C# Signature:

[DllImport("kernel32.dll")]
[return: MarshalAs(UnmanagedType.Bool)]
static extern bool CreateDirectory(string lpPathName,
   IntPtr lpSecurityAttributes);

User-Defined Types:

None.

Notes:

Advantage - Doesn't require the running assembly to have permissions to the entire folder path.

Disadvantage - Only creates the end-most directory, not the full path.

Tips & Tricks:

Iteratively Create a Directory Tree

I updated the code in the Microsoft.Esxperimental.IO project (http://bcl.codeplex.com/releases/view/42783) so some of the classes referenced here (NativeMethods, LongPathCommon) are taken from that project.

    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);
    }

    public static void Create(string path)
    {
        if (String.IsNullOrEmpty(path))
        {
        throw new ArgumentException("Path cannot be an empty string.");
        }

        int startPos = 1;   // First part of the path to create.

        // Split the given string into components which will have to be individually created.
        String[] pathComponents = path.Split(new char[] { '\\', '/' }, StringSplitOptions.RemoveEmptyEntries);

        // Get the first path component, either "c:" or "\\servername".
        String partPath = pathComponents[0];

        // If the path is a UNC path, then add the share to the initial starting path.  
        if (partPath.StartsWith(@"\\"))
        {
        if (pathComponents.Length < 3)
        {
            throw new ArgumentException("Invalid path specified: " + path);
        }

        // Add the share name.
        partPath = Path.Combine(partPath, pathComponents[1]);
        startPos = 2;
        }

        // If the path is the root of a drive, then add the directoy separator char.
        if (partPath.Contains(":"))
        {
        if (pathComponents.Length < 2)
        {
            throw new ArgumentException("Invalid path specified: " + path);
        }

        // Add the directory separator char here since Path.Combine will not add it if a drive specifier (ie: "C:") is given.
        // See Remarks in http://msdn.microsoft.com/en-us/library/fyy7a5kt.aspx
        // Also see http://msdn.microsoft.com/en-us/library/aa365247%28VS.85%29.aspx#relative%5Fpaths
        partPath += Path.DirectorySeparatorChar;
        }

        // Create the individual path components.
        for (int i = startPos; i < pathComponents.Length; i++)
        {
        // Add the path component to create.
        partPath = System.IO.Path.Combine(partPath, pathComponents[i]);

        string normalizedPath = LongPathCommon.NormalizeLongPath(partPath, "partPath");

        // Create the path.
        if (!NativeMethods.CreateDirectory(normalizedPath, IntPtr.Zero))
        {
            // To mimic Directory.CreateDirectory, we don't throw if the directory (not a file) already exists
            int errorCode = Marshal.GetLastWin32Error();
            if (errorCode != NativeMethods.ERROR_ALREADY_EXISTS || !LongPathDirectory.Exists(partPath))
            {
            throw LongPathCommon.GetExceptionFromWin32Error(errorCode);
            }
        }
        }
    }

Sample Code:

string path = "c:\\test\\newDir";
CreateDirectory(path, IntPtr.Zero);

Alternative Managed API: