UE4 C# Multiplayer Core APIs

Multiplayer Core APIs with UE4 C#

This sample demonstrates essential concepts for building multiplayer functionalities in Unreal Engine 4 using C#. We'll cover basic replication, RPCs (Remote Procedure Calls), and state synchronization.

Understanding Replication

Replication is the process by which properties and events are synchronized between the server and clients in a multiplayer game. In UE4 C#, this is primarily managed through the `Replicated` keyword and the `RepNotify` mechanism.

Replicated Properties

Properties marked with the `[Replicated]` attribute will automatically be replicated from the server to all connected clients. Changes made on the server are automatically sent to clients.

                
using UnrealEngine.Runtime;
using UnrealEngine.Engine;
using UnrealEngine.Core;

public class MyReplicatedActor : AActor
{
    // Property replicated from server to clients
    [Replicated]
    public int Health { get; set; }

    // Property that triggers a notification function when replicated
    [ReplicatedUsing = "OnRep_PlayerName"]
    public string PlayerName { get; set; }

    protected override void OnConstruction(FTransform Transform)
    {
        base.OnConstruction(Transform);
        // Initialize replicated properties on server
        if (IsServer)
        {
            Health = 100;
            PlayerName = "Player_Default";
        }
    }

    // RepNotify function for PlayerName
    public void OnRep_PlayerName()
    {
        // This function is called on clients when PlayerName is replicated
        UE_LOG.Info("Player name updated on client: {0}", PlayerName);
    }

    // Example of changing a replicated property on the server
    public void Server_SetHealth(int NewHealth)
    {
        if (IsServer)
        {
            Health = NewHealth;
            UE_LOG.Info("Server Health updated to: {0}", Health);
        }
    }
}
                
            

Remote Procedure Calls (RPCs)

RPCs allow clients to call functions on the server, or the server to call functions on clients. They are defined using specific attributes like `[ServerRpc]` and `[ClientRpc]`.

Server RPCs

Functions marked with `[ServerRpc]` can only be called by clients, and they execute on the server. This is crucial for actions that require server authority, like player input or firing a weapon.

Client RPCs

Functions marked with `[ClientRpc]` