GenericFake.cs

[TestMethod]
public async Task TestGenericOperation
{
      var fakeThing = new StubIThingWithGeneric();
      
      fakeThing.GenericOperationAsyncOf1(() => new SomeOtherThing();
      
      var target = new TestTarget(fakeThing);
      
      var actual = await target.DoSomething();
      
      Assert.IsNotNull(actual);
}