CustomValidator to validate multiple date formats

Question

I'm using a CustomValidator field in an Asp.net Webform to validate a date field. How can I validate multiple date formats? For example, this would be valid: 01/05/2019 and this would be valid: 1/05/2019 and this would be valid: 2019/05/01. Here is the code behind that doesn't work when trying to validate all three date types but does work when using only one date format:

protected void CustomValidator1_ServerValidate(object source, ServerValidateEventArgs args)
{
    //Issue Date
    if (Regex.IsMatch(txtIssueDate.Text, "(((0|1)[0-9]|2[0-9]|3[0-1])\\/(0[1-9]|1[0-2])\\/((19|20)\\d\\d))$"))
    {
        DateTime dt;
        args.IsValid = DateTime.TryParseExact(args.Value, "dd/MM/yyyy", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);
        args.IsValid = DateTime.TryParseExact(args.Value, "d/MM/yyyy", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);
        args.IsValid = DateTime.TryParseExact(args.Value, "yyyy/MM/dd", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);
        if (args.IsValid)
        {
            args.IsValid = true;
        }
    }
    else
    {
        args.IsValid = false;
        ScriptManager.RegisterStartupScript(UpdatePanel2, UpdatePanel2.GetType(), "myFunction", "alertInvalidDate();", true);
    }
}

EDIT I took Sach's advice and used this method to check the date fields - forgetting the Custom Validator altogether:

var formats = new string[] { "dd/MM/yyyy", "d/MM/yyyy", "yyyy/MM/dd" };            

var isValidFormat = DateTime.TryParseExact(txtIssueDate.Text, formats, CultureInfo.CurrentCulture, DateTimeStyles.None, out DateTime res1);
if (isValidFormat)
{
    //It's all good.
}
else
{
    ScriptManager.RegisterStartupScript(UpdatePanel2, UpdatePanel2.GetType(), "myFunction", "alertInvalidDate();", true);
}

Answer 1

You just want to use TryParseExact() with multiple formats.

static void Main()
{
    var formats = new string[] { "dd/MM/yyyy", "d/MM/yyyy", "yyyy/MM/dd" };

    DateTime.TryParseExact("01/05/2019", formats, CultureInfo.CurrentCulture, DateTimeStyles.None, out DateTime res1);
    Console.WriteLine(res1.ToLongDateString());
    DateTime.TryParseExact("1/05/2019", formats, CultureInfo.CurrentCulture, DateTimeStyles.None, out DateTime res2);
    Console.WriteLine(res2.ToLongDateString());
    DateTime.TryParseExact("2019/05/01", formats, CultureInfo.CurrentCulture, DateTimeStyles.None, out DateTime res3);
    Console.WriteLine(res3.ToLongDateString());
    Console.ReadLine();
}

Output:

Wednesday, May 01, 2019

Wednesday, May 01, 2019

Wednesday, May 01, 2019

Answer 2

Your code doesn't work because it takes into account the result of the last TryParseExact line ONLY :

args.IsValid = DateTime.TryParseExact(args.Value, "yyyy/MM/dd", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);

Previous results are lost.

Also, this piece is completely meaninglessness ( I hope you understand why ):

if (args.IsValid)
{
    args.IsValid = true;
}

Try this instead:

if (Regex.IsMatch(txtIssueDate.Text, "(((0|1)[0-9]|2[0-9]|3[0-1])\\/(0[1-9]|1[0-2])\\/((19|20)\\d\\d))$"))
{
        DateTime dt;
        args.IsValid = (DateTime.TryParseExact(args.Value, 
                            "dd/MM/yyyy", 
                            new CultureInfo("en-GB"), 
                            DateTimeStyles.None, 
                            out dt) || 
                        DateTime.TryParseExact(args.Value, 
                            "d/MM/yyyy", 
                            new CultureInfo("en-GB"), 
                            DateTimeStyles.None, 
                            out dt) ||
                        DateTime.TryParseExact(args.Value, 
                            "yyyy/MM/dd", 
                            new CultureInfo("en-GB"), 
                            DateTimeStyles.None, 
                            out dt));
}
else
{
    args.IsValid = false;
    ScriptManager.RegisterStartupScript(UpdatePanel2, UpdatePanel2.GetType(), "myFunction", "alertInvalidDate();", true);
}