private void toolBar1_ButtonClick(object sender, System.Windows.Forms.ToolBarButtonClickEventArgs e) {
switch(toolBar1.Buttons.IndexOf(e.Button)) {
case 0:
richTextBox1.SelectionFont= new Font(richTextBox1.SelectionFont, (richTextBox1.SelectionFont.Style| System.Drawing.FontStyle.Bold));
break;
case 1:
richTextBox1.SelectionFont= new Font(richTextBox1.SelectionFont, (richTextBox1.SelectionFont.Style| System.Drawing.FontStyle.Italic));
break;
case 2:
richTextBox1.SelectionFont= new Font(richTextBox1.SelectionFont, (richTextBox1.SelectionFont.Style| System.Drawing.FontStyle.Underline));
break;
}
}

private void richTextBox1_SelectionChanged(object sender, System.EventArgs e) {
if (richTextBox1.SelectionFont.Bold) {
toolBarButton1.Pushed=true;
}
else {
toolBarButton1.Pushed=false;
}
if (richTextBox1.SelectionFont.Italic) {
toolBarButton2.Pushed=true;
}
else {
toolBarButton2.Pushed=false;
}
if (richTextBox1.SelectionFont.Underline) {
toolBarButton3.Pushed=true;
}
else {
toolBarButton3.Pushed=false;
}