Issue in downloading video from Youtube

Hi.
I am trying to make a video download application for desktop.
Now the problem is that following code works fine

        WebClient webOne = new WebClient();
        string temp1 = " http://www.c-sharpcorner.com/UploadFile/shivprasadk/visual-studio-and-net-tips-and-tricks-15/Media/Tip15.wmv";
        webOne.DownloadFile(new Uri(temp1), "video.wmv");
           
but following doesn't:

        temp1="http://www.youtube.com/watch?v=Y_UmWdcTrrc&feature=plcp";
        (in this case a 200-400 kilobyte junk file gets downloaded )

difference between the two url's is obvious, first one contains exact name for file while other seems to be encrypted in some way,...

I was unable to find any solution to the problem so I would highly appreciate a little help here, Thanks.