Question

This is a bit wierd. Parsing a text field with a valid timespan fails if I try to be precise!

const string tmp = "17:23:24";
//works
var t1 = TimeSpan.Parse(tmp);
//fails
var t2 = TimeSpan.ParseExact(tmp, "hh:mm:ss", System.Globalization.CultureInfo.InvariantCulture);

The second parse fails with an exception “Input string was not in a correct format.” from DateTime.

Answer 1

From the documentation:

Any other unescaped character in a format string, including a
white-space character, is interpreted as a custom format specifier. In
most cases, the presence of any other unescaped character results in a
FormatException.

There are two ways to include a literal character in a format string:

• Enclose it in single quotation marks (the literal string delimiter).

• Precede it with a backslash (“\”), which is interpreted as an escape character. This means that, in C#, the format string must
  either be @-quoted, or the literal character must be preceded by an
  additional backslash.

The .NET Framework does not define a grammar for separators in time
intervals. This means that the separators between days and hours,
hours and minutes, minutes and seconds, and seconds and fractions of a
second must all be treated as character literals in a format string.

So, the solution is to specify the format string as

TimeSpan.ParseExact(tmp, "hh\\:mm\\:ss", CultureInfo.InvariantCulture)