.NET - "%5C" Showing Up on URLs

Are you a developer, who is accepting a file or some string of text and putting it into a cloud? Are you, also, per chance, retrieving its URL, and notcing this pesky "%5C popping up in our URLs? Well, I know recently I was. It was a pretty strange, out of the blue error with an interesting cause. Here are some details for you:

What Causes This?

The actual causes of this problem are numerous. The bottom line is, '%5C' in a URL means that the backslash '\' got encoded in the URL. Somewhere along the way, either unintentionally or on purpose, a string containing one wound up in the encoding step.

The culprit for many users is using .NET's Path utility for trying to parse URLs. Here's an example:

Let's look at expectation vs Reality

Code: Path.GetDirectoryName("I/want/to/upload/to/this/place")

Expected: "I/want/to/upload/to/this/"

Actual: "I\want\to\upload\to\this"

This is because Path uses a field called Path.DirectorySeperatorChar, which on Windows is '\'... A bit backwards that Path can READ forward slashes as directory names, yet only output backslashes in the return value.

How Do I Fix This?

There are plenty of good solutions to fix this. However, with so many different causes to the problem, it's hard to say exactly.

To Fix it in a simple string

It's possible that all you need to do is Path.GetDirectoryName(your_string).Replace(Path.DirectorySeperatorChar, Path.AltDirectorySeperatorChar), and you're good to go.

Hopefully this shed some light on what the problem may be in your case. Hopefully, moving forward, Microsoft is not afraid to add a .NET feature like PathFlags or something so that this doesn't crop up as much as it currently does.