<TextBox Height="38" TextWrapping="Wrap" VerticalAlignment="Top"
 Margin="16,23,0,0" HorizontalAlignment="Left" Width="226"
 FontSize="20" Background="Black" BorderThickness="2" Name="tb"
 Text="" KeyUp="KeyDownHandler">

private void KeyDownHandler(object sender, KeyRoutedEventArgs e)
{
     if (e.Key == Windows.System.VirtualKey.Enter)
     {
          result();
     }
}

private void result()
{
     String op;
     int iOp = 0;
     if (tb.Text.Contains("+"))
     {
         iOp = tb.Text.IndexOf("+");
     }
     else if (tb.Text.Contains("-"))
     {
         iOp = tb.Text.IndexOf("-");
     }
     else if (tb.Text.Contains("*"))
     {
         iOp = tb.Text.IndexOf("*");
     }
     else if (tb.Text.Contains("/"))
     {
         iOp = tb.Text.IndexOf("/");
     }
     else
     {
         //error
     }

     op = tb.Text.Substring(iOp, 1);
     double num1 = Convert.ToDouble(tb.Text.Substring(0, iOp));
     double num2 = Convert.ToDouble(tb.Text.Substring(iOp + 1, tb.Text.Length - iOp - 2));

     if (op == "+")
     {
         tb.Text = Convert.ToString(num1 + num2);
     }
     else if (op == "-")
     {
         tb.Text = Convert.ToString(num1 - num2);
     }
     else if (op == "*")
     {
         tb.Text = Convert.ToString(num1 * num2);
     }
     else if (op == "/")
     {
         tb.Text = Convert.ToString(num1 / num2);
     }
}

double num2 = Convert.ToDouble(tb.Text.Substring(iOp + 1, tb.Text.Length - iOp - 2));

double num2 = Convert.ToDouble(tb.Text.Substring(iOp + 1, tb.Text.Length - iOp - 1));