Create custom .NET Aspire client integrations

This article is a continuation of the Create custom .NET Aspire hosting integrations article. It guides you through creating a .NET Aspire client integration that uses MailKit to send emails. This integration is then added into the Newsletter app you previously built. The previous example omitted the creation of a client integration 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.

Prerequisites

If you're following along, you should have a Newsletter app from the steps in the Create custom .NET Aspire hosting integration article.

Create library for integration

.NET Aspire integrations 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 integration and reference it as a project. .NET Aspire integration 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.

   dotnet new classlib -o MailKit.Client
   

2. Add the project to the solution.

   dotnet sln ./MailDevResource.sln add MailKit.Client/MailKit.Client.csproj
   

The next step is to add all the NuGet packages that the integration 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.

<ItemGroup>
  <PackageReference Include="MailKit" Version="4.8.0" />
  <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="8.0.2" />
  <PackageReference Include="Microsoft.Extensions.Resilience" Version="8.10.0" />
  <PackageReference Include="Microsoft.Extensions.Hosting.Abstractions" Version="8.0.1" />
  <PackageReference Include="Microsoft.Extensions.Diagnostics.HealthChecks" Version="8.0.10" />
  <PackageReference Include="OpenTelemetry.Extensions.Hosting" Version="1.9.0" />
</ItemGroup>

Define integration settings

Whenever you're creating a .NET Aspire integration, 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. When adding health checks, you should use any established or existing health checks where possible. Otherwise, you might consider implementing your own in the integration. Add the following code to the MailKit.Client project in a file named MailKitClientSettings.cs:

using System.Data.Common;

namespace MailKit.Client;

/// <summary>
/// Provides the client configuration settings for connecting MailKit to an SMTP server.
/// </summary>
public sealed class MailKitClientSettings
{
    internal const string DefaultConfigSectionName = "MailKit:Client";

    /// <summary>
    /// Gets or sets the SMTP server <see cref="Uri"/>.
    /// </summary>
    /// <value>
    /// The default value is <see langword="null"/>.
    /// </value>
    public Uri? Endpoint { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the database health check is disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableHealthChecks { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the OpenTelemetry tracing is disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableTracing { get; set; }

    /// <summary>
    /// Gets or sets a boolean value that indicates whether the OpenTelemetry metrics are disabled or not.
    /// </summary>
    /// <value>
    /// The default value is <see langword="false"/>.
    /// </value>
    public bool DisableMetrics { get; set; }

    internal void ParseConnectionString(string? connectionString)
    {
        if (string.IsNullOrWhiteSpace(connectionString))
        {
            throw new InvalidOperationException($"""
                    ConnectionString is missing.
                    It should be provided in 'ConnectionStrings:<connectionName>'
                    or '{DefaultConfigSectionName}:Endpoint' key.'
                    configuration section.
                    """);
        }

        if (Uri.TryCreate(connectionString, UriKind.Absolute, out var uri))
        {
            Endpoint = uri;
        }
        else
        {
            var builder = new DbConnectionStringBuilder
            {
                ConnectionString = connectionString
            };
            
            if (builder.TryGetValue("Endpoint", out var endpoint) is false)
            {
                throw new InvalidOperationException($"""
                        The 'ConnectionStrings:<connectionName>' (or 'Endpoint' key in
                        '{DefaultConfigSectionName}') is missing.
                        """);
            }

            if (Uri.TryCreate(endpoint.ToString(), UriKind.Absolute, out uri) is false)
            {
                throw new InvalidOperationException($"""
                        The 'ConnectionStrings:<connectionName>' (or 'Endpoint' key in
                        '{DefaultConfigSectionName}') isn't a valid URI.
                        """);
            }

            Endpoint = uri;
        }
    }
}

The preceding code defines the MailKitClientSettings class with:

• Endpoint property that represents the connection string to 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:ConnectionString: The connection string to the SMTP server.

If neither of these values are provided, an exception is thrown.

Expose client functionality

The goal of .NET Aspire integrations 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 integrations. 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:

using MailKit.Net.Smtp;

namespace MailKit.Client;

/// <summary>
/// A factory for creating <see cref="ISmtpClient"/> instances
/// given a <paramref name="smtpUri"/> (and optional <paramref name="credentials"/>).
/// </summary>
/// <param name="settings">
/// The <see cref="MailKitClientSettings"/> settings for the SMTP server
/// </param>
public sealed class MailKitClientFactory(MailKitClientSettings settings) : IDisposable
{
    private readonly SemaphoreSlim _semaphore = new(1, 1);

    private SmtpClient? _client;

