Path.Combine absolute with relative path strings

Path.GetFullPath(@"c:\windows\temp\..\system32")?

Path.GetFullPath() does not work with relative paths.

Here's the solution that works with both relative + absolute paths. It works on both Linux + Windows and it keeps the .. as expected in the beginning of the text (at rest they will be normalized). The solution still relies on Path.GetFullPath to do the fix with a small workaround.

It's an extension method so use it like text.Canonicalize()

/// <summary>
///     Fixes "../.." etc
/// </summary>
public static string Canonicalize(this string path)
{
    if (path.IsAbsolutePath())
        return Path.GetFullPath(path);
    var fakeRoot = Environment.CurrentDirectory; // Gives us a cross platform full path
    var combined = Path.Combine(fakeRoot, path);
    combined = Path.GetFullPath(combined);
    return combined.RelativeTo(fakeRoot);
}
private static bool IsAbsolutePath(this string path)
{
    if (path == null) throw new ArgumentNullException(nameof(path));
    return
        Path.IsPathRooted(path)
        && !Path.GetPathRoot(path).Equals(Path.DirectorySeparatorChar.ToString(), StringComparison.Ordinal)
        && !Path.GetPathRoot(path).Equals(Path.AltDirectorySeparatorChar.ToString(), StringComparison.Ordinal);
}
private static string RelativeTo(this string filespec, string folder)
{
    var pathUri = new Uri(filespec);
    // Folders must end in a slash
    if (!folder.EndsWith(Path.DirectorySeparatorChar.ToString())) folder += Path.DirectorySeparatorChar;
    var folderUri = new Uri(folder);
    return Uri.UnescapeDataString(folderUri.MakeRelativeUri(pathUri).ToString()
        .Replace('/', Path.DirectorySeparatorChar));
}

Be careful with Backslashes, don't forget them (neither use twice:)

string relativePath = "..\\bling.txt";
string baseDirectory = "C:\\blah\\";
//OR:
//string relativePath = "\\..\\bling.txt";
//string baseDirectory = "C:\\blah";
//THEN
string absolutePath = Path.GetFullPath(baseDirectory + relativePath);