Client-server architecture in Unity

Introduction

Client-server architecture helps computers talk to each other. One computer (server) shares information, and others (clients) ask for it.

When making a multiplayer game where players connect to a central game server.

When your game needs to save player progress on a remote server.

When you want to get live data like scores or chat messages from a server.

When you want to separate game logic on a server and show results on players' devices.

Syntax

Unity

using UnityEngine;
using System.Net.Sockets;
using System.Text;

public class SimpleClient : MonoBehaviour {
    TcpClient client;
    NetworkStream stream;

    void Start() {
        client = new TcpClient("127.0.0.1", 5500);
        stream = client.GetStream();

        byte[] data = Encoding.ASCII.GetBytes("Hello Server");
        stream.Write(data, 0, data.Length);

        byte[] responseData = new byte[256];
        int bytes = stream.Read(responseData, 0, responseData.Length);
        string response = Encoding.ASCII.GetString(responseData, 0, bytes);
        Debug.Log("Server says: " + response);
    }

    void OnApplicationQuit() {
        stream?.Close();
        client?.Close();
    }
}

This example shows a simple client connecting to a server using TCP.

Unity uses Debug.Log to print messages in the Console window.

Examples

Connects the client to the server at the given IP and port.

Unity

TcpClient client = new TcpClient("server_ip", port_number);

Gets the stream to send and receive data.

Unity

NetworkStream stream = client.GetStream();

Sends a message to the server.

Unity

byte[] data = Encoding.ASCII.GetBytes("message");
stream.Write(data, 0, data.Length);

Reads the server's response.

Unity

int bytes = stream.Read(buffer, 0, buffer.Length);
string response = Encoding.ASCII.GetString(buffer, 0, bytes);

Sample Program

This Unity script connects to a server at localhost on port 5500, sends a greeting, and prints the server's reply in the Console.

Unity

using UnityEngine;
using System.Net.Sockets;
using System.Text;

public class SimpleClient : MonoBehaviour {
    TcpClient client;
    NetworkStream stream;

    void Start() {
        try {
            client = new TcpClient("127.0.0.1", 5500);
            stream = client.GetStream();

            byte[] data = Encoding.ASCII.GetBytes("Hello Server");
            stream.Write(data, 0, data.Length);

            byte[] responseData = new byte[256];
            int bytes = stream.Read(responseData, 0, responseData.Length);
            string response = Encoding.ASCII.GetString(responseData, 0, bytes);
            Debug.Log("Server says: " + response);
        } catch (SocketException e) {
            Debug.Log("SocketException: " + e.Message);
        }
    }

    void OnApplicationQuit() {
        stream?.Close();
        client?.Close();
    }
}

Important Notes

Always close your network streams and clients to free resources.

Client-server communication needs matching server code running to accept connections.

Network errors can happen; handle exceptions to avoid crashes.

Summary

Client-server architecture lets your game talk to a central server.

Clients send requests; servers send back responses.

Unity uses TcpClient and NetworkStream for network communication.

Practice

1. In Unity's client-server architecture, what is the main role of the client?

easy

A. Send requests to the server and receive responses

B. Store all game data permanently

C. Control the server hardware

D. Manage other clients directly

Solution

1. Step 1: Understand client role in client-server

   The client initiates communication by sending requests to the server.

2. Step 2: Understand server response

   The server processes requests and sends back responses to the client.

3. Final Answer:

   Send requests to the server and receive responses -> Option A

4. Quick Check:

   Client = Request sender [OK]

Hint: Clients always ask; servers always answer [OK]

Common Mistakes:

• Thinking client stores all data permanently
• Confusing client with server hardware control
• Assuming clients manage other clients

2. Which of the following is the correct way to create a TcpClient in Unity C#?

easy

A. TcpClient client = TcpClient("127.0.0.1", 8080);

B. TcpClient client = new TcpClient.connectTo("127.0.0.1", 8080);

C. TcpClient client = new TcpClient.connect("127.0.0.1", 8080);

D. TcpClient client = new TcpClient("127.0.0.1", 8080);

Solution

1. Step 1: Recall TcpClient constructor syntax

   In C#, creating a new TcpClient requires the 'new' keyword and constructor with IP and port.

2. Step 2: Check each option's syntax

   Only TcpClient client = new TcpClient("127.0.0.1", 8080); uses 'new TcpClient("127.0.0.1", 8080);' correctly.

