Auto mocking services

[egil]

A fallback service provider added to bUnit's root service provider is invoked if the root service provider cannot resolve a GetService request. With that info in mind, we can use Moq (or another mocking framework in a similar fashion shown below) and a little reflection trickery to create a fallback service provider, which is really just something that implements the IServiceProvider interface, that will use Moq to create a mocked version of a requested service, when its GetService method is called.

AutoMockingServiceProvider

The auto moclking service provider will use Moq to create a mock of a requested service type once, and any subsequent requests will get the same type returned (they are saved in the mockedTypes dictionary).

The reason for saving the mocked services is that you can retrieve the same mocked instance of a type in both your test and in the component under test, which allows you to configure the mock in your test.

The GetMockedService extension method below makes it easy to pull out a Mock from the service provider.

using System; using System.Collections.Generic; using System.Linq; using System.Reflection; using Microsoft.Extensions.DependencyInjection; using Moq; public class AutoMockingServiceProvider : IServiceProvider { private static readonly MethodInfo GenericMockFactory = typeof(Mock).GetMethods().First(x => x.Name == "Of"); private readonly Dictionary<Type, object> mockedTypes = new(); public object? GetService(Type serviceType) => GetMockedService(serviceType); public object GetMockedService<T>() => GetMockedService(typeof(T)); public object GetMockedService(Type serviceType) { if (!mockedTypes.TryGetValue(serviceType, out var service)) { var mockFactory = GenericMockFactory.MakeGenericMethod(serviceType); service = mockFactory.Invoke(null, Array.Empty<object>())!; mockedTypes.Add(serviceType, service); } return service; } } internal static class ServiceProviderExtensions { public static Mock<T> GetMockedService<T>(this IServiceProvider services) where T : class => Mock.Get<T>(services.GetService<T>()!); }

NOTE: This code doesn't deal with any edge cases, so it might not work in all cases, but should serve as a good starting point.

Example usage

Suppose we have the following component, <MyComp>:

@inject IPerson Person @Person.Name

That depends on this interface:

public interface IPerson { public string Name { get; } } Then it can be tested like this: ```c# [Fact] public void Test1() { using var ctx = new TestContext(); // Add the AutoMockingServiceProvider as the fallback service provider ctx.Services.AddFallbackServiceProvider(new AutoMockingServiceProvider()); // Retrieves the mocked person from the service collection and configures it.  var mockedPerson = ctx.Services.GetMockedService<IPerson>(); mockedPerson.SetupGet(x => x.Name).Returns("Foo Bar"); // Render component var cut = ctx.RenderComponent<MyComp>(); // Verify content cut.MarkupMatches("Foo Bar"); }

This was tested with .NET 6 rc.1, Moq 4.16.1, and bunit 1.2.49.