ASP.NET MVC unit testing with MOQ object

If you want to mock ProductService first you need to inject this dependency.

Constructor injection is the most common approach for controllers in ASP.NET MVC.

public class YourController : Controller
{
    private readonly IProductService ProductService;

    /// <summary>
    /// Constructor injection
    /// </summary>
    public YourController(IProductService productService)
    {
        ProductService = productService;
    }

    /// <summary>
    /// Code of this method has not been changed at all.
    /// </summary>
    public ActionResult Products()
    {
        ViewBag.Title = "Company Product";
        IEnumerable<ProductDetailDto> productList = ProductService.GetAllEffectiveProductDetails();
        ProductModels.ProductCategoryListModel model = new ProductModels.ProductCategoryListModel
        {
            //the type of ProductDetails => IEnumerable<productDetailDto>  
            ProductDetails = ProductService.GetAllEffectiveProductDetails(),

            //the type of ProductCategoryList => IEnumerable<selectlistitem>
            ProductCategoryList = productList.Select(x => new SelectListItem
            {
                Value = x.FKProductId.ToString(),
                Text = x.Name
            })
        };
        return View(model);
    }
}

#region DataModels

public class ProductDetailDto
{
    public int FKProductId { get; set; }
    public string Name { get; set; }
}

public class ProductModels
{
    public class ProductCategoryListModel
    {
        public IEnumerable<ProductDetailDto> ProductDetails { get; set; }
        public IEnumerable<SelectListItem> ProductCategoryList { get; set; }
    }
}

#endregion

#region Services

public interface IProductService
    {
        IEnumerable<ProductDetailDto> GetAllEffectiveProductDetails()
    }

public class ProductService : IProductService
{
    public IEnumerable<ProductDetailDto> GetAllEffectiveProductDetails()
    {
        throw new NotImplementedException();
    }
}

#endregion

Then you easily create a mock instance of IProductService, pass it into constructor of YourController, setup GetAllEffectiveProductDetails method and check returned ActionResult and its model.

[TestClass]
public class YourControllerTest
{
    private Mock<IProductService> productServiceMock;

    private YourController target;

    [TestInitialize]
    public void Init()
    {
        productServiceMock = new Mock<IProductService>();

        target = new YourController(
            productServiceMock.Object);
    }

    [TestMethod]
    public void Products()
    {
        //arrange
        // There is a setup of 'GetAllEffectiveProductDetails'
        // When 'GetAllEffectiveProductDetails' method is invoked 'expectedallProducts' collection is exposed.
        var expectedallProducts = new List<ProductDetailDto> { new ProductDetailDto() };
        productServiceMock
            .Setup(it => it.GetAllEffectiveProductDetails())
            .Returns(expectedallProducts);

        //act
        var result = target.Products();

        //assert
        var model = (result as ViewResult).Model as ProductModels.ProductCategoryListModel;
        Assert.AreEqual(model.ProductDetails, expectedallProducts);
        /* Any other assertions */
    }
}