C#

private void btnBold_Click(object sender, System.EventArgs e)
{
	Font oldFont;
	Font newFont;
				
	// Get the font that is being used in the selected text
	oldFont = this.rtfText.SelectionFont;

	// If the font is using bold style at the moment, we should remove it
	if (oldFont.Bold)
	       newFont = new Font(oldFont, oldFont.Style & ~FontStyle.Bold);
	else
        	newFont = new Font(oldFont, oldFont.Style | FontStyle.Bold);

	// Insert the new font
	this.rtfText.SelectionFont = newFont;
	this.rtfText.Focus();
}