posting tweets and uploading pictures to twitterusing c#

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C#

twitter

hello i created a c# application that posts tweets and pictures on an account on twitter but im stuck with an error i can't seem to understand.
if someone ca help identify the problem i would appreciate it thanks!

here is the code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

using TweetSharp;
using Hammock;

namespace TweetNow
{
    class TweetManager
    {

        static public TwitterService initiate()
        {
   String _consumerKey = "epG4Cd8W06r8pxlt05I8Hg";
            String _consumerSecret = "aattB5375aOn0tfpWSj0rdNUiYCwIfsQzwLGG1yVGFE";

            String _accessToken = "479270385-Nda8X27zzi5xBKfUioNm9U8qCOzOvakVSGkS4YJS";
            String _accessTokenSecret = "8rSKr6ph6oRJ4pISjZDny2mMyn4ldX1AoSHwtzx4";
            // Pass your credentials to the service
            TwitterService service = new TwitterService(_consumerKey, _consumerSecret);

            //  Retrieve an OAuth Request Token
            service.AuthenticateWith(_accessToken, _accessTokenSecret);

            return service;
           

        }

 


        public static void sendTweet(String msg,String picname,TwitterService serv) {
           

            RestRequest request = serv.PrepareEchoRequest();
            request.Path = "uploadAndPost.xml";
            request.AddFile("media", "smile", "Images/"+picname, "image/jpeg");
           


            //request.AddField("key", "b9deee8ea5af8a6e1bac2aeb9d541c46"); 
            request.AddField("key", "560adb8b7b328ae25ae3e7d3436de399"); 
            request.AddField("message", "Emote with a Pic!");

        
            RestClient client = new RestClient { Authority = "http://api.twitpic.com/", VersionPath = "2" };
            RestResponse response = client.Request(request);
            Console.WriteLine(response.Content);

            String str = response.Content;
            int first = str.IndexOf("<url>");
            int last = str.LastIndexOf("</url>");
            string str2 = str.Substring(first + 5, last - first - 5);

            serv.SendTweet(msg+" "+str2+" .");
        }
    }
}

and the error i'm getting is :
system.ArgumentOutOfRangeException: Lenght cannot be less than yero.
Parameter name : length
at System.String.InternalSubStringWithChecks(int32 startIndex,Int32 length,Boolean fAlwaysCopy)
at TweetNow.TweetManager.sendTweet(String msg, String picname,TwitterService serv) in
d: ....