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Create custom extensions

This article is for developers building custom test frameworks or extensions for Microsoft.Testing.Platform.

Note

For complete sample code, see the TestingPlatformExamples in the Microsoft Test Framework repository.

Extension point summary

Extension point In/Out of process Purpose
ITestFramework In-process The only mandatory extension. Discovers and runs tests.
IDataConsumer In-process Subscribes to and processes test data from the message bus.
ITestSessionLifetimeHandler In-process Runs code before and after a test session.
ITestApplicationLifecycleCallbacks In-process Runs code at the very start and very end of the test host.
ICommandLineOptionsProvider Both Adds custom command-line options.
ITestHostEnvironmentVariableProvider Out-of-process Sets environment variables before the test host starts.
ITestHostProcessLifetimeHandler Out-of-process Observes the test host process externally.

In-process vs out-of-process extensions

Extensions are categorized into two types:

  • In-process extensions run inside the test host process, alongside the test framework. Register them through builder.TestHost:

    var builder = await TestApplication.CreateBuilderAsync(args);
builder.TestHost.AddXXX(/* ... */);

  • Out-of-process extensions run in a separate process that observes the test host. Register them through builder.TestHostControllers:

    var builder = await TestApplication.CreateBuilderAsync(args);
builder.TestHostControllers.AddXXX(/* ... */);

Out-of-process extensions are needed when:

  • You need to set environment variables before the test host starts.
  • You need to monitor the test host externally because user code might crash or hang the process.

When any out-of-process extension is registered, the platform starts a second process automatically.

The IExtension interface

All extension points inherit from IExtension, which provides identification and opt-in/opt-out:

public interface IExtension
{
    string Uid { get; }
    string Version { get; }
    string DisplayName { get; }
    string Description { get; }
    Task<bool> IsEnabledAsync();
}
  • Uid: A unique identifier for the extension. Choose a unique value to avoid conflicts.
  • Version: The version of the extension, using semantic versioning.
  • DisplayName: A user-friendly name that appears in logs and --info output.
  • Description: A description that appears in --info output.
  • IsEnabledAsync(): Return false to exclude the extension from the session. Typically decisions are based on configuration or command-line options.
  • Build a test framework: Create a custom ITestFramework implementation, handle requests, and report test results.
  • Build extensions: Create in-process and out-of-process extensions such as data consumers, session handlers, and process monitors.
  • VSTest Bridge: Simplify migration of existing VSTest-based test frameworks to Microsoft.Testing.Platform.
  • Capabilities: Declare and query framework and extension capabilities.
  • Services: Access configuration, logging, message bus, and other platform services.
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