Why is System.IO implemented this way

Question

Took me a while to find this bug in my code. Trying to create a temporary test directory:

Path.Combine("C:", "test"); 
Directory.CreateDirectory(path);

So this doesn't create the directory C:\test. It seems to just create the directory test in the local folder of the executing assembly.

This is because Path.Combine("C:", "test"); returns C:test, and not C:\test.

I don't pretend to know much about file systems so maybe the reason for this should be obvious, but I don't get it. I'm not sure why they implemented Path that way, or why CreateDirectory doesn't complain about an invalid directory name.

Answer 1

Well, you're combining a drive letter and a name. Under Windows that means "the name under the current directory of that drive". So for example:

C:\> cd foo
c:\foo> d:
d:\> mkdir c:bar

that would create c:\foo\bar. So it seems consistent with the design of Windows (harking back to MS-DOS) to me - that isn't necessarily a sensible design for Windows, but it makes sense for .NET to be consistent with it.

Answer 2

Path.Combine behaves this way, because : is a valid VolumeSeparatorChar - so a \ doesn't get appended to C:.

From MSDN - Path.Combine Method (String, String):

If path1 is not a drive reference (that is, "C:" or "D:") and does not end with a valid separator character as defined in DirectorySeparatorChar, AltDirectorySeparatorChar, or VolumeSeparatorChar, DirectorySeparatorChar is appended to path1 before concatenation.

As for the behavior of CreateDirectory - see the answer by Jon Skeet.