Generic c# casting

Question

interface ISample
{
    int Id { get; set; }
}
public class MyClass : ISample
{
    public int Id
    {
       get;
       set;
    }
    public string Name { get; set; }
}

The above is a class which implements a interface ISample. Using generics i have added the below method to convert an object of given type to another type .

private TResult Sample<TSource, TResult>(TSource source)
            where TSource : TResult
{
   return (TResult)(source);
}

using Generics how do i achieve the reverse of the same that is given an ISample object i want to convert it back to MyClass object where ISample is the TSource and TResult is MyClass where TResult implements TSource.

Answer 1

You would simply have to switch from

where TSource : TResult

to

where TResult : TSource

ending up with code that might look like

private TResult ToInterface<TSource, TResult>(TSource source) where TSource : TResult
{
    return source;
}

private TResult FromInterface<TSource, TResult>(TSource source) where TResult : TSource
{
    return source as TResult;
}

Be aware that the FromInterface function will throw an InvalidCastException if the 'source' parameter is not of type TResult or does not inherit from TResult.

---

Edit:

See the answer by Lajos Arpad.

Answer 2

The compiler will not know in advance that you intend to do downcasting from TSource to TResult . You will need to cast your TSource instance into something that is surely a superclass of TResult , so you will need to cast TSource to object first:

(TResult)((object)source)

EDIT:

As pointed out in the comment section by @Zdeněk Jelínek, Eric Lippert has written a blog post about this.