Replace a component that the system under test (SUT) depends on with a much lighter-weight implementation.

Test doubles - Fake.cs

using System.Collections.Generic;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace TestDoubles.Examples.Fake
{
    [TestClass]
    public class Invoice_Test
    {
        [TestMethod]
        public void AddItemQuantity_should_add_item_quantity()
        {
            // Arrange
            var fakeRepository = new FakeRepository();
            var invoice = new Invoice(fakeRepository);
            var product = new Product { Id = 1 };

            // Act 
            invoice.AddItemQuantity(product, 10);

            // Assert
            Assert.AreEqual(product, fakeRepository.GetProduct(1));
        } 
    }

    public class Invoice
    {
        private readonly IProductRepository _repository;

        public Invoice(IProductRepository repository)
        {
            _repository = repository;
        }
        /* Methods that use repository*/

        public void AddItemQuantity(Product product, int i)
        {
            _repository.AddProduct(product, i);
        }
    }

    public class Product
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    public interface IProductRepository
    {
        Product AddProduct(Product product, int i);
        Product GetProduct(int productId);
    }

    public class FakeRepository : IProductRepository
    {
        private List<Product> _list = new List<Product>();

        public Product AddProduct(Product product, int i)
        {
            _list.Add(product);
            /* Some additional logic */
            return product;
        }

        public Product GetProduct(int productId)
        {
            return _list.SingleOrDefault(p => p.Id == productId);
        }
    }
}