    /// <summary>
    /// Gets an <see cref="ISmtpClient"/> instance in the connected state
    /// (and that's been authenticated if configured).
    /// </summary>
    /// <param name="cancellationToken">Used to abort client creation and connection.</param>
    /// <returns>A connected (and authenticated) <see cref="ISmtpClient"/> instance.</returns>
    /// <remarks>
    /// Since both the connection and authentication are considered expensive operations,
    /// the <see cref="ISmtpClient"/> returned is intended to be used for the duration of a request
    /// (registered as 'Scoped') and is automatically disposed of.
    /// </remarks>
    public async Task<ISmtpClient> GetSmtpClientAsync(
        CancellationToken cancellationToken = default)
    {
        await _semaphore.WaitAsync(cancellationToken);

        try
        {
            if (_client is null)
            {
                _client = new SmtpClient();

                await _client.ConnectAsync(settings.Endpoint, cancellationToken)
                             .ConfigureAwait(false);
            }
        }
        finally
        {
            _semaphore.Release();
        }       

        return _client;
    }

    public void Dispose()
    {
        _client?.Dispose();
        _semaphore.Dispose();
    }
}

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. 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 MailKitExtensions.cs:

using MailKit;
using MailKit.Client;
using MailKit.Net.Smtp;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;

namespace Microsoft.Extensions.Hosting;

/// <summary>
/// Provides extension methods for registering a <see cref="SmtpClient"/> as a
/// scoped-lifetime service in the services provided by the <see cref="IHostApplicationBuilder"/>.
/// </summary>
public static class MailKitExtensions
{
    /// <summary>
    /// Registers 'Scoped' <see cref="MailKitClientFactory" /> for creating
    /// connected <see cref="SmtpClient"/> instance for sending emails.
    /// </summary>
    /// <param name="builder">
    /// The <see cref="IHostApplicationBuilder" /> to read config from and add services to.
    /// </param>
    /// <param name="connectionName">
    /// A name used to retrieve the connection string from the ConnectionStrings configuration section.
    /// </param>
    /// <param name="configureSettings">
    /// An optional delegate that can be used for customizing options.
    /// It's invoked after the settings are read from the configuration.
    /// </param>
    public static void AddMailKitClient(
        this IHostApplicationBuilder builder,
        string connectionName,
        Action<MailKitClientSettings>? configureSettings = null) =>
        AddMailKitClient(
            builder,
            MailKitClientSettings.DefaultConfigSectionName,
            configureSettings,
            connectionName,
            serviceKey: null);

    /// <summary>
    /// Registers 'Scoped' <see cref="MailKitClientFactory" /> for creating
    /// connected <see cref="SmtpClient"/> instance for sending emails.
    /// </summary>
    /// <param name="builder">
    /// The <see cref="IHostApplicationBuilder" /> to read config from and add services to.
    /// </param>
    /// <param name="name">
    /// The name of the component, which is used as the <see cref="ServiceDescriptor.ServiceKey"/> of the
    /// service and also to retrieve the connection string from the ConnectionStrings configuration section.
    /// </param>
    /// <param name="configureSettings">
    /// An optional method that can be used for customizing options. It's invoked after the settings are
    /// read from the configuration.
    /// </param>
    public static void AddKeyedMailKitClient(
        this IHostApplicationBuilder builder,
        string name,
        Action<MailKitClientSettings>? configureSettings = null)
    {
        ArgumentNullException.ThrowIfNull(name);

        AddMailKitClient(
            builder,
            $"{MailKitClientSettings.DefaultConfigSectionName}:{name}",
            configureSettings,
            connectionName: name,
            serviceKey: name);
    }

    private static void AddMailKitClient(
        this IHostApplicationBuilder builder,
        string configurationSectionName,
        Action<MailKitClientSettings>? configureSettings,
        string connectionName,
        object? serviceKey)
    {
        ArgumentNullException.ThrowIfNull(builder);

        var settings = new MailKitClientSettings();

        builder.Configuration
               .GetSection(configurationSectionName)
               .Bind(settings);

        if (builder.Configuration.GetConnectionString(connectionName) is string connectionString)
        {
            settings.ParseConnectionString(connectionString);
        }

        configureSettings?.Invoke(settings);

        if (serviceKey is null)
        {
            builder.Services.AddScoped(CreateMailKitClientFactory);
        }
        else
        {
            builder.Services.AddKeyedScoped(serviceKey, (sp, key) => CreateMailKitClientFactory(sp));
        }

        MailKitClientFactory CreateMailKitClientFactory(IServiceProvider _)
        {
            return new MailKitClientFactory(settings);
        }

        if (settings.DisableHealthChecks is false)
        {
            builder.Services.AddHealthChecks()
                .AddCheck<MailKitHealthCheck>(
                    name: serviceKey is null ? "MailKit" : $"MailKit_{connectionName}",
                    failureStatus: default,
                    tags: []);
        }

        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));
        }
    }
}

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.

