How can I extend the richtextbox control using system namespace only

Question

Is there a way to extend the richtextbox control in c# using an equivalent method like below :

namespace System
{
    public static class StringExtensions
    {
        public static string PadBoth(this string str, int length)
        {
            int spaces = length - str.Length;
            int padLeft = spaces / 2 + str.Length;
            return str.PadLeft(padLeft).PadRight(length);
        }
    }
}

So something like :

namespace System.Windows.Controls
{
    public static class RichTextBoxExtensions
    {
        public static string MyCustomMethod()
        {
             return "It works!";
        }
    }
}

I know how to extend it using the old way by creating a class and inheriting the richtextbox object, however what I would prefer to do is the reverse as the above adds the functionality to the base RichTextBox object without the need to create a new custom usercontrol to extend it's functionality.

To be clear, I am not looking to do the following (or similar) :

public class Foo : RichTextBox { }

I am not sure what this method of extending is called or if it even has a specific name / classification, but it just feels more natural when objects are extended in this manner than creating new controls filling up an already bloated toolbar of hundreds of controls.

Answer 1

What you want is called an extension method , for instance, this method will extend the RichTextBox:

public static class RichTextBoxExtensions
{
    public static void MyCustomMethod(this RichTextBox self)
    {
        MessageBox.Show("It works, this textbox has " + self.Text + " as the text!");
    }
}

Answer 2

Just do like your string extension, but use a RichTextBox as the first argument:

public static string MyCustomMethod(this RichTextBox richTextBox)
{
    return richTextBox.Text;
}

Also you don't need to have the same namespace as the control, you can use your own/your project's namespace without a problem.