Select text in VS 2012

Hi, how do I select text in a text box between point A to Z and then continue with the loop

eg:

TextBox1.Focus()
TextBox1.Selectionstart(at point A)
TextBox1.SelectionLength(ends at point

Then

TextBox1.SelectedText = "Hello"

But when there is more than 1 point A and Z (the thing I can't get right) is when This happens and doesn't continues to the next point A

A hi my name is ### Z sdljglsgsjlk A hello guys Z

So "hi my name is ###" should then (with "hello guys") turn to Hello
So it must be "Hello sdljglsgsjlk Hello"