using System;

using System.Net;

public class Program

{

    public static void Main()

    {

        string filePath = @"C:\test folder\A+B\testfile.txt";

        var encoded = WebUtility.UrlEncode(filePath.Replace('\\', '/'));

        var decoded = WebUtility.UrlDecode(encoded.Replace('/', '\\'));

        Console.WriteLine(decoded); 

    }

}