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Socket Programming in The .NET Framework Using C#

March 25, 2022

Let us say there are two devices on a network that need to communicate with each other. To enhance communication between these devices, we set one device on a socket of a specific port and an IP address.

The other device will be able to communicate with the first device through the same port and IP address if they are on the same socket or network. In the .NET framework, we have the Socket class that enables network programming. This class handles both synchronous and asynchronous modes of network programming.

In asynchronous programming, our program can continue doing other tasks and receiving data while it waits for other tasks to be executed. In synchronous programming, tasks are only carried out one at a time and follow the order in which they come in.

In this state, our program can only handle one task until it completes, then it accepts another task. Therefore, this calls us to look at how asynchronous socket programming is handled and how we can develop one in C#. To do this, we will build a simple server-side console application.

Table of contents


To follow through this tutorial, you must:

Getting started

To create our application, we will start by creating a new project. Open Microsoft Visual Studio and click on create new project as shown in the image below:

New Project

Since we are creating a console application, we will click on the Console Application which uses C# as the programming language, and click next.

Console application

On the next screen, we are required to enter the name of our application. In this case, we will use ConsoleApp1 as the name of our application.

Name of our project

On the next screen, we are required to select the target framework for our application, we will use .NET 5.0 and then click on create to create our application.

Target framework

In this application, we will mainly look at server-side socket programming.

C# namespaces

This project will use the following C# namespaces:

using System;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using System.Net;

Server-side socket programming

We will start by creating the server-side of our application. To begin, we will write our code in the program.cs file.

The first thing we need to do is to create a public class and call it AppState. Inside this class, we will define a socket, name it socket and set it to null. We will also define an integer and give it a data size of 1024 bytes.

Next, we will define a byte array that will be able to hold the new data and pass in the data size we initialized above. The last thing we need to do in this class is to create a string builder and initialize it new:

public class ProgramState
            public Socket socket = null;
            public const int dataSize = 1024;
            public StringBuilder sBuilder = new StringBuilder();

We will again create another class which will be a socket listener. In this class we need do the following:

  • Set a reset event for all completed tasks and set its initial parameter to false.
  • Add a method to start the listener. In this same method, we will also add a byte array and set the new instance to 1024 bytes
  • Add an IP host entry and get the host entry by passing in the hostname of the DNS server.
  • Next, we set the IP address of the host and pick its first element.
  • Afterwards, we will set the endpoint IP address and set it to a new IP on port 80.
  • We then initialize another socket, call a listener and pass in the IP address family. Afterwards, we set a stream and a protocol.
  • Finally, we can start our application and catch any errors in the application. We will do this by using the try...catch method.

In our try method, we will bind our listener to the local endpoint and set the listen backlog 111. We will use a while loop to wait for any incoming connections.

In the catch method, we will print the message on the screen using the e.Message method:

 public class AsyncSocketListener
     public static ManualResetEvent completed = new ManualResetEvent(false);
     public static void StartListener()
         byte[] dataSize = new byte[1024];
         IPHostEntry hostEntry = Dns.GetHostEntry(Dns.GetHostName());
         IPAddress ip = hostEntry.AddressList[0];
         IPEndPoint localEnd = new IPEndPoint(ip, 4444);
         Socket eventListener = new Socket(ip.AddressFamily, SocketType.Stream,ProtocolType.Tcp);
             while (true)
                 Console.WriteLine($"Waiting for new connections...");
                 eventListener.BeginAccept(new AsyncCallback(AcceptCallBack), eventListener);
         catch(Exception e)

In the while loop we created above, we realize that we are getting an error that requires us to create another method, the method we will be creating to remove the error is the AcceptCallBack. In this method, we will set the completed tasks to void by using empty brackets.

In this same method, we will create a socket as our listener. This listener will be initialized with the asyncState. We will create another socket as our handler. This method will be initialized with the EndAccept method. The program state which tells us the state of the connection we have currently will be initialized by the new keyword.

Next, we set the state of our handler, which we will use to start receiving data. With this, we can use the .BeginReceive method to pass in the data, program state, and the callback event as parameters:

 private static void AcceptCallBack(IAsyncResult ar)
                Socket eventListener = (Socket)ar.AsyncState;
                Socket handler = eventListener.EndAccept(ar);
                ProgramState state = new ProgramState();
                state.socket = handler;
                handler.BeginReceive(, 0, ProgramState.dataSize, 0, new AsyncCallback(ReadCallBack), state);

From the method above, we realize that we are getting errors when we pass in the ReadCallBack function as a parameter to the begin receive method. To remove this error, we will create another method to handle the ReadCallBack even. Inside this method, we will create a string, call it content, and set it to empty using the Empty method.

We will create the program state and initialize it with the ar.AsyncState method. We need to define where the data read will be stored after the connection is established. So we will initialize the data read variable with the handler.EndReceive method.

We need to use an if statement to print the data read on the screen in case the data is greater than zero. We will need to convert this data into string because in socket programming, data encoding is necessary when it is presented in bytes format. In this case, we will use the ASCII encoding method to convert it to string format before printing it on the screen.

If the connection is set and data is read, we need to send a response to the client. Because of this, we will create the Send event and pass in the handler, and content as parameters:

 private static void ReadCallBack(IAsyncResult ar)
                string content = string.Empty;
                ProgramState state = (ProgramState)ar.AsyncState;
                Socket handler = state.socket;
                int dataRead = handler.EndReceive(ar);
                if(dataRead > 0)
                    state.sBuilder.Append(Encoding.ASCII.GetString(, 0, dataRead));
                    content = state.sBuilder.ToString();
                    if (content.IndexOf("<EOF>", StringComparison.Ordinal) > -1)
                        Console.WriteLine($"Read:{content.Length} data from \n socket data:{content}");
                        Send(handler, content);
                    handler.BeginReceive(, 0, ProgramState.dataSize, 0, new AsyncCallback(ReadCallBack), state);

When we create the method above, we realise that we are getting an error message that requires us to create the Send method. In the Send method, we will create a byte array, name it sizeOfData, and pass in the content of our data.

The BeginSend function below in our handler receives the sizeOfData, data length, and handler as parameters. new AsyncCallBack is a parameter in our function that receives SendCallBack as another parameter:

private static void Send(Socket handler, string content)
                byte[] sizeOfData = Encoding.ASCII.GetBytes(content);
                handler.BeginSend(sizeOfData, 0, sizeOfData.Length, 0, new AsyncCallback(SendCallBack), handler);

The SendCallBack argument in the method above generates an error message that requires us to create another method called SendCallBack. In the SendCallBack method, we will use a try...catch method, add a socket handler, and define the data sent to the client.

We will be closing the connections on both ends once data is sent and the catch method is used to check for any errors in the connection:

private static void SendCallBack(IAsyncResult ar)
                    Socket handler = (Socket)ar.AsyncState;
                    int dataSent = handler.EndSend(ar);
                    Console.WriteLine($"Send:{dataSent} to client");
                catch (Exception e)

The last thing we need to do is to implement our application in the main function. We will use the Console.WriteLine method to display a message on the screen.

The Console.ReadLine will be used to take data from the user. With that, we can start our listener in the main function:

        static void Main(string[] args)
            Console.WriteLine("Press enter key to continue...");


From this article, we can deduce that socket programming is a better way for devices to communicate over a network. For example, if a device sends data on port 4444 using the TCP protocol, the data can be received on the other end. This is because our device will be acting as a recipient and the sender will be acting as a server.

Peer Review Contributions by: Dawe Daniel