Create custom .NET Aspire component

docs/extensibility/custom-component.md

@@ -0,0 +1,173 @@
+---
+title: Create custom .NET Aspire component
+description: Learn how to create a custom .NET Aspire component for an existing containerized application.
+ms.date: 07/16/2024
+ms.topic: how-to
+---
+
+# Create custom .NET Aspire component
+
+This article is a continuation of the [Create custom resource types for .NET Aspire](custom-resources.md) article. It guides you through creating a .NET Aspire component that uses [MailKit](https://github.com/jstedfast/MailKit) to send emails. This component is then integrated into the Newsletter app you previously built. The previous example, omitted the creation of a component and instead relied on the existing .NET `SmtpClient`. It's best to use MailKit's `SmtpClient` over the official .NET `SmtpClient` for sending emails, as it's more modern and supports more features/protocols. For more information, see [.NET SmtpClient: Remarks](/dotnet/api/system.net.mail.smtpclient#remarks).
+
+## Prerequisites
+
+If you're following along, you should have a Newsletter app from the steps in the [Create custom resource types for .NET Aspire](custom-resources.md) article.
+
+> [!TIP]
+> This article is inspired by existing .NET Aspire components, and based on the teams official guidance. There are places where said guidance varies, and it's important to understand the reasoning behind the differences. For more information, see [.NET Aspire component requirements](https://github.com/dotnet/aspire/blob/f38b6cba86942ad1c45fc04fe7170f0fd4ba7c0b/src/Components/Aspire_Components_Progress.md#net-aspire-component-requirements).
+
+## Create library for component
+
+[.NET Aspire components](../fundamentals/components-overview.md) are delivered as NuGet packages, but in this example, it's beyond the scope of this article to publish a NuGet package. Instead, you create a class library project that contains the component and reference it as a project. .NET Aspire component packages are intended to wrap a client library, such as MailKit, and provide production-ready telemetry, health checks, configurability, and testability. Let's start by creating a new class library project.
+
+1. Create a new class library project named `MailKit.Client` in the same directory as the _MailDevResource.sln_ from the previous article.
+
+    ```dotnetcli
+    dotnet new classlib -o MailKit.Client
+    ```
+
+1. Add the project to the solution.
+
+    ```dotnetcli
+    dotnet sln /MailDevResource.sln add MailKit.Client/MailKit.Client.csproj
+    ```
+
+The next step is to add all the NuGet packages that the component relies on. Rather than having you add each package one-by-one from the .NET CLI, it's likely easier to copy and paste the following XML into the _MailKit.Client.csproj_ file.
+
+```xml
+  <ItemGroup>
+    <PackageReference Include="MailKit" Version="4.7.0" />
+    <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="8.0.2" />
+    <PackageReference Include="Microsoft.Extensions.Resilience" Version="8.7.0" />
+    <PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.0" />
+    <PackageReference Include="Microsoft.Extensions.Diagnostics.HealthChecks" Version="8.0.7" />
+    <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.9.0" />
+  </ItemGroup>
+```
+
+## Define component settings
+
+Whenever you're creating a .NET Aspire component, it's best to understand the client library that you're mapping to. With MailKit, you need to understand the configuration settings that are required to connect to a Simple Mail Transfer Protocol (SMTP) server. But it's also important to understand if the library has support for _health checks_, _tracing_ and _metrics_. MailKit supports _tracing_ and _metrics_, through its [`Telemetry.SmtpClient` class](https://github.com/jstedfast/MailKit/blob/master/MailKit/Telemetry.cs#L112-L189). When adding _health checks_, you should use any established or existing health checks where possible. Otherwise, you might consider implementing your own in the component. Add the following code to the `MailKit.Client` project in a file named _MailKitClientSettings.cs_:
+
+:::code source="snippets/MailDevResource/MailKit.Client/MailKitClientSettings.cs":::
+
+The preceding code defines the `MailKitClientSettings` class with:
+
+- `Endpoint` property that represents the connection string to the SMTP server.
+- `Credentials` property that represents the credentials to authenticate with the SMTP server.
+- `DisableHealthChecks` property that determines whether health checks are enabled.
