paste cliboard to textbox where keyboard is focused

Assuming this is winforms, or C# has no access to the user clipboard anyway:

void myTimer_Tick(object sender, EventArgs e)
    {
    textBox1.Text = "";
    textBox2.Text = "";
    textBox3.Text = "";
    TextBox tb = ActiveControl as TextBox;
    if (tb != null)
        {
        tb.Text = Clipboard.GetText();
        }
    }

(Obviously, this can't be done from a button click or any other control!)