Create a directory, but fail if it already exists

Question

The standard .NET Directory.CreateDirectory(string path) method creates a directory (including parent directories, if needed), if they don't already exists, and returns a DirectoryInfo object for the created directory.

And if the directory does already exist, it returns a DirectoryInfo object for the directory that was already there.

I need a method that will create a directory, if it doesn't already exist, but will fail - returning an error or throwing an exception - if the directory already exists.

And no, Directory.Exists(string path) will not serve. I need something that is atomic. I'm interfacing with C++ apps that were designed to the the presence or absence of certain directories as locks, and doing DirectoryExists() followed by Directory.CreateDirectory() leaves a window that can be stepped on.

At the filesystem level, creating a directory is an atomic operation. If two processes both attempt it, only one will succeed. And the API available to C++ programs reflects this. The standard API in .NET hides this. Is there some lower-level API that is available that does not?

AlexD · Accepted Answer · 2018-07-17 19:23:51Z

Consider using native CreateDirectory via Interop:

internal static class NativeMethods { [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)] [return: MarshalAs(UnmanagedType.Bool)] internal static extern bool CreateDirectory(string lpPathName, IntPtr lpSecurityAttributes); }

If it fails because the directory exists, the function returns false and Marshal.GetLastWin32Error should return ERROR_ALREADY_EXISTS (decimal 183 or 0xB7).

ERROR_ALREADY_EXISTS The specified directory already exists.

Please note @stijn's remark however:

Small caveat: CreateDirectory also returns if lpPathName points to an existing file so one might wrongly assume it is a directory.

That works. One thing - if you aren't specifying security attributes, instead of passing null, pass IntPtr.Zero.
Small caveat: CreateDirectory also returns if lpPathName points to an existing file so one might wrongly assume it is a directory.
@stijn Good point, thanks! I incorporated it into the answer.

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