3. Final Answer:

   TcpClient client = new TcpClient("127.0.0.1", 8080); -> Option D

4. Quick Check:

   Use 'new' keyword with constructor [OK]

Hint: Always use 'new' to create objects in C# [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using incorrect method names like connect or connectTo
• Wrong constructor call syntax

3. What will the following Unity C# code output if the server sends back the string "Hello Client"?

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

TcpClient client = new TcpClient("127.0.0.1", 9000);
NetworkStream stream = client.GetStream();
byte[] data = new byte[256];
int bytes = stream.Read(data, 0, data.Length);
string response = Encoding.ASCII.GetString(data, 0, bytes);
Console.WriteLine(response);

medium

A. System.Byte[]

B. Hello Client

C. null

D. Exception thrown

Solution

1. Step 1: Understand data reading from NetworkStream

   The code reads bytes from the stream into a byte array and counts bytes read.

2. Step 2: Convert bytes to string

   It converts the bytes read to a string using ASCII encoding, which matches the server's sent string.

3. Final Answer:

   Hello Client -> Option B

4. Quick Check:

   Read bytes + decode = server message [OK]

Hint: Decode bytes read from stream to get server message [OK]

Common Mistakes:

• Printing byte array directly instead of decoding
• Ignoring number of bytes read
• Assuming null if no data

4. Identify the error in this Unity C# client code snippet:

TcpClient client = new TcpClient();
client.Connect("192.168.1.10", 7000);
NetworkStream stream = client.GetStream();
byte[] message = Encoding.ASCII.GetBytes("Ping");
stream.Write(message, 0, message.Length);
stream.Close();
client.Close();

medium

A. Missing try-catch block for network exceptions

B. Calling stream.Close() before client.Close() causes error

C. TcpClient must be created with IP and port in constructor

D. No error; code is correct

Solution

1. Step 1: Review TcpClient creation and connection

   Creating TcpClient with default constructor and then calling Connect is valid.

2. Step 2: Check for error handling

   Network operations can fail; missing try-catch means exceptions are unhandled.

3. Final Answer:

   Missing try-catch block for network exceptions -> Option A

4. Quick Check:

   Always handle network exceptions [OK]

Hint: Always wrap network calls in try-catch [OK]

Common Mistakes:

• Assuming stream.Close() before client.Close() is wrong
• Thinking TcpClient must have IP in constructor
• Ignoring exception handling

5. You want to build a Unity client that connects to a server and sends a player's score only if it is above 1000. Which code snippet correctly implements this logic using TcpClient and NetworkStream?

hard

A. if(score > 1000) {
TcpClient client = new TcpClient("server.com");
NetworkStream stream = client.GetStream();
byte[] data = Encoding.ASCII.GetBytes(score.ToString());
stream.Write(data, 0, data.Length);
stream.Close();
client.Close();
}

B. TcpClient client = new TcpClient("server.com", 5000);
NetworkStream stream = client.GetStream();
if(score > 1000) {
byte[] data = Encoding.ASCII.GetBytes(score.ToString());
stream.Write(data, 0, data.Length);
}
stream.Close();
client.Close();

C. TcpClient client = new TcpClient();
if(score > 1000) {
client.Connect("server.com", 5000);
NetworkStream stream = client.GetStream();
byte[] data = Encoding.ASCII.GetBytes(score.ToString());
stream.Write(data, 0, data.Length);
stream.Close();
client.Close();
}

D. if(score > 1000) {
TcpClient client = new TcpClient();
client.Connect("server.com", 5000);
NetworkStream stream = client.GetStream();
byte[] data = Encoding.ASCII.GetBytes(score);
stream.Write(data, 0, data.Length);
stream.Close();
client.Close();
}

Solution

1. Step 1: Check conditional connection and sending

   TcpClient client = new TcpClient(); if(score > 1000) { client.Connect("server.com", 5000); NetworkStream stream = client.GetStream(); byte[] data = Encoding.ASCII.GetBytes(score.ToString()); stream.Write(data, 0, data.Length); stream.Close(); client.Close(); } creates TcpClient without parameters, connects only if score > 1000, then sends data.

2. Step 2: Verify data conversion and method calls

   Converts score to string before encoding to bytes, uses correct Connect() and stream methods.

3. Final Answer:

   new TcpClient(); client.Connect() inside if(score > 1000) -> Option C

4. Quick Check:

   Connect and send inside if condition [OK]

Hint: Connect and send only inside the if block [OK]

Common Mistakes:

• Sending data without converting score to string
• Connecting outside the if condition
• Using score directly as bytes without encoding