Generic class and downcasting (typecasting)

public interface ISomeDTOInterface 
{
    ...
}

public class    SomeDTOClass : ISomeDTOInterface
{
    ...
}

public interface DataStoreChanges<T>
{
    IEnumerable<T> data {get;}
    IEnumerable<Guid> removedItems {get;}
    IEnumerable<Guid> newItems {get;}
    IEnumerable<Guid> modifiedItems {get;}
}

now I have a method which expects

DataStoreChanges<ISomeDTOInterface>

, so I try to pass an instance of DataStoreChanges<SomeDTOClass>, which generates a type error. It wont let me开发者_开发百科 downcast to the interface:) so whats wrong here.

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In C# 4, you can make the interface covariant. To do this, change the interface declaration to:

public interface DataStoreChanges<out T>

Otherwise, you can make the method generic and add the relevant type constraint.

void SomeMethod<T> (DataStoreChanges<T> x)  where T : ICommonInterface
{
    ...
}

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You're having issues because by default you're not specifying that the method should allow a more derived type of T, in that sense its not covariant. By changing your interface definition to:

public interface DataStoreChanges<out T> 

This will allow covariance.

If that is not possible, I guess you could also use a generic constraint on where T : ICommonInterface

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I think you are lacking type constrains - but could you actually add the non-compiling code?

where T:ICommonInterface

But this is just an educated guess based on your other information.