Question

So, I’m pretty new to all this network programming, and I have a few questions…

I’m building a client-server chat application, wherein the server is running, the client(s) connect(s) to the server, and then when a client sends a message to the server, the server relays it to all the clients. The server is a console application, and the client is a Windows Form Application.

The error I’m getting is in my client, at the very top of my form, I have a textbox to take in a user’s name and a button to “submit” it and connect to the server with that username.

Anyways, this is my button connect code:

    private void btnConnect_Click(object sender, EventArgs e)
    {
        readData = "Connecting to chat server...";
        msg();

        try
        {
            sck_client.Connect("127.0.0.1", 8888);
            sw = new StreamWriter(sck_client.GetStream());
            string toSend = txtUsername.Text;

            sw.Write(toSend);
            sw.Flush();

            chatThread = new Thread(GetMessages);
            chatThread.Start();
        }
        catch (Exception ex)
        {
            readData = ex.ToString(); 
            msg();
        }
    }

msg() quite simply takes the value in readData and prints it to the screen (in a richtextbox). sw is a streamwriter that has been declared publicly, outside of the method, so is sck_client (a TcpClient) and chatThread (a Thread).

Basically, the issue is, when I run my program and try to connect, it throws Exception ex, as though it cannot connect. It throws a NullReferenceException with the text:

System.NullReferenceException: Object reference not set to an instance
of an object. at Chat_Client.Main.btnConnect_Click(Object sender,
EventArgs e) in filepath\Chat_Client\Main.cs:line36

That occurs even when my server is running and listening to port 8888. So, what should I do to fix it?

If you need any more of my code to solve the problem, let me know in a comment and I’ll post it.

To show where the code is instantiated:

public partial class Main : Form // new class for the form itself
    {
        // all of these are declared outside any method:
        TcpClient sck_client = default(TcpClient);
        Thread chatThread = default(Thread);
        string readData = null;
        StreamWriter sw = default(StreamWriter);
        StreamReader sr = default(StreamReader);
        ...

Answer 1

TcpClient sck_client = default(TcpClient);
...
sck_client.Connect("127.0.0.1", 8888);

at some point, you will need to give it a value other than null (the default for TcpClient is null). Also, you probably don’t need a StreamWriter just to send a string – I’d look at using Encoding (to get the bytes; typically UTF8), and a length-prefix of the size (in bytes).