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

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.

According to this doc on Path.Combine the remark states it's best to use Path.Join() for this scenario where the user may have entered values. Looks like it was introduced with .NET Core 3.0

Path.Join(basePath, relativePath);

Seems like the only difference is this explicit scenario where we don't want to regard any of the paths we are combining as potential root paths.

Unlike the Combine method, the Join method does not attempt to root the returned path. (That is, if path2 or path2 or path3 is an absolute path, the Join method does not discard the previous paths as the Combine method does.

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('/', '\\'));