Mock IMemoryCache in unit test

IMemoryCache.Set Is an extension method and thus cannot be mocked using Moq framework.

The code for the extension though is available here

public static TItem Set<TItem>(this IMemoryCache cache, object key, TItem value, MemoryCacheEntryOptions options)
{
    using (var entry = cache.CreateEntry(key))
    {
        if (options != null)
        {
            entry.SetOptions(options);
        }

        entry.Value = value;
    }

    return value;
}

For the test, a safe path would need to be mocked through the extension method to allow it to flow to completion. Within Set it also calls extension methods on the cache entry, so that will also have to be catered for. This can get complicated very quickly so I would suggest using a concrete implementation

//...
using Microsoft.Extensions.Caching.Memory;
using Microsoft.Extensions.DependencyInjection;
//...

public Test GetSystemUnderTest() {
    var services = new ServiceCollection();
    services.AddMemoryCache();
    var serviceProvider = services.BuildServiceProvider();

    var memoryCache = serviceProvider.GetService<IMemoryCache>();
    return new Test(memoryCache);
}

[Fact]
public void TestCache() {
    //Arrange
    var sut = GetSystemUnderTest();

    //Act
    sut.SetCache("key", "value");

    //Assert
    //...
}

So now you have access to a fully functional memory cache.