c# - How to aill cause it to ces when I try to d -

my program crashes when try decimal calculation starting decimal point. example, ".9" cause crash whereas "0.9" works fine.

here's code:

    private void textbox1_keypress(object sender, keypresseventargs e)     {         if (!char.iscontrol(e.keychar)              && !char.isdigit(e.keychar)              && (e.keychar != '.'))         {             e.handled = true;         }          if ((e.keychar == '.') && ((sender textbox).text.indexof('.') > -1))         {             e.handled = true;         }         if (e.keychar == convert.tochar(keys.return))         {             textbox2.text = area1.tostring();         }     }     private void textbox1_textchanged(object sender, eventargs e)     {         if (textbox1.text == string.empty)         {             textbox2.text = " ";         }         else             diameter1 = double.parse(textbox1.text);         area1 = (math.pi * math.pow((diameter1 / 2), 2));     } 

as updating value while typing, fail when have typed period , textbox contains ".". when parse that, cause exception.

use tryparse attempt parse value, , else if parsing fails. example:

private void textbox1_textchanged(object sender, eventargs e) {   if (textbox1.text == string.empty) {     textbox2.text = " ";   } else {     if (double.tryparse(textbox1.text, out diameter1)) {       area1 = math.pi * math.pow((diameter1 / 2), 2);       textbox2.text = area1.tostring();     } else {       textbox2.text = "-not numeric-";     }   } }