Question

I am attempting to validate a date text box to make sure the correct date format was entered.

I converted this vb6 code:

If (IsDate(txtBirthDate)) And (IsNumeric(Right(txtBirthDate, 4))))

into this C# code –

int output = 1;
DateTime output2;

if ((! DateTime.TryParse(txtBirthDate.Text, out output2)) & (!int.TryParse((txtBirthDate.Text.Substring(txtBirthDate.Text.Length - 5)), out output))) 
{
    MessageBox.Show("error")
}

What I am attempting to do is make sure that the last 4 digits of the date text box are numeric (the year – i.e 1990 in 05/10/1990) and if it is not a number, then show error. Although I cannot verify everything is numeric due to the “/” in the date format.

The code does not show an error and builds. But when I debug the application I receive an error. The error states:

Index and length must refer to a location within the string.
Parameter name: length.

Any ideas on how to accomplish this?

Answer 1

Edit: It sounds like the cause of your error is your string is too short. Test the string length before you test the last 4 characters.

Three other issues:

1. The And operator in C# is &&. You are using & which is a bitwise operator.
2. To check the last four characters of the string, you should use .Length - 4, not 5.
3. You are negating the return values in your C#, but not in your VB. To match the VB, omit the !. But, it looks like that’s not what you are actually trying to do. It looks like you want to show an error message if either the string is not parsable as a date or the year portion is not 4 digits. In that case use a an or comparison (||):

if (!DateTime.TryParse(txtBirthDate.Text, out output2) ||
    txtBirthDate.Text.Length < 4 ||
    !int.TryParse((txtBirthDate.Text.Substring(txtBirthDate.Text.Length - 4)), out output))
{
    MessageBox.Show("error")
}

Keep in mind that yyyy/M/d is also parsable as a Date, contains a 4-digit year, but will fail your test. Is that what you want?