Delphi Tpath.Combine('c:', 'myfile.txt') leaves out DirSeperator - delphi

Question

So when I run
TPath.Combine('c:', 'myfile.txt');
in Delphi XE2 then I get 'C:myfile.txt' in return. This is not what I expect and it is not a valid path in windows. I would expect TPath.Combine to be a call to the windows API ( http://msdn.microsoft.com/en-us/library/fyy7a5kt%28v=vs.110%29.aspx ) or to have the same behaviour as the API.
Is there something I'm doing wrong? Can I "fix" the behaviour of TPath.Combine? Or do I have to search all uses in my code and replace it with a string concatenation with a '\' in between?

Answer 1

I think that the behaviour is correct, and as designed. There is a difference between C:myfile.txt and C:\myfile.txt. The Windows documentation calls this out quite explicitly:
If a file name begins with only a disk designator but not the
backslash after the colon, it is interpreted as a relative path to the
current directory on the drive with the specified letter. Note that
the current directory may or may not be the root directory depending
on what it was set to during the most recent "change directory"
operation on that disk. Examples of this format are as follows:
"C:tmp.txt" refers to a file named "tmp.txt" in the current directory on drive C.
"C:tempdir\tmp.txt" refers to a file in a subdirectory to the current directory on drive C.
If the RTL function TPath.Combine added a separator after a drive designator, then there would be no way for you to use TPath.Combine to produce a path like "C:tmp.txt". So, if you want a directory separator, you'll need to supply one yourself:
TPath.Combine('c:\', 'myfile.txt');
Note that the .net framework method Path.Combine on which the Delphi RTL class TPath is loosely modelled behaves the same was the the Delphi RTL equivalent.
Related:
Why Path.Combine doesn't add the Path.DirectorySeparatorChar after the drive designator?
Path.Combine() does not add directory separator after drive letter

Answer 2

When combining folder names and folder files it is always good (if you don't want the default behaviour when a drive designator is given as a path) to put folder name through IncludeTrailingPathDelimiter method. This method will add trailing delimiter to your path if there isn't one
TPath.Combine(IncludeTrailingPathDelimiter('c:'), 'myfile.txt');

Answer 3

The Path.Combine and CombinePath functions have problems with very long path names, furthermore when the path is not physically on the drive (but for example in a zip file) then it also doesn't work. This implementation works for me:
function ExpandFileNameEx(const BasePath, RelativeFileName: string): string;
var
p:integer;
s,folder:string;
begin
{ Check if Relative file name is a fully qualified path: }
if (pos(':\', RelativeFileName) > 0) or (copy(RelativeFileName,1,2) = '\\') then
Result := RelativeFileName
{ Check if Relative file name is a root path assignment: }
else if copy(RelativeFileName,1,1) = '\' then
begin
Result := IncludeTrailingPathDelimiter(ExtractFileDrive(BasePath))
+ Copy(RelativeFileName,2,Length(RelativeFileName));
end else
begin
{ Check all sub paths in Relative file name: }
Result := BasePath;
s := RelativeFileName;
repeat
p := pos('\', s);
if p > 0 then
begin
folder := Copy(s,1,p-1);
Delete(s, 1,p);
end else
begin
folder := s;
s := '';
end;
if folder <> EmptyStr then
begin
if Folder = '..' then
Result := ExtractFileDir(Result)
else if Folder = '.' then
{ No action }
else
Result := IncludeTrailingPathDelimiter(Result) + Folder;
end;
until p = 0;
end;
end;

Answer 4

That is why you should always use IncludeTrailingBackslash (or IncludeTrailingPathDelimiter if you're a masochist):
Before: TPath.Combine('c:', 'myfile.txt');
After: IncludeTrailingBackslash('c:')+'myfile.txt';
It avoids the unknown nuances that someone else was thinking, or edge cases they didn't bother to handle, or quirks they didn't bother to document, when they designed their abstraction.