Question

Path.GetFullPath(path); works fine. But Directory.CreateDirectory(path); throws a path too long exception. Can it be that the backslashes are counted differently for the two methods?

Answer 1

In ILSpy it seems that GetFullPath uses the private const MaxDirectoryLength(255) whereas CreateDirectory uses 248.

Path.GetFullPath -> GetFullPathInternal -> NormalizePath

// System.IO.Path
private static readonly int MaxDirectoryLength = 255;

// ...
if (num8 - num5 > Path.MaxDirectoryLength)
{
    throw new PathTooLongException(Environment.GetResourceString("IO.PathTooLong"));
}
// ...

Directory.CreateDirectory -> InternalCreateDirectory (btw, NormalizePath is also called before InternalCreateDirectory)

// ...
string text2 = list[list.Count - 1];
list.RemoveAt(list.Count - 1);
if (text2.Length >= 248)
{
    throw new PathTooLongException(Environment.GetResourceString("IO.PathTooLong"));
}
// ...

So it seems that a folder name cannot be longer than 248 chars whereas the full path (incl. each subfolder) can be longer.