private void button1_Click(object sender, System.EventArgs e)
{
    this.textBox1_MouseDown(new object(),new MouseEventArgs(MouseButtons.Left,1,0,0,0));
} private void textBox1_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
{
if(e.Button==System.Windows.Forms.MouseButtons.Left)
{
this.textBox1.Text="left";
}
else
{
    this.textBox1.Text="right";
}
}