Why can this generic type not be converted?

Question

Say I have the following three classes/interfaces:

public interface IImportViewModel
{

}

public class TestImportViewModel : IImportViewModel
{

}

public class ValidationResult<TViewModel> where TViewModel : IImportViewModel
{

}

As TestImportViewModel implements IImportViewModel, why will the following not compile?

ValidationResult<IImportViewModel> r = new ValidationResult<TestImportViewModel>();

I understand what the error message "Cannot implicitly convert type 'ValidationResult' to 'ValidationResult'" means. I just don't understand why this is the case. Would this not be covariance?

Answer 1

Would this not be covariance?

Yes, except that in C# 4.0 covariance works on interfaces only. So you will have to make your ValidationResult implement a covariant interface (one for which the generic parameter is defined as out):

public interface IImportViewModel
{
}

public class TestImportViewModel : IImportViewModel
{
}

public interface IValidationResult<out TViewModel> where TViewModel : IImportViewModel
{
}

public class ValidationResult<TViewModel> : IValidationResult<TViewModel> where TViewModel : IImportViewModel 
{
}

and now you can do this:

IValidationResult<IImportViewModel> r = new ValidationResult<TestImportViewModel>();