Client-server architecture in Unity - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Unity client-server message exchange?

Consider a simple Unity client-server setup where the server sends a message to the client, and the client logs it. What will be printed in the Unity console?

Unity

using UnityEngine;

public class Server : MonoBehaviour {
    public delegate void MessageHandler(string message);
    public static event MessageHandler OnMessageSent;

    void Start() {
        SendMessageToClient("Hello from Server");
    }

    void SendMessageToClient(string msg) {
        OnMessageSent?.Invoke(msg);
    }
}

public class Client : MonoBehaviour {
    void OnEnable() {
        Server.OnMessageSent += ReceiveMessage;
    }

    void OnDisable() {
        Server.OnMessageSent -= ReceiveMessage;
    }

    void ReceiveMessage(string msg) {
        Debug.Log("Client received: " + msg);
    }
}

A Client received: Hello from Server

B Server received: Hello from Server

C No output because events are not subscribed

D NullReferenceException at runtime

💡 Hint

Think about which class subscribes to the event and what message is passed.

The Server class sends a message via an event. The Client subscribes to this event and logs the received message. So the output is the client logging the server's message.

🧠 Conceptual

intermediate

Which statement best describes client-server roles in Unity networking?

In a Unity multiplayer game using client-server architecture, which statement is true about the roles of client and server?

A Clients and server have identical responsibilities

B Clients manage game state and server only relays messages

C The server manages game state and clients send input commands

D Server only displays graphics, clients handle logic

💡 Hint

Think about who has authority over the game world.

The server is authoritative and manages the game state. Clients send their input to the server, which processes and updates the state.

🔧 Debug

advanced

Why does this Unity client never receive server messages?

Given the following Unity code, why does the client never log the server's message?

Unity

public class Server : MonoBehaviour {
    public delegate void MessageHandler(string message);
    public static event MessageHandler OnMessageSent;

    void Start() {
        SendMessageToClient("Hello Client");
    }

    void SendMessageToClient(string msg) {
        OnMessageSent?.Invoke(msg);
    }
}

public class Client : MonoBehaviour {
    void Start() {
        Server.OnMessageSent += ReceiveMessage;
    }

    void ReceiveMessage(string msg) {
        Debug.Log("Received: " + msg);
    }
}

A Event delegate is not static, so client can't subscribe

B Client subscribes too late; event fired before subscription

C Client unsubscribes immediately after subscribing

D Server never invokes the event

💡 Hint

Check the order of event subscription and invocation.

The Server fires the event in Start(), but the Client subscribes in its Start(), which may run after the Server's Start(). So the event is fired before the client subscribes.

📝 Syntax

advanced

Which option correctly declares a Unity event for client-server messaging?

Which of the following is the correct syntax to declare a static event in Unity C# for sending string messages from server to clients?

A public event static Action<string> OnMessageSent;

B public static delegate event Action<string> OnMessageSent;

C static public event void OnMessageSent(string message);

D public static event Action<string> OnMessageSent;

💡 Hint

Remember the order: access modifier, static, event, delegate type, name.

Option D correctly declares a static event using the Action delegate with a string parameter.

🚀 Application

expert

How many clients receive this server broadcast in Unity?

In a Unity server script, the server broadcasts a message to all connected clients using a static event. If 3 clients are connected and subscribed to the event, how many clients will receive the message?

Unity

using System;
using UnityEngine;

public class Server : MonoBehaviour {
    public static event Action<string> OnBroadcast;

    public void Broadcast(string msg) {
        OnBroadcast?.Invoke(msg);
    }
}

public class Client : MonoBehaviour {
    void OnEnable() {
        Server.OnBroadcast += Receive;
    }

    void OnDisable() {
        Server.OnBroadcast -= Receive;
    }

    void Receive(string msg) {
        Debug.Log("Client received: " + msg);
    }
}

A 3

B Depends on network latency

C 0

D 1

💡 Hint

Think about how events work with multiple subscribers.

Each subscribed client receives the event invocation, so all 3 clients get the message.