Why does DateTime.ToString("h") cause exception?

Question

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new DateTime(2010,01,01).ToString("h")

Gives:

new DateTime(2010,01,01).ToString("h") threw an exception of type System.FormatException

Yet...

new DateTime(2010,01,01).ToString("h ")

Gives: "12 "

So why does an extra space stop this format exception from happening? Is this a bug?

Answer 1

It's because it thinks it's a standard format string instead of a custom one, due to being a single character.

A better way of fixing this is to use %:

string text = DateTime.Now.ToString("%h");

From the docs on custom format strings:

A custom date and time format string consists of two or more characters. Date and time formatting methods interpret any single-character string as a standard date and time format string. If they do not recognize the character as a valid format specifier, they throw a FormatException. For example, a format string that consists only of the specifier "h" is interpreted as a standard date and time format string. However, in this particular case, an exception is thrown because there is no "h" standard date and timeformat specifier.

To use any of the custom date and time format specifiers as the only specifier in a format string (that is, to use the "d", "f", "F", "g", "h", "H", "K", "m", "M", "s", "t", "y", "z", ":", or "/" custom format specifier by itself), include a space before or after the specifier, or include a percent ("%") format specifier before the single custom date and time specifier.

For example, "%h" is interpreted as a custom date and time format string that displays the hour represented by the current date and time value. You can also use the " h" or "h " format string, although this includes a space in the result string along with the hour. The following example illustrates these three format strings.