Hi mahesh213,

You need to check the enpty value inside IsLetter and IsDate method.

If the value is empty then return false else proceed for further validation checking.

private bool IsLetter(string value)
{
    Regex regexLetter = new Regex(@"^[a-zA-Z]+$");
    return regexLetter.IsMatch(value);
}
 
private bool IsDate(string value)
{
    Regex regex = new Regex(@"(((0|1)[0-9]|2[0-9]|3[0-1])\/(0[1-9]|1[0-2])\/((19|20)\d\d))$");
    //Verify whether date entered in dd/MM/yyyy format.
    bool isValid = regex.IsMatch(value);
    //Verify whether entered date is Valid date.
    DateTime dt;
    isValid = DateTime.TryParseExact(value, "dd/MM/yyyy", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);
    return isValid;
}

Change above code with below.

private bool IsLetter(string value)
{
    bool isValid = false;
    if (!string.IsNullOrEmpty(value))
    {
        Regex regexLetter = new Regex(@"^[a-zA-Z]+$");
        isValid = regexLetter.IsMatch(value);
    }

    return isValid;
}

private bool IsDate(string value)
{
    bool isValid = false;
    if (!string.IsNullOrEmpty(value))
    {
        Regex regex = new Regex(@"(((0|1)[0-9]|2[0-9]|3[0-1])\/(0[1-9]|1[0-2])\/((19|20)\d\d))$");
        //Verify whether date entered in dd/MM/yyyy format.
        isValid = regex.IsMatch(value);
        //Verify whether entered date is Valid date.
        DateTime dt;
        isValid = DateTime.TryParseExact(value, "dd/MM/yyyy", new CultureInfo("en-GB"), DateTimeStyles.None, out dt);
    }

    return isValid;
}