private void CustomTextSourceSample_Shown(object sender, EventArgs e) { var s = CreateExtraLargeString(); MessageBox.Show("Extralarge string is created.\nPress OK to assign string to the FastColoredTextbox"); //create our custom TextSource var ts = new StringTextSource(fctb); //open source string ts.OpenString(s); //assign TextSource to the component fctb.TextSource = ts; }