Using Moq to verify a parameter of type List?

You can get rid of the call to Setup and Verifiable on your mock. Just use Verify.

I created a little test-project, and this worked for me:

using System;
using System.Collections.Generic;
using System.Linq;
using Moq;

namespace csharp
{
    public class SomeDTO { }

    public class SomeRepository { public virtual void Write(List<SomeDTO> list) { } }

    public class MainClass
    {
        private SomeRepository someRepository;

        public MainClass(SomeRepository someRepository) { this.someRepository = someRepository; }

        public void DoRepositoryWrite(List<SomeDTO> list) { this.someRepository.Write(list); }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var mockSomeRepository = new Mock<SomeRepository>();
            var someList = Enumerable.Repeat(new SomeDTO(), 25).ToList();

            var mainClass = new MainClass(mockSomeRepository.Object);
            mainClass.DoRepositoryWrite(someList);

            mockSomeRepository.Verify(m => m.Write(It.IsAny<List<SomeDTO>>()), Times.Once(), "Write was not called");
            mockSomeRepository.Verify(m => m.Write(It.Is<List<SomeDTO>>(l => l.Count == 25)), Times.Once(), "Write was not called with a 25-element-list");
        }
    }
}