Responding to Invalid TextBox Entries

Question

Users enter numbers into textboxes, and the values are displayed on a graph. I want to be sure users type numbers in the textboxes, so I tried:

double xValue;
double yValue;

Double.TryParse(xTextBox.Text, out xValue);
Double.TryParse(yTextBox.Text, out yValue);

chart1.Series["data1"].Points.AddXY(xValue, yValue);

Double.TryParse() validates whether users type numbers, but I'm not sure how to code for when users don't type numbers. Double.TryParse() will assign xValue or yValue a 0--but I also want users to be able to enter 0 values so I cannot just code something like

if (xValue!=0 && yValue!=0)...

Something like

if (xTextBox.Text!="0") {
  Double.TryParse(xTextBox.Text, out xValue);
  }

seems like awkward code that will just get more. What's the standard way to make sure users entered a double?

Answer 1

TryParse returns a boolean which is true if the conversion succeeded, and false if it failed.

if (!Double.TryParse(xTextBox.Text, out xValue)) {
     // handle a failure to convert here
}

Answer 2

Double.TryParse returns a boolean if parsing was a success, and you should check the results of Double.TryParse not the xValue and yValue.

Answer 3

First of all you need to take into consideration the user locale settings. For example in USA the decimal separator is . and in my country it is ,. So you need to specify the format when user enters double values or replace , with . and then parse values with Invariant culture.

Second Double.TryParse method returns bool value which you can use to check whether the entered value is successfully parsed as double.