You can simply subscribe the same event delegate method for all the textBoxes.
this.textBox1.MouseDown += new System.Windows.Forms.MouseEventHandler(this.textBox_MouseDown);
this.textBox1.MouseDown += new System.Windows.Forms.MouseEventHandler(this.textBox_MouseDown);
private void textBox_MouseDown(object sender, MouseEventArgs e)
{
((TextBox)sender).Clear()
}