technical details

Secure networking

The `ReferenceExpression` and `EndpointReference` type

Manifest publishing

Summary

Next steps

Files

custom-resources.md

Latest commit

History

custom-resources.md

File metadata and controls

Create custom resource types for .NET Aspire

Key elements of a .NET Aspire custom resource

A practical example: MailDev

Setup the starter project

Create library for resource extension

Define the resource types

Define the resource extensions

Validate custom integration inside the app host

Add a .NET service project to the app host for testing

Use connection string to send messages

Unix

Windows

Unix

Windows

Technical details

Secure networking

The ReferenceExpression and EndpointReference type

Manifest publishing

Summary

Next steps

The `ReferenceExpression` and `EndpointReference` type