C# - Casting an object to an interface

Question

Suppose we have an interface:

interface ICustomShape
{
}

and we have a class that inherits from the Shape class, and implements the interface:

public class CustomIsocelesTriangle : Shape, ICustomShape
{

}

How would I go about casting a CustomIsocelesTriangle to a ICustomShape object, for use on an "interface level?"

ICustomShape x = (ICustomShape)canvas.Children[0]; //Gives runtime error: Unable to cast object of type 'program_4.CustomIsocelesTriangle' to type 'program_4.ICustomShape'.

Answer 1

If you are sure that:

1. canvas.Children[0] returns a CustomIsocelesTriangle.
   Use the debugger to verify, or print the type to the console:

   var shape = canvas.Children[0];
   Console.WriteLine(shape.GetType());
   // Should print "program_4.CustomIsocelesTriangle"
   

2. You're casting to ICustomShape (not CustomShape).

3. CustomIsocelesTriangle implements ICustomShape.
   Try this to verify (it should compile):

   ICustomShape shape = new CustomIsocelesTriangle(/* Fake arguments */);
   

Then perhaps:

• you have CustomIsocelesTriangle in a different project or assembly, and you forgot to save and rebuild it after you made it implement ICustomShape;
• or, you reference an older version of the assembly;
• or, you have two interfaces named ICustomShape or two classes CustomIsocelesTriangle (perhaps in different namespaces), and you (or the compiler) got them mixed up.