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

Question

From the Immediate Window in Visual Studio     gt  Path Combine   C  x    y    C   x  y   gt  Path Combine   C  x      y      y    It seems that they should both be the same     The old FileSystemObject BuildPath   didn t work this way

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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     gt cd C   gt cd  mydironC apath  gt cd D   gt cd  mydironD bpath  gt cd C   gt cd  gt C  mydironC apath

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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    gt  Path Combine x  y TrimStart

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These two methods should save you from accidentally joining two strings that both have the delimiter in them       public static string Combine string x  string y  char delimiter            return     x TrimEnd delimiter     delimiter    y TrimStart delimiter                 public static string Combine string   xs  char delimiter            if  xs Length  lt  1  return string Empty          if  xs Length    1  return xs 0           var x   Combine xs 0   xs 1   delimiter           if  xs Length    2  return x          var ys   new List lt string gt             ys Add x           ys AddRange xs Skip 2  ToList             return Combine ys ToArray    delimiter

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Following Christian Graus  advice in his  Things I Hate about Microsoft  blog titled  Path Combine is essentially useless    here is my solution   public static class Pathy       public static string Combine string path1  string path2                if  path1    null  return path2         else if  path2    null  return path1         else return path1 Trim   TrimEnd System IO Path DirectorySeparatorChar               System IO Path DirectorySeparatorChar              path2 Trim   TrimStart System IO Path DirectorySeparatorChar              public static string Combine string path1  string path2  string path3                return Combine Combine path1  path2   path3             Some advise that the namespaces should collide       I went with Pathy  as a slight  and to avoid namespace collision with System IO Path   Edit  Added null parameter checks

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I wanted to solve this problem   string sample1    configuration config xml   string sample2     configuration config xml   string sample3      configuration config xml    string dir1    c   temp   string dir2    c   temp     string dir3    c   temp     string path1   PathCombine dir1  sample1   string path2   PathCombine dir1  sample2   string path3   PathCombine dir1  sample3    string path4   PathCombine dir2  sample1   string path5   PathCombine dir2  sample2   string path6   PathCombine dir2  sample3    string path7   PathCombine dir3  sample1   string path8   PathCombine dir3  sample2   string path9   PathCombine dir3  sample3     Of course  all paths 1-9 should contain an equivalent string in the end  Here is the PathCombine method I came up with   private string PathCombine string path1  string path2        if  Path IsPathRooted path2                 path2   path2 TrimStart Path DirectorySeparatorChar           path2   path2 TrimStart Path AltDirectorySeparatorChar              return Path Combine path1  path2       I also think that it is quite annoying that this string handling has to be done manually  and I d be interested in the reason behind this

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This code should do the trick                       string strFinalPath   string Empty          string normalizedFirstPath   Path1 TrimEnd new char                      string normalizedSecondPath   Path2 TrimStart new char                      strFinalPath    Path Combine normalizedFirstPath  normalizedSecondPath           return strFinalPath

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This   means  the root directory of the current drive   In your example it means the  test  folder in the current drive s root directory  So  this can be equal to  c  test

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Reason   Your second URL is considered an absolute path  The Combine method will only return the last path if the last path is an absolute path   Solution  Just remove the starting slash   of your second Path   SecondPath to SecondPath   Then it works as you excepted

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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

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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

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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   amp  amp            ch    AltDirectorySeparatorChar    amp  amp            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  gt   1   amp  amp                                             path 0     DirectorySeparatorChar                            path 0     AltDirectorySeparatorChar                                                                         length  gt   2   amp  amp                 path 1     VolumeSeparatorChar                                      return true                      return false

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This actually makes sense  in some way  considering how  relative  paths are treated usually   string GetFullPath string path         string baseDir     C  Users Foo Bar        return Path Combine baseDir  path         Get full path for RELATIVE file path GetFullPath  file txt         C  Users Foo Bar file txt     Get full path for ROOTED file path GetFullPath   C  Temp file txt         C  Temp file txt   The real question is  Why are paths  which start with      considered  rooted   This was new to me too  but it works that way on Windows   new FileInfo   windows       FullName   C  Windows  Exists   True new FileInfo  windows       FullName   C  Users Foo Bar Windows  Exists   False

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If you want to combine both paths without losing any path you can use this    Path Combine   C  test      test  Substring 0  1               test  Substring 1     test  Length - 1       test      Or with variables   string Path1     C  Test   string Path2      test   string FullPath   Path Combine Path1  Path2 IsRooted     Path2 Substring 1  Path2 Length - 1    Path2     Both cases return  C  test test    First  I evaluate if Path2 starts with   and if it is true  return Path2 without the first character  Otherwise  return the full Path2

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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  amp  amp  ch    AltDirectorySeparatorChar  amp  amp              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

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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

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Remove the starting slash       in the second parameter  path2  of Path Combine

[c#] Why does Path.Combine not properly concatenate filenames that start with Path.DirectorySeparatorChar?

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