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?

I'm assuming your class _TestImportViewModel_ is supposed to be _TViewModel_? (The code above does not show class TViewmModel at all). — Sam Goldberg, Jul 02 '12 at 13:23
Yes. TestImportViewModel implements IImportViewModel and thus __should__ be able to be used as a TViewModel implementation as TViewModel : IImportViewModel. — devlife, Jul 02 '12 at 13:28
@Sam TViewmModel(aka ) is a type of IImportViewModel its not a class . — Renatas M., Jul 02 '12 at 13:28
@SamGoldberg, It's the generic parameter, often seen as `T` but in this case has a much more descriptive name. — Filip Ekberg, Jul 02 '12 at 13:28
[Here is](http://stackoverflow.com/a/4653199/540345) a explanation of similar problem. — NaveenBhat, Jul 02 '12 at 13:37

score 8 · Accepted Answer · answered Jul 02 '12 at 13:32

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>();

You still cannot assign to `ValidationResult` you can only assign to `IValidationResult`. This is unfortunate. — John Alexiou, Jul 02 '12 at 13:40

Why can this generic type not be converted?

1 Answers1