Formatting nullable DateTime

October 25, 2024

This is my function:

private static string strTimeStampMsg(string strMsg, DateTime? dtTimestamp = null) {
    if (strMsg == null || strMsg.Length == 0) {
        return string.Empty;
    }
    string strDateTime;
    if (dtTimestamp != null) {
        strDateTime = dtTimestamp.ToString(CSTR.dtFormat);
    } else {
        strDateTime = DateTime.Now.ToString(CSTR.dtFormat);
    }
    return strDateTime + "> " + strMsg;
}

CSTR.dtFormat is defined in a DLL:

public const string dtFormat = "dd-MM-yyyy HH:mm:ss.fff";

Today I’ve added the optional, the problem is the line:

strDateTime = dtTimestamp.ToString(CSTR.dtFormat);

Has a red underline under ToString, why and how do I resolve whilst being able to format using the required format?

>Solution :

Has a red underline under ToString, why and how do I resolve whilst being able to format using the required format?

Well, you could use dtTimestamp.Value.ToString(...) so that you’re trying to call DateTime.ToString instead of Nullable<DateTime>.ToString().

Note that this isn’t really about the value coming from an optional parameter – it’s about it being a Nullable<DateTime> rather than a DateTime. If you’d obtained that Nullable<DateTime> value from anywhere else, without using optional parameters, you’d still face the same issue.

But there’s a better solution which avoid code duplication:

string strDateTime = (dtTimestamp ?? DateTime.Now).ToString(CSTR.dtFormat);

That uses the null-coalescing operator – ?? – to effectively use DateTime.Now as a default.

I view this as cleaner code than, because it makes it clear that the only difference between explicitly providing the DateTime value and using the default value is in which part is formatted – you do the same operation with the result either way. In your current code, both branches perform an operation on the chosen DateTime, and while they’re the same right now it would be easy for them to end up inconsistent.