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

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;

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.

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();