Question

I’m having a bit of an issue casting my objects in C# to be able to use the additional object methods besides the ones declared in the interface. Below is a simple example of what I am speaking about.

public interface IShape
{
    void Print();
}

public class Square : IShape
{
    #region IShape Members

    public void Print()
    {
        HttpContext.Current.Response.Write("Square Print Called"); 
    }   

    #endregion

    public void PrintMore()
    {
        HttpContext.Current.Response.Write("Square Print More Called"); 
    }        
}

Why when this code below is called am I not able to access PrintMore()?

IShape s = (Square)shape;
s.PrintMore() // This is not available. only Print() is. 

Any help and explanation would be helpful?

Answer 1

Your s variable is still of type IShape. Just because you happen to have used a cast when assigning to it doesn’t change the type as far as the compiler’s concerned. You’d need:

Square s = (Square) shape;
s.PrintMore();

Of course, this will only work if shape really is a Square (assuming there aren’t custom conversions going on).

I would advise you to think carefully before going down this route though. Usually a cast like this indicates that you’re breaking abstraction somewhat – if you only know about shape as an IShape, you should (usually) be able to do what you need just with the members of IShape. If that’s not the case, you can:

• Make IShape more powerful (give it more members)
• Change your code to accept a Square instead of an IShape
• Cast if absolutely necessary