Question

I am using a ValueConverter to display double types as currency. I want to make sure the entry is valid, so I tried using this validation method I made up. My only problem seems to be that I can’t select the whole value (ie $500.00) and type anything. If the ‘$’ is not selected, everything seems fine. How can I fix this?

XAML:

<TextBox Text="{Binding Converter={StaticResource MoneyConverter}, Path=fr}"
  PreviewTextInput="ValidateMoney" DataObject.Pasting="TextBox_Pasting"/>

Code behind:

private void ValidateMoney(object sender, TextCompositionEventArgs e) {
  Regex rgx = new Regex(@"^(\$)[0-9]*[.]{0,1}[0-9]*$|^[.][0-9]+$|^[0-9]*[.]{0,1}[0-9]*$");
  e.Handled = !rgx.IsMatch((sender as TextBox).Text.Insert((sender as TextBox).SelectionStart, e.Text));
}
public class MoneyConverter : IValueConverter {
  public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
    if (value != null) {
      double d;
      string temp = value.ToString();
      if (double.TryParse(temp, out d)) {
        return String.Format("{0:C}", d);
      } else {
        return value;
      }
    } else {
      return value;
    }
  }
  public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
    if (value != null) {
      double d;
      string temp = value.ToString();
      if (double.TryParse(temp, out d)) {
        return String.Format("{0:C}", d);
      } else {
        return value;
      }
    } else {
      return value;
    }
  }
}

Answer 1

Your approach to including the dollar sign is correct, though the parentheses are unnecessary. It is sufficient to write this: ^\$.

You can simplify your regular expression a lot:

• You don’t need three separate cases in your alternation. There is some unnecessary repetition there.
• {0,1} can be replaced with ?.
• You probably want to ensure that there is at least one digit.

Try this:

@"^\$?(?:[0-9]+(?:\.[0-9]+)?|\.[0-9]+)$"

Or using capturing groups to reduce clutter:

@"^\$?([0-9]+(\.[0-9]+)?|\.[0-9]+)$"

Some other points:

• You should be aware that TryParse is affected by the current culture. 1.990 means different things depending on your culture.
• ToString also depends on culture. The "{0:C}" format doesn’t guarantee a dollar sign.
• Your regular expression allows things like $.990. That might be what you want, but it looks a little odd to me.
• You should probably use decimal rather than double to store monetary amounts. The double type can’t store 1.99 exactly, but decimal can.
• I replaced [.] with the equivalent \. but that is just a matter of taste. Both do the same thing.