Question

I have a class that extends to Control:

public foo : Control 
{
  //.. 
}

And then I get a control:

var baa = (((foo)((Control)Controls.Find(controlName, true).First()));
baa.etc = ..; 

But when I do:

 ((PictureBox)((Control)controlImg)).MyExtensionMethod(..) 

I get an exception:

Unable to cast object of type ‘System.Windows.Forms.PictureBox’ to type ‘ControlExtensions.foo’.

How to fix this exception and let me know.

Thank you.

Answer 1

There is no way to fix this. You class foo is correct. The error message explains it all. foo does not inherit PictureBox. If foo is a picture box of some sort, implement the PictureBox class and not Control.

To give you a real life example:

interface IAnimal { }
class Dog : IAnimal { public static void Bark() { } }
class Cat : IAnimal { public static void Meow() { } }

The signature of Cat is different than Dog as Cat defines Cat.Meow(), whereas Dog does not. Dog defines Dog.Bark(), whereas Cat does not. For example, the following code with comments will help you wrap your head around this:

class Program
{
    static void Main(string[] args)
    {
        Dog myDog = new Dog(); // myDog contains definition for Bark
        IAnimal myPet = (IAnimal)myDog; // Cast not necessary.
                                        // myPet's signiature is of Dog, but typeof(myPet)
                                        //   is Animal as it was boxed (google for help on this)
        Cat myCat = (Cat)myPet // now try and imagine this in real life
                               // (turning a dog into a cat) [I don't even]
                               // It doesn't work because Cat doesn't
                               //   contain a definition for Bark()
        Cat myDimentedCat = (Cat)(IAnimal)myDog; // which is why this code fails.
    }
}

---

What I am trying to show is the same thing as a square is a rectangle, but a rectangle isn't always a square:

interface IRectangle { }
interface ISquare : IRectangle { }