+- `DisableTracing` property that determines whether tracing is enabled.
+- `DisableMetrics` property that determines whether metrics are enabled.
+
+### Parse connection string logic
+
+The settings class also contains a `ParseConnectionString` method that parses the connection string into a valid `Uri`. The configuration is expected to be provided in the following format:
+
+- `ConnectionStrings:<connectionName>`: The connection string to the SMTP server.
+- `MailKit:Client:Endpoint`: The connection string to the SMTP server.
+
+If neither of these values are provided, an exception is thrown. Likewise, if there's a value but it's not a valid URI, an exception is thrown.
+
+## Expose component wrapper functionality
+
+The goal of .NET Aspire components is to expose the underlying client library to consumers through dependency injection. With MailKit and for this example, the `SmtpClient` class is what you want to expose. You're not wrapping any functionality, but rather mapping configuration settings to an `SmtpClient` class. It's common to expose both standard and keyed-service registrations for components. Standard registrations are used when there's only one instance of a service, and keyed-service registrations are used when there are multiple instances of a service. Sometimes, to achieve multiple registrations of the same type you use a factory pattern. Add the following code to the `MailKit.Client` project in a file named _MailKitClientFactory.cs_:
+
+:::code source="snippets/MailDevResource/MailKit.Client/MailKitClientFactory.cs":::
+
+The `MailKitClientFactory` class is a factory that creates an `ISmtpClient` instance based on the configuration settings. It's responsible for returning an `ISmtpClient` implementation that has an active connection to a configured SMTP server and optionally authenticated. Next, you need to expose the functionality for the consumers to register this factory with the dependency injection container. Add the following code to the `MailKit.Client` project in a file named _MailKitClientServiceCollectionExtensions.cs_:
+
+:::code source="snippets/MailDevResource/MailKit.Client/MailKitClientServiceCollectionExtensions.cs":::
+
+The preceding code adds two extension methods on the `IHostApplicationBuilder` type, one for the standard registration of MailKit and another for keyed-registration of MailKit.
+
+> [!TIP]
+> Extension methods for .NET Aspire components should extend the `IHostApplicationBuilder` type and follow the `Add<MeaningfulName>` naming convention where the `<MeaningfulName>` is the type or functionality you're adding.
+
+Both extensions ultimately rely on the private `AddMailKitClient` method to register the `MailKitClientFactory` with the dependency injection container as a [scoped service](/dotnet/core/extensions/dependency-injection#scoped). The reason for registering the `MailKitClientFactory` as a scoped service is because the connection (and authentication) operations are considered expensive and should be reused within the same scope where possible. In other words, for a single request, the same `ISmtpClient` instance should be used. The factory holds on to the instance of the `SmtpClient` that it creates and disposes of it.
+
+### Configuration binding
+
+One of the first things that the private implementation of the `AddMailKitClient` methods does, is to bind the configuration settings to the `MailKitClientSettings` class. The settings class is instantiated and then `Bind` is called with the specific section of configuration. Then the optional `configureSettings` delegate is invoked with the current settings. This allows the consumer to further configure the settings, ensuring that manual code settings are honored over configuration settings. After that, depending on whether the `serviceKey` value was provided, the `MailKitClientFactory` should be registered with the dependency injection container as either a standard or keyed service.
+
+> [!IMPORTANT]
+> It's intentional that the `implementationFactory` overload is called when registering services. The `CreateMailKitClientFactory` method throws when the configuration is invalid. This ensures that creation of the `MailKitClientFactory` is deferred until it's needed and it prevents the app from erroring out before logging is available.
+
+The registration of health checks, and telemetry are described in a bit more detail in the following sections.
+
+### Add health checks
+
+[Health checks](../fundamentals/health-checks.md) are a way to monitor the health of a component. With MailKit, you can check if the connection to the SMTP server is healthy. Add the following code to the `MailKit.Client` project in a file named _MailKitHealthCheck.cs_:
