How can I test ModelState?

You don't have to use a Mock if you're using the Repository Pattern for your data, of course.

Some examples: http://www.singingeels.com/Articles/Test_Driven_Development_with_ASPNET_MVC.aspx

// Test for required "FirstName".
   controller.ViewData.ModelState.Clear();

   newCustomer = new Customer
   {
       FirstName = "",
       LastName = "Smith",
       Zip = "34275",    
   };

   controller.Create(newCustomer);

   // Make sure that our validation found the error!
   Assert.IsTrue(controller.ViewData.ModelState.Count == 1, 
                 "FirstName must be required.");

//[Required]
//public string Name { get; set; }
//[Required]
//public string Description { get; set; }

ProductModelEdit model = new ProductModelEdit() ;
//Init ModelState
var modelBinder = new ModelBindingContext()
{
    ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(
                      () => model, model.GetType()),
    ValueProvider=new NameValueCollectionValueProvider(
                        new NameValueCollection(), CultureInfo.InvariantCulture)
};
var binder=new DefaultModelBinder().BindModel(
                 new ControllerContext(),modelBinder );
ProductController.ModelState.Clear();
ProductController.ModelState.Merge(modelBinder.ModelState);

ViewResult result = (ViewResult)ProductController.CreateProduct(null,model);
Assert.IsTrue(result.ViewData.ModelState["Name"].Errors.Count > 0);
Assert.True(result.ViewData.ModelState["Description"].Errors.Count > 0);
Assert.True(!result.ViewData.ModelState.IsValid);

For testing Web API, use the Validate method on the controller:

var controller = new MyController();
controller.Configuration = new HttpConfiguration();
var model = new MyModel();

controller.Validate(model);
var result = controller.MyMethod(model);

Ran into this problem for .NetCore 2.1 Here's my solution:

Extension Method

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Linq;

namespace MyExtension
{
    public static void BindViewModel<T>(this Controller controller, T model)
    {
        if (model == null) return;

        var context = new ValidationContext(model, null, null);
        var results = new List<ValidationResult>();

        if (!Validator.TryValidateObject(model, context, results, true))
        {
            controller.ModelState.Clear();
            foreach (ValidationResult result in results)
            {
                var key = result.MemberNames.FirstOrDefault() ?? "";
                controller.ModelState.AddModelError(key, result.ErrorMessage);
            }
        }
    }
}

View Model

public class MyViewModel
{
    [Required]
    public string Name { get; set; }
}

Unit Test

public async void MyUnitTest()
{
    // helper method to create instance of the Controller
    var controller = this.CreateController();

    var model = new MyViewModel
    {
        Name = null
    };

    // here we call the extension method to validate the model
    // and set the errors to the Controller's ModelState
    controller.BindViewModel(model);

    var result = await controller.ActionName(model);

    Assert.NotNull(result);
    var viewResult = Assert.IsType<BadRequestObjectResult>(result);
}

How can I test ModelState?

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