TextBox tb = new TextBox();
this.Controls.Add(tb);

private void tb_KeyPress(object sender, KeyPressEventArgs e)
{
    if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) )
    {
        e.Handled = true;
    }
}

tb.KeyPress += tb_KeyPress;

TextBox tb = new TextBox();
tb.KeyPress  += tb_KeyPress; //add this
this.Controls.Add(tb);

private void tb_KeyPress(object sender, KeyPressEventArgs e)
{
   if (!char.IsControl(e.KeyChar) && !char.IsDigit(e.KeyChar) )
   {
    e.Handled = true;
   }
}