The Of extension method to confiture existing a mocked instance using LINQ query syntax.

MockExtensions.cs

namespace Jwc.MoqExtensions
{
    using Moq;
    using System;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Reflection;
    using Xunit;

    public interface IFoo
    {
        string GetName();
    }

    public class MockExtensionsTest
    {
        [Fact]
        public void OfCorrectlyConfiguresMockedInstance()
        {
            var mocked = new Mock<IFoo>().Object;
            mocked.Of(x => x.GetName() == "name");
            Assert.Equal("name", mocked.GetName());
        }
    }

    public static class MockExtensions
    {
        private readonly static Type TypeOfMockQueryable
            = typeof(Mock).Assembly.GetTypes().Single(t => t.Name == "MockQueryable`1");

        public static T Of<T>(this T mocked, Expression<Func<T, bool>> predicate) where T : class
        {
            return CreateMockQuery<T>(mocked).First(predicate);
        }

        private static IQueryable<T> CreateMockQuery<T>(T mocked) where T : class
        {
            return (IQueryable<T>)MockExtensions.GetConstructorOfMockQueryable<T>()
                .Invoke(new object[] { MockExtensions.GetMethodCallExpression(mocked) });
        }

        private static ConstructorInfo GetConstructorOfMockQueryable<T>()
        {
            return MockExtensions.TypeOfMockQueryable
                .MakeGenericType(typeof(T))
                .GetConstructor(new[] { typeof(MethodCallExpression) });
        }

        private static MethodCallExpression GetMethodCallExpression<T>(T mocked) where T : class
        {
            return Expression.Call(
                new Func<T, IQueryable<T>>(m => new[] { m }.AsQueryable()).Method,
                Expression.Constant(mocked));
        }
    }
}