RichTextBox changing letters color - speed problem

Question

My file is 450 503 letters text. I have to change every letter (eg. 'b' - user choice) on another and mark it by set red color. When I do it in that way:

 for(int i=0; i<lenght; ++i) {
    this.rtb.Select(i, 1);
    this.rtb.SelectionColor = Color.Red;
    this.rtb.SelectedText = this.rtb.SelectedText;
    this.rtb.DeselectAll();
 }

It's too slooow - actually it never finished... (17min awaiting). I have no idea how to speed it up.

Answer 1

It's because you are forcing it to to redraw everytime it happens.

Wrap this in a SuspendLayout and ResumeLayout

Answer 2

Try calling rtb.SuspendLayout(); before running your logic and rtb.ResumeLayout(); afterwards. Like this:

rtb.SuspendLayout();

for(int i=0; i<lenght; ++i) {
    this.rtb.Select(i, 1);
    this.rtb.SelectionColor = Color.Red;
    // you shouldn't need these lines:
    // this.rtb.SelectedText = this.rtb.SelectedText;
    // this.rtb.DeselectAll();
 }

rtb.ResumeLayout();

Optimization aside you do at some point have to check whether the selected letter is the one you want. The current loop will attempt to color every single letter red.