Event returns null. and throws object reference not set to instance when event is triggered

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Hello everyone,

I have an problem with events and delegates. Basically i have two projects under same solution. One project is the server and other is the client. The client form contains a textbox for login and login button, rich textbox and button for whisper. Now when the user clicks on whisper the RTB of the client to whom message is sent has to be updated with the message but in my case i am recieving an exception saying object not set to instance. The code for server and client are has follows

Client code

C#

namespace ChatClient
{
    [CallbackBehavior(UseSynchronizationContext = false)]
    public partial class Client : Form,IChatCallback
    {
        public delegate void ReceiveMsg(string sender, string message);
        public event ReceiveMsg Receivemessage;

        IChat chatter = null;
        DuplexChannelFactory<ichat> connect;
        InstanceContext instanceContext = null;

        
        public string CurrentUser;
        public Client()
        {
            InitializeComponent();
            
        }

        

        private void login_btn_Click(object sender, EventArgs e)
        {
            //CallBackHandler instance = new CallBackHandler();
            //instanceContext = new InstanceContext(new CallBackHandler());
            Client instance = new Client();
            instanceContext = new InstanceContext(new Client());
          
            connect = new DuplexChannelFactory<ichat>(instanceContext, "TcpBinding");
            chatter = connect.CreateChannel();
            
            if (!string.IsNullOrEmpty(login_txt.Text))
            {
                CurrentUser = login_txt.Text;
                chatter.JoinChat(CurrentUser);
               
            }
            else
            {
                MessageBox.Show("Please Enter Valid UserName", "WARNING", MessageBoxButtons.OK, MessageBoxIcon.Warning);
            }
        }

        private void whispher_btn_Click(object sender, EventArgs e)
        {
            
            chatter.SendMessage(login_txt.Text, rtxt_message.Text);
            //Trigger event
            Receivemessage(login_txt.Text, rtxt_message.Text);
            //after execution of this statement it throws object reference not set
            //to the instance
                      
            
        }



        #region IChatCallback Members

        public void ReceiveMessage(string from, string message)
        {
            
             rtxt_message.AppendText("hellO");
            
        }
</ichat></ichat>

Server code

<pre lang="c#">namespace WCFChat
{
    [ServiceBehavior(InstanceContextMode=InstanceContextMode.PerCall,ConcurrencyMode=ConcurrencyMode.Reentrant)]
    class ChatServer:IChat
    {
        //public delegate void ReceiveMsg(string sender, string message);
        //public event ReceiveMsg Receivemessage;

        IChatCallback callback = null;
        Client cs = new Client();
        public static Dictionary<string,> ListofUsers = new Dictionary<string,>();
        public void JoinChat(string username)
        {
            callback = OperationContext.Current.GetCallbackChannel<ichatcallback>();
            if (CheckUserExists(username) == true)
            {
                ListofUsers.Add(username, callback);
                
            }
            else
            {
                MessageBox.Show("Username already exixts!: Please select different Name");
            }
            ListofUsers.ToArray();
            
        }

        public void SendMessage(string sender, string message)
        {
            foreach (string str in ListofUsers.Keys)
            {
                if (str == "abc")
                {
                    
                    IChatCallback icall = ListofUsers[str];
                    icall.ReceiveMessage(sender,message);
                    //registering for the event on client side
                    cs.Receivemessage+=new Client.ReceiveMsg(icall.ReceiveMessage);
                    
                    
                }
            }
        }

        public void LeaveChat(string name)
        {
        }

        //public string[] GetOnlineUsers()
        //{
        //    return ;
        //}

        public bool CheckUserExists(string username)
        {
            foreach (string name in ListofUsers.Keys)
            {
                if(username.Equals(name,StringComparison.OrdinalIgnoreCase))
                     return false;
            }
            return true;
        }
    }
}</ichatcallback>

Posted 22-Oct-11 18:34pm

manu g m

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BillWoodruff 23-Oct-11 3:31am

Is this an ASP.NET project: is there an actual server (IIS ?) being used here ?

manu g m 23-Oct-11 3:32am

no this is not ASP.net its windows project.

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OriginalGriff · Answer 1 · 2011-10-22T21:31:00

It's difficult to tell from that code fragment, but if

rtxt_message.AppendText("hellO");

causes an event to be fired then there are two thing you need to check:
1) Is rtxt_message set to an instance of your class?
2) In AppendText, do you check for an event handler before triggering it?
My default format for an event trigger is:

C#

public event EventHandler Changed;

protected virtual void OnChanged(EventArgs e)
   {
   EventHandler eh = Changed;
   if (eh != null)
      {
      eh(this, e);
      }
   }

If you do not check for an handler before trying to trigger the event, you will get a null reference exception.

If this doesn't help, then you are going to have to either give us better information, or use a breakpoint and single step into each line until you find the null value.