xaml - C# Visual Studio Calculator App Crash when using "Enter" Key -

i trying write graphical calculator, far calculator works, want use key "enter" perform operation , give answer. crashes whenever press enter. xaml code:

<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"> 

here code handles enter key event.

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);      } } 

i beginner in c#, xaml , visual studio. right way of handling enter.key event?

this exception thrown:

exception thrown: 'system.formatexception' in mscorlib.ni.dll

change this:

double num2 = convert.todouble(tb.text.substring(iop + 1, tb.text.length - iop - 2)); 

to this:

double num2 = convert.todouble(tb.text.substring(iop + 1, tb.text.length - iop - 1));