Question

I want a generic function that would work with types that have Top, Bottom, Right and Rect read-only properties – I have a lot of such classes in a third-party library.

I wrote this:

internal class MyTemplate<WhatType> {
    internal static void Work( WhatType what )
    {
        int left = what.Left;
    }
};

and I expect it to work – equivalent code in C++ would work okay. However C# objects:

error CS1061: ‘WhatType’ does not contain a definition for ‘Left’ and no extension method ‘Left’ accepting a first argument of type ‘WhatType’ could be found (are you missing a using directive or an assembly reference?)

I don’t get it – why would it try to instantiate the template before I even call it? Of course, type WhatType is not yet known and so no properties can be found.

What am I doing wrong and how do I fix that?

Answer 1

C# generics look similar to C++ templates, but they’re actually quite different. C# generics are strongly typed, so you can’t call a member that is not statically known.

In order to be able to call Left on an object of type WhatType, you have to specify that WhatType implements an interface or inherits from a class that defines Left, using a generic constraint. For instance:

interface IHasPosition
{
    int Left { get; }
    int Top { get; }
}

internal class MyTemplate<WhatType> where WhatType : IHasPosition
{
    internal static void Work( WhatType what )
    {
        int left = what.Left;
    }
};