TryValidateModel in asp.net core throws Null Reference Exception while performing unit test

Question

I'm trying to write unit tests for ModelState validation for an Asp.Net Core Web API.

I read that, the best way to do so is to use TryValidateModel function. But, every time I run the unit test, it throws NullReference exception.
I found many articles suggesting controller.ModelState.AddModelError("",""), but I'm not interested in this, as I believe that it beats the actual purpose of the real model validation.

[TestMethod]
public void TestMethod1()
{
    var controller = new TestController();

    controller.Post(new Model());
}


public class TestController : Controller
{
    public IActionResult Post(Model model)
    {
        bool b = TryValidateModel(model)

        return Ok();
    }
}

TryValidateModel(model) always throws NullReference Exception from TryValidateModel(model, prefix) function.

Appreciate any help.

Answer 1

It's configuration/integration issue.

You can see some additional info in the issue in ASP.NET Core repo and another one on github. But I can tell you the easiest fix (I used it once)

        var objectValidator = new Mock<IObjectModelValidator>();
        objectValidator.Setup(o => o.Validate(It.IsAny<ActionContext>(), 
                                          It.IsAny<ValidationStateDictionary>(), 
                                          It.IsAny<string>(), 
                                          It.IsAny<Object>()));
        controller.ObjectValidator = objectValidator.Object;

Answer 2

As I figured how to fix Null Reference Exception thanks to @Egorikas, I noticed that it still doesn't actually validate the model and always returns a true.

I found that we could just use Validator class in System.ComponentModel.DataAnnotationsnamespace.

[TestMethod]
public void TestMethod1()
{
    var model = new Person();
    var validationResultList = new List<ValidationResult>();


    bool b1 = Validator.TryValidateObject(model, new ValidationContext(model), validationResultList);
}

You can directly validate it from the Test method itself, rather than having to call the controller, if ModelState validation is your intention.

Hope this helps.

Answer 3

Based on Andrew Van Den Brink answer's, but with actually having the validation errors set in the ModelState.

private class ObjectValidator : IObjectModelValidator
{

    public void Validate(ActionContext actionContext, ValidationStateDictionary validationState, string prefix, object model)
    {
        var context = new ValidationContext(model, serviceProvider: null, items: null);
        var results = new List<ValidationResult>();

        bool isValid = Validator.TryValidateObject(
            model, context, results,
            validateAllProperties: true
        );

        if (!isValid)
            results.ForEach((r) =>
            {
                // Add validation errors to the ModelState
                actionContext.ModelState.AddModelError("", r.ErrorMessage);
            });
    }
}

Then, simply set the ObjectValidator in your controller:

controller.ObjectValidator = new ObjectValidator();