Mocking of extension method result in System.NotSupportedException

Question

I'm unit testing a ClientService that uses the IMemoryCache interface:

ClientService.cs:

public string Foo()
{        
    //... code

    _memoryCache.Set("MyKey", "SomeValue", new TimeSpan(0, 0, 60));
}

When I try to mock the IMemoryCache's Set extension with:

AutoMock mock = AutoMock.GetLoose();

var memoryCacheMock = _mock.Mock<IMemoryCache>();

string value = string.Empty;

// Attempt #1:
memoryCacheMock
     .Setup(x => x.Set<string>(It.IsAny<object>(), It.IsAny<string>(), It.IsAny<TimeSpan>()))
     .Returns("");

// Attempt #2:
memoryCacheMock
    .Setup(x => x.Set(It.IsAny<object>(), It.IsAny<object>(), It.IsAny<TimeSpan>()))
    .Returns(new object());

It throw an exception of:

System.NotSupportedException: Unsupported expression: x => x.Set(It.IsAny(), It.IsAny(), It.IsAny()) Extension methods (here: CacheExtensions.Set) may not be used in setup / verification ex

This is the Cache extension of the namespace Microsoft.Extensions.Caching.Memory

public static class CacheExtensions
{
   public static TItem Set<TItem>(this IMemoryCache cache, object key, TItem value, TimeSpan absoluteExpirationRelativeToNow);
}

Answer 1

Extension methods are actually static methods and they cannot be mocked using moq. What you could mock are the methods used by the extension method itself...

In your case Set uses CreateEntry which is the method defined by IMemoryCache and it could be mocked. Try something like this:

memoryCacheMock
    .Setup(x => x.CreateEntry(It.IsAny<object>()))
    .Returns(Mock.Of<ICacheEntry>);