DateTime格式字符串（无效的格式字符串）

Question

I am trying to format Datetime string using

date.ToString(format)

If user feeds in wrong format, eg "YYYY MM DDr" I would like to know whether I can convert datetime using that format, rather than returning

2015 04 DDr

since

DateTime.ToString(format)

always returns a valid String.

For example, is there any method that perhaps throw an exception on failed conversion so that I can catch and decide not display my output string instead of displaying something like

2015 04 DDr

Answer 1

If you assume that all the letters inserted in your format are either separators or letters that should be converted to a DatePart, you can check if after converting the date you still have non separator chars that were not converted, as follows:

public static class DateTimeExtension
    {
        public static string ToStringExt(this DateTime p_Date, String format)
        {
            char[] separators = { ' ', '/', '-' };

            String stringDate = p_Date.ToString(format);

            foreach (char dateChar in format)
            {
                if (stringDate.Contains(dateChar) && !separators.Contains(dateChar))
                {
                    throw new FormatException("Format Error");
                }
            }
            return stringDate;
        }
    }

Edited after @Vladimir Mezentsev observation:

This code assumes that you are converting only to Numbers, if you are doing something that will convert to Day strings like Tuesday, the logic may fail. To address this scenario the code would get a little more complicated but can also be achieved with something like this:

public static string ToStringExt(this DateTime p_Date, String format)
{
    foreach (string dateFormatPart in getFormatStrings(format))
    {
        if (p_Date.ToString(dateFormatPart) == dateFormatPart)
        {
            throw new FormatException("Format Error");
        }
    }
    return p_Date.ToString(format);
}

private static IEnumerable<string> getFormatStrings(String format)
{
    char[] separators = { ' ', '/', '-' };
    StringBuilder builder = new StringBuilder();

    char previous = format[0];
    foreach (char c in format)
    {
        if (separators.Contains(c) || c != previous)
        {
            string formatPart = builder.ToString();
            if (!String.IsNullOrEmpty(formatPart))
            {
                yield return formatPart;
                builder.Clear();
            }
        }
        if(!separators.Contains(c))
        {
            builder.Append(c);
        }
        previous = c;
    }
    if (builder.Length > 0)
        yield return builder.ToString();
}

Answer 2

Have a look at https://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx . Particularly this part...

If you're wanting to validate the string used to format the DateTime object, then you'll probably have to write your own using the link provided to know what formats are acceptable, and treat any other characters as errors.

Answer 3

There is no invalid format, because you can parse formatted string with exact same format. Even if after parsing you have not loss in any part of date, that could not form the final decision - valid or invalid format.

You should carefully consider what can be appropriate for user, even give him an opportunity to construct format from some predefined blocks. Maybe show sample conversion with confirmation.

For specific formats you can create some extension method where you can apply your business rules and throw exceptions when you need it.