Why does Path.Combine not properly concatenate filenames that start with Path.DirectorySeparatorChar?

This is kind of a philosophical question (which perhaps only Microsoft can truly answer), since it's doing exactly what the documentation says.

System.IO.Path.Combine

"If path2 contains an absolute path, this method returns path2."

Here's the actual Combine method from the .NET source. You can see that it calls CombineNoChecks, which then calls IsPathRooted on path2 and returns that path if so:

public static String Combine(String path1, String path2) {
    if (path1==null || path2==null)
        throw new ArgumentNullException((path1==null) ? "path1" : "path2");
    Contract.EndContractBlock();
    CheckInvalidPathChars(path1);
    CheckInvalidPathChars(path2);

    return CombineNoChecks(path1, path2);
}

internal static string CombineNoChecks(string path1, string path2)
{
    if (path2.Length == 0)
        return path1;

    if (path1.Length == 0)
        return path2;

    if (IsPathRooted(path2))
        return path2;

    char ch = path1[path1.Length - 1];
    if (ch != DirectorySeparatorChar && ch != AltDirectorySeparatorChar &&
            ch != VolumeSeparatorChar) 
        return path1 + DirectorySeparatorCharAsString + path2;
    return path1 + path2;
}

I don't know what the rationale is. I guess the solution is to strip off (or Trim) DirectorySeparatorChar from the beginning of the second path; maybe write your own Combine method that does that and then calls Path.Combine().

This is the disassembled code from .NET Reflector for Path.Combine method. Check IsPathRooted function. If the second path is rooted (starts with a DirectorySeparatorChar), return second path as it is.

public static string Combine(string path1, string path2)
{
    if ((path1 == null) || (path2 == null))
    {
        throw new ArgumentNullException((path1 == null) ? "path1" : "path2");
    }
    CheckInvalidPathChars(path1);
    CheckInvalidPathChars(path2);
    if (path2.Length == 0)
    {
        return path1;
    }
    if (path1.Length == 0)
    {
        return path2;
    }
    if (IsPathRooted(path2))
    {
        return path2;
    }
    char ch = path1[path1.Length - 1];
    if (((ch != DirectorySeparatorChar) &&
         (ch != AltDirectorySeparatorChar)) &&
         (ch != VolumeSeparatorChar))
    {
        return (path1 + DirectorySeparatorChar + path2);
    }
    return (path1 + path2);
}


public static bool IsPathRooted(string path)
{
    if (path != null)
    {
        CheckInvalidPathChars(path);
        int length = path.Length;
        if (
              (
                  (length >= 1) &&
                  (
                      (path[0] == DirectorySeparatorChar) ||
                      (path[0] == AltDirectorySeparatorChar)
                  )
              )

              ||

              ((length >= 2) &&
              (path[1] == VolumeSeparatorChar))
           )
        {
            return true;
        }
    }
    return false;
}

In my opinion this is a bug. The problem is that there are two different types of "absolute" paths. The path "d:\mydir\myfile.txt" is absolute, the path "\mydir\myfile.txt" is also considered to be "absolute" even though it is missing the drive letter. The correct behavior, in my opinion, would be to prepend the drive letter from the first path when the second path starts with the directory separator (and is not a UNC path). I would recommend writing your own helper wrapper function which has the behavior you desire if you need it.

From MSDN:

If one of the specified paths is a zero-length string, this method returns the other path. If path2 contains an absolute path, this method returns path2.

In your example, path2 is absolute.

Reason:

Your second URL is considered an absolute path, and the Combine method will only return the last path if the last path is an absolute path.

Solution:

Just remove the leading slash / from your second Path (/SecondPath to SecondPath), and it would work as excepted.

Not knowing the actual details, my guess is that it makes an attempt to join like you might join relative URIs. For example:

urljoin('/some/abs/path', '../other') = '/some/abs/other'

This means that when you join a path with a preceding slash, you are actually joining one base to another, in which case the second gets precedence.

I used aggregate function to force paths combine as below:

public class MyPath    
{
    public static string ForceCombine(params string[] paths)
    {
        return paths.Aggregate((x, y) => Path.Combine(x, y.TrimStart('\\')));
    }
}

Remove the starting slash ('\') in the second parameter (path2) of Path.Combine.

As mentiond by Ryan it's doing exactly what the documentation says.

From DOS times, current disk, and current path are distinguished. \ is the root path, but for the CURRENT DISK.

For every "disk" there is a separate "current path". If you change the disk using cd D: you do not change the current path to D:\, but to: "D:\whatever\was\the\last\path\accessed\on\this\disk"...

So, in windows, a literal @"\x" means: "CURRENTDISK:\x". Hence Path.Combine(@"C:\x", @"\y") has as second parameter a root path, not a relative, though not in a known disk... And since it is not known which might be the «current disk», python returns "\\y".

>cd C:
>cd \mydironC\apath
>cd D:
>cd \mydironD\bpath
>cd C:
>cd
>C:\mydironC\apath