CSharp

🔗 WebSocket with C# (.NET) - A Complete Guide 🚀

🔍 Introduction

WebSockets enable real-time, bidirectional communication between a server and multiple clients. In C#, WebSockets are supported in ASP.NET Core, allowing efficient messaging in web applications.

🎯 What You’ll Learn

✅ How to create a WebSocket server using ASP.NET Core
✅ How to create a WebSocket client in C#
✅ How to send and receive messages
✅ How to handle multiple clients
✅ How to use WebSockets with HTML & JavaScript

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🏗️ 1. Setting Up a WebSocket Server in C#

We will create an ASP.NET Core Web API that supports WebSocket connections.

📌 Step 1: Create an ASP.NET Core Project

1️⃣ Open Visual Studio
2️⃣ Create a new ASP.NET Core Web API project
3️⃣ Select .NET 6+
4️⃣ Remove the default WeatherForecast files

📌 Step 2: Add WebSocket Support

Modify Program.cs to enable WebSockets:

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// Enable WebSockets
app.UseWebSockets();

app.Map("/ws", async (HttpContext context) =>
{
    if (context.WebSockets.IsWebSocketRequest)
    {
        using WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
        await HandleWebSocketConnection(webSocket);
    }
    else
    {
        context.Response.StatusCode = 400; // Bad Request
    }
});

app.Run();

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📡 2. Handling WebSocket Messages

Create a new method HandleWebSocketConnection to manage WebSocket communication.

using System.Net.WebSockets;
using System.Text;

async Task HandleWebSocketConnection(WebSocket webSocket)
{
    var buffer = new byte[1024 * 4];

    while (webSocket.State == WebSocketState.Open)
    {
        var result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
        if (result.MessageType == WebSocketMessageType.Close)
        {
            await webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, "Closed", CancellationToken.None);
            break;
        }

        string message = Encoding.UTF8.GetString(buffer, 0, result.Count);
        Console.WriteLine($"Received: {message}");

        string response = $"Server: {message.ToUpper()}";
        byte[] responseBytes = Encoding.UTF8.GetBytes(response);
        await webSocket.SendAsync(new ArraySegment<byte>(responseBytes), WebSocketMessageType.Text, true, CancellationToken.None);
    }
}

How It Works

• Receives messages from the client.
• Logs the message to the console.
• Sends back a response (converted to uppercase).

🏃 Run the WebSocket Server

1. Open Terminal in Visual Studio.
2. Run the project with:

   dotnet run

3. The server starts at ws://localhost:5000/ws.

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💻 3. Creating a WebSocket Client in C#

Now, let's create a C# WebSocket client to connect to the server.

📌 Step 1: Create a New Console App

1️⃣ Open Visual Studio
2️⃣ Create a new Console Application
3️⃣ Add System.Net.WebSockets for WebSocket support

📜 Step 2: Implement the WebSocket Client

using System;
using System.Net.WebSockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

class Program
{
    static async Task Main(string[] args)
    {
        using var client = new ClientWebSocket();
        await client.ConnectAsync(new Uri("ws://localhost:5000/ws"), CancellationToken.None);
        Console.WriteLine("Connected to WebSocket Server");

        _ = Task.Run(async () =>
        {
            var buffer = new byte[1024];
            while (client.State == WebSocketState.Open)
            {
                var result = await client.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
                string message = Encoding.UTF8.GetString(buffer, 0, result.Count);
                Console.WriteLine($"Received: {message}");
            }
        });

        while (true)
        {
            string message = Console.ReadLine();
            if (message.ToLower() == "exit") break;

            byte[] bytes = Encoding.UTF8.GetBytes(message);
            await client.SendAsync(new ArraySegment<byte>(bytes), WebSocketMessageType.Text, true, CancellationToken.None);
        }

        await client.CloseAsync(WebSocketCloseStatus.NormalClosure, "Closing", CancellationToken.None);
        Console.WriteLine("WebSocket closed");
    }
}

How It Works

• Connects to ws://localhost:5000/ws
• Reads messages from the console and sends them to the server
• Receives messages and displays them

🏃 Run the WebSocket Client

1. Start the WebSocket Server.
2. Run the client in another terminal with:

   dotnet run

3. Type a message and press Enter.
4. The server will respond with an uppercase message.

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🌐 4. WebSocket with JavaScript (HTML)

We can also connect a web client to our C# WebSocket server.

📜 Step 1: Create index.html

<!DOCTYPE html>
<html>
<head>
    <title>WebSocket Client</title>
</head>
<body>
    <h2>WebSocket Chat</h2>
    <div id="messages" style="border:1px solid black; padding:10px; height:200px; overflow:auto;"></div>
    <input type="text" id="msg" placeholder="Enter message">
    <button onclick="sendMessage()">Send</button>

    <script>
        const socket = new WebSocket("ws://localhost:5000/ws");

        socket.onopen = () => console.log("Connected to server");
        socket.onmessage = (event) => {
            let messages = document.getElementById("messages");
            messages.innerHTML += `<div>${event.data}</div>`;
        };

        function sendMessage() {
            let msg = document.getElementById("msg").value;
            socket.send(msg);
            document.getElementById("msg").value = "";
        }
    </script>
</body>
</html>

🏃 Run the HTML WebSocket Client

1. Open the file in a browser.
2. Type a message and click Send.
3. The server will respond, and the message will appear on the page.

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🔒 5. WebSocket Security Best Practices

🔹 Use HTTPS/WSS (wss://) to secure connections.
🔹 Authenticate users (JWT tokens, API keys).
🔹 Limit message size to prevent DoS attacks.
🔹 Validate messages to prevent injection attacks.
🔹 Close inactive connections to free resources.

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🎉 6. Conclusion

✅ Built a WebSocket server in C# (ASP.NET Core).
✅ Created a C# WebSocket client.
✅ Implemented message handling & broadcasting.
✅ Connected a JavaScript client.
✅ Applied security best practices.