+
+:::code source="snippets/MailDevResource/MailKit.Client/MailKitHealthCheck.cs":::
+
+The preceding health check implementation:
+
+- Implements the `IHealthCheck` interface.
+- Accepts the `MailKitClientFactory` as a primary constructor parameter.
+- Satisfies the `CheckHealthAsync` method by:
+  - Attempting to get an `ISmtpClient` instance from the `factory`. If successful, it returns `HealthCheckResult.Healthy`.
+  - If an exception is thrown, it returns `HealthCheckResult.Unhealthy`.
+
+As previously shared in the registration of the `MailKitClientFactory`, the `MailKitHealthCheck` is conditionally registered with the `IHeathChecksBuilder`:
+
+```csharp
+if (settings.DisableHealthChecks is false)
+{
+    builder.Services.AddHealthChecks()
+        .AddCheck<MailKitHealthCheck>(
+            name: serviceKey is null ? "MailKit" : $"MailKit_{connectionName}",
+            failureStatus: default,
+            tags: []);
+}
+```
+
+The consumer could choose to omit health checks by setting the `DisableHealthChecks` property to `true` in the configuration. A common pattern for components is to have optional features and .NET Aspire components strongly encourages these types of configurations. For more information on health checks and a working sample that includes a user interface, see [.NET Aspire ASP.NET Core HealthChecksUI sample](/samples/dotnet/aspire-samples/aspire-health-checks-ui/).
+
+### Wire up telemetry
+
+As a best practice, the [MailKit client library exposes telemetry](https://github.com/jstedfast/MailKit/blob/master/Telemetry.md). .NET Aspire can take advantage of this telemetry and display it in the [.NET Aspire dashboard](../fundamentals/dashboard/overview.md). Depending on whether or not tracing and metrics are enabled, telemetry is wired up as shown in the following code snippet:
+
+```csharp
+if (settings.DisableTracing is false)
+{
+    builder.Services.AddOpenTelemetry()
+        .WithTracing(
+            traceBuilder => traceBuilder.AddSource(
+                Telemetry.SmtpClient.ActivitySourceName));
+}
+
+if (settings.DisableMetrics is false)
+{
+    // Required by MailKit to enable metrics
+    Telemetry.SmtpClient.Configure();
+
+    builder.Services.AddOpenTelemetry()
+        .WithMetrics(
+            metricsBuilder => metricsBuilder.AddMeter(
+                Telemetry.SmtpClient.MeterName));
+}
+```
+
+## Update the Newsletter service
+
+With the component library created, you can now update the Newsletter service to use the MailKit client. The first step is to add a reference to the `MailKit.Client` project. Add the _MailKit.Client.csproj_ project reference to the `MailDevResource.NewsletterService` project:
+
+```dotnetcli
+dotnet add ./MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj reference MailKit.Client/MailKit.Client.csproj
+```
+
+The final step is to replace the existing _Program.cs_ file in the `MailDevResource.NewsletterService` project with the following C# code:
+
+:::code source="snippets/MailDevResource/MailDevResource.NewsletterService/Program.cs":::
+
+The most notable changes in the preceding code are:
+
+- The updated `using` statements that include the `MailKit.Client`, `MailKit.Net.Smtp`, and `MimeKit` namespaces.
+- The replacement of the registration for the official .NET `SmtpClient` with the call to the `AddMailKitClient` extension method.
+- The replacement of both `/subscribe` and `/unsubscribe` map post calls to instead inject the `MailKitClientFactory` and use the `ISmtpClient` instance to send the email.
+
+## Summary
+
+In this article, you learned how to create a .NET Aspire component that uses MailKit to send emails. You also learned how to integrate this component into the Newsletter app you previously built. You learned about the core principles of .NET Aspire components, such as exposing the underlying client library to consumers through dependency injection, and how to add health checks and telemetry to the component. You also learned how to update the Newsletter service to use the MailKit client.
+
+Go forth and build your own .NET Aspire components. If you believe that there's enough community value in the component you're building, consider publishing it as a [NuGet package](/dotnet/standard/library-guidance/nuget) for others to use. Furthermore, consider submitting a pull request to the [.NET Aspire GitHub repository](https://github.com/dotnet/aspire) for consideration to be included in the official .NET Aspire components.

docs/extensibility/custom-resources.md

@@ -1,9 +1,8 @@
 ---
 title: Create custom resource types for .NET Aspire
 description: Learn how to create a custom resource for an existing containerized application.
-ms.date: 05/29/2024
+ms.date: 07/15/2024
 ms.topic: how-to
-ms.custom: devx-track-extended-azdevcli
 ---
 
 # Create custom resource types for .NET Aspire
@@ -230,7 +229,7 @@ In order to test the end-to-end scenario, you need a .NET project which we can i
 1. Create a new .NET project named _:::no-loc text="MailDevResource.NewsletterService":::_.
 
     ```dotnetcli
-    dotnet new webapi --use-minimal-apis --no-openapi -o MailDevResource.NewsletterService
+    dotnet new webapi --use-minimal-apis -o MailDevResource.NewsletterService
     ```
 
 1. Add a reference to the _:::no-loc text="MailDev.Hosting":::_ project.
@@ -265,11 +264,45 @@ The preceding screenshot shows the environment variables for the `newsletterserv
 
 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":::
+```csharp
+builder.Services.AddSingleton<SmtpClient>(sp =>
+{
+    var smtpUri = new Uri(builder.Configuration.GetConnectionString("maildev")!);
 
-To test the client, add two simple `subscribe` and `unsubscribe` GET methods to the newsletter service. Add the following code after the `MapGet` call in the _:::no-loc text="Program.cs":::_ file of the _MailDevResource.NewsletterService_ project to setup the ASP.NET Core routes:
+    var smtpClient = new SmtpClient(smtpUri.Host, smtpUri.Port);
 
-:::code source="snippets/MailDevResource/MailDevResource.NewsletterService/Program.cs" id="subs":::
+    return smtpClient;
+});
+```
+
+> [!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](https://github.com/jstedfast/MailKit), see [Create custom .NET Aspire component](custom-component.md).
+
+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:
+
+```csharp
+app.MapPost("/subscribe", async (SmtpClient smtpClient, string email) =>
+{
+    using var message = new MailMessage("newsletter@yourcompany.com", email)
+    {
+        Subject = "Welcome to our newsletter!",
+        Body = "Thank you for subscribing to our newsletter!"
+    };
+
+    await smtpClient.SendMailAsync(message);
+});
+
+app.MapPost("/unsubscribe", async (SmtpClient smtpClient, string email) =>
+{
+    using var message = new MailMessage("newsletter@yourcompany.com", email)
+    {
+        Subject = "You are unsubscribed from our newsletter!",
+        Body = "Sorry to see you go. We hope you will come back soon!"
+    };
+
+    await smtpClient.SendMailAsync(message);
+});
+```
 
 > [!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.
@@ -293,7 +326,7 @@ curl -H "Content-Type: application/json" --request POST https://localhost:7251/s
 ## [Windows](#tab/windows)
 
 ```powershell
-curl -H "Content-Type: application/json" --request POST https://localhost:7251/subscribe?email=test@test.com
+curl -H @{ ContentType = "application/json" } -Method POST https://localhost:7251/subscribe?email=test@test.com
 ```
 
 ---
@@ -317,7 +350,7 @@ curl -H "Content-Type: application/json" --request POST https://localhost:7251/u
 ## [Windows](#tab/windows)
 
 ```powershell
-curl -H "Content-Type: application/json" --request POST https://localhost:7251/unsubscribe?email=test@test.com
+curl -H @{ ContentType = "application/json" } -Method POST https://localhost:7251/unsubscribe?email=test@test.com
 ```
 
 ---
@@ -482,3 +515,8 @@ Careful consideration should be given as to whether the resource should be prese
 ## 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 components, but it's possible to build out custom components 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.
+
+## Next steps
+
+> [!div class="nextstepaction"]
+> [Create custom .NET Aspire component](custom-component.md)

docs/extensibility/snippets/MailDevResource/MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj

@@ -1,6 +1,12 @@
 <Project Sdk="Microsoft.NET.Sdk.Web">
 
   <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="8.0.7" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="6.6.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MailKit.Client\MailKit.Client.csproj" />
     <ProjectReference Include="..\MailDev.Hosting\MailDev.Hosting.csproj" />
     <ProjectReference Include="..\MailDevResource.ServiceDefaults\MailDevResource.ServiceDefaults.csproj" />
   </ItemGroup>