title: InvalidArgument with SelectionStart when handling backspace
body: <p>Just starting out in C#. I've had some success in creating a simple text editor in Windows Forms, that loads a text file into a Read Only, rich text box. If I choose to edit the file using a "Edit" button, then Form2 pops up and allows me to append or delete characters.
  The problem I am having is if the cursor is at the beginning of the text file and I press the backspace button that I made, I get an exception "InvalidArgument=Value of '-1' is not valid for 'SelectionStart'."
  I have tried many things and after a whole day of trying different things, I'm left to wonder what can be done to resolve this.</p>
  
  <pre><code>        private void button32_Click(object sender, EventArgs e)     //BackSpace button
      {
  
          int index = richTextBox.SelectionStart - 1;
          richTextBox.SelectionStart = index;
          if (richTextBox.Text.Length &lt; 1) MessageBox.Show("  Nothing to Undo  ");
  
              //Otherwise, delete character when button press, (one character at a time).
          else
  
         richTextBox.Text = richTextBox.Text.Remove(richTextBox.SelectionStart - 1, 1);
        richTextBox.Focus();
      }
  </code></pre>
  
  <p>Any help will be much appreciated.
  Thanks.</p>