C#四舍五入

double txtnum = Convert.ToDouble(this.textBox1.Text);
if (txtnum < 0)
{
     txtnum = Math.Round(txtnum + 5 / Math.Pow(10, 3), 2, MidpointRounding.AwayFromZero);
}
else
{
     txtnum = Math.Round(Convert.ToDouble(txtnum), 2, MidpointRounding.AwayFromZero);
}
MessageBox.Show(txtnum.ToString());