Find & Highlight Text? | Text Editor

Hello Programmers.

I’m making my first C# WinForms Application and it’s a Text Editor so I can practice some code.

I am trying to do something to find and highlight with another color so it distinguishes (highlights). I think you got it.

Here’s the code of that part:

This is the error (same for textbox1)

If you need the full source to check something just ask.
If you know how I can fix this that would be very helpful,

Cheers.

C# is case sensitive. RichTextBox1 is not the same as richtextbox1 and TextBox1 is not the same as textbox1