Good start. One thing you should make a habit in C# is properly naming controls. TextBox1, TexBox2, Button1, etc gets confusing real quick for bigger apps or other people who view your source.

The way controls are usually named in C# is like this;
Texboxes:
- txtSource
- txtTarget

Buttons:
- btnOpen
- btnConvert

Listboxes etc:
- lstWhatever

Naming controls like this makes it instantly obvious what type of control it is and what its purpose is. And it causes the VS IntelliSense to group them nicely together by type.