Why does Path.Combine produce this result with a relative path?

Question

To my surprise, this code does not produce expected results:

var basePath = @"\\server\BaseFolder";
var relativePath = @"\My\Relative\Folder";

var combinedPath = Path.Combine(basePath, relativePath);

The result is \My\Relative\Folder instead of the expected \\server\BaseFolder\My\Relative\Folder.

Why is this? What's the best way to combine relative paths that may or may not have a slash in them?

EDIT: I'm aware that I can just do string manipulation on relativePath to detect and remove a starting slash. Is there a safer way of doing this (I thought Path.Combine was supposed to be the safe way) that will account for backslashes and frontslashes?

Answer 1

Drop the leading slash on relativePath and it should work.

The reason why this happens is that Path.Combine is interpreting relativePath as a rooted (absolute) path because, in this case, it begins with a \. You can check if a path is relative or rooted by using Path.IsRooted().

From the doc:

If the one of the subsequent paths is an absolute path, then the combine operation resets starting with that absolute path, discarding all previous combined paths.

Answer 2

Paths that start with a slash are interpreted as being absolute rather than relative. Simply trim the slash off if you want to guarantee that relativePath will be treated as relative.

var basePath = @"\\server\BaseFolder";
var relativePath = @"\My\Relative\Folder";

var combinedPath = Path.Combine(basePath, relativePath.TrimStart('/', '\\'));