Both extensions ultimately rely on the private AddMailKitClient method to register the MailKitClientFactory with the dependency injection container as a scoped service. The reason for registering the MailKitClientFactory as a scoped service is because the connection 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.

The registration of health checks, and telemetry are described in a bit more detail in the following sections.

Add health checks

Health checks are a way to monitor the health of an integration. 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:

using Microsoft.Extensions.Diagnostics.HealthChecks;

namespace MailKit.Client;

internal sealed class MailKitHealthCheck(MailKitClientFactory factory) : IHealthCheck
{
    public async Task<HealthCheckResult> CheckHealthAsync(
        HealthCheckContext context,
        CancellationToken cancellationToken = default)
    {
        try
        {
            // The factory connects (and authenticates).
            _ = await factory.GetSmtpClientAsync(cancellationToken);

            return HealthCheckResult.Healthy();
        }
        catch (Exception ex)
        {
            return HealthCheckResult.Unhealthy(exception: ex);
        }
    }
}

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:

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 integrations is to have optional features and .NET Aspire integrations 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.

Wire up telemetry

As a best practice, the MailKit client library exposes telemetry. .NET Aspire can take advantage of this telemetry and display it in the .NET Aspire dashboard. Depending on whether or not tracing and metrics are enabled, telemetry is wired up as shown in the following code snippet:

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 integration 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:

dotnet add ./MailDevResource.NewsletterService/MailDevResource.NewsletterService.csproj reference MailKit.Client/MailKit.Client.csproj

Next, add a reference to the ServiceDefaults project:

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

The final step is to replace the existing Program.cs file in the MailDevResource.NewsletterService project with the following C# code:

using System.Net.Mail;
using MailKit.Client;
using MailKit.Net.Smtp;
using MimeKit;

var builder = WebApplication.CreateBuilder(args);

builder.AddServiceDefaults();

builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

// Add services to the container.
builder.AddMailKitClient("maildev");

var app = builder.Build();

app.MapDefaultEndpoints();

// Configure the HTTP request pipeline.

app.UseSwagger();
app.UseSwaggerUI();
app.UseHttpsRedirection();

app.MapPost("/subscribe",
    async (MailKitClientFactory factory, string email) =>
{
    ISmtpClient client = await factory.GetSmtpClientAsync();

    using var message = new MailMessage("newsletter@yourcompany.com", email)
    {
        Subject = "Welcome to our newsletter!",
        Body = "Thank you for subscribing to our newsletter!"
    };

    await client.SendAsync(MimeMessage.CreateFromMailMessage(message));
});

app.MapPost("/unsubscribe",
    async (MailKitClientFactory factory, string email) =>
{
    ISmtpClient client = await factory.GetSmtpClientAsync();

    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 client.SendAsync(MimeMessage.CreateFromMailMessage(message));
});

app.Run();

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.

Run the sample

Now that you've created the MailKit client integration and updated the Newsletter service to use it, you can run the sample. From your IDE, select F5 or run dotnet run from the root directory of the solution to start the application—you should see the .NET Aspire dashboard:

Once the application is running, navigate to the Swagger UI at https://localhost:7251/swagger and test the /subscribe and /unsubscribe endpoints. Select the down arrow to expand the endpoint:

Then select the Try it out button. Enter an email address, and then select the Execute button.

Repeat this several times, to add multiple email addresses. You should see the email sent to the MailDev inbox:

Stop the application by selecting Ctrl+C in the terminal window where the application is running, or by selecting the stop button in your IDE.

View MailKit telemetry

The MailKit client library exposes telemetry that can be viewed in the .NET Aspire dashboard. To view the telemetry, navigate to the .NET Aspire dashboard at https://localhost:7251. Select the newsletter resource to view the telemetry on the Metrics page:

Open up the Swagger UI again, and make some requests to the /subscribe and /unsubscribe endpoints. Then, navigate back to the .NET Aspire dashboard and select the newsletter resource. Select a metric under the mailkit.net.smtp node, such as mailkit.net.smtp.client.operation.count. You should see the telemetry for the MailKit client:

Summary

In this article, you learned how to create a .NET Aspire integration that uses MailKit to send emails. You also learned how to integrate this integration into the Newsletter app you previously built. You learned about the core principles of .NET Aspire integrations, such as exposing the underlying client library to consumers through dependency injection, and how to add health checks and telemetry to the integration. You also learned how to update the Newsletter service to use the MailKit client.

Go forth and build your own .NET Aspire integrations. If you believe that there's enough community value in the integration you're building, consider publishing it as a NuGet package for others to use. Furthermore, consider submitting a pull request to the .NET Aspire GitHub repository for consideration to be included in the official .NET Aspire integrations.

Next steps