Time and culture

The problem

Yesterday we find a cool bug in code. The code is really simple and was working on 3 of 4 machines. The problematic code is following:

var path = @" E:\tmp\test_" + DateTime.Today.ToShortDateString()+".tmp";

using (var file = new FileStream(path, FileMode.Create, FileAccess.Write))
{
    //something
}

The exception was System.IO.DirectoryNotFoundException in FileStream and we of course checked that E:\tmp\ exists. Moreover the error was only one one machine

The solution

You can run above code on your machine and I am almost sure that if it works correctly. But If you add following line

CultureInfo.DefaultThreadCurrentCulture = new CultureInfo("en-us");

it will throw an error. Why? Because directory: E:\tmp\test_2\13\ does not exist. In polish culture the file E:\tmp\2015-02-13.tmp is created without problems, because it does not contains any \

Thing to remember

Be careful we culture specific date formating, especially when we use them in paths.

The funny thing is that any unit test did not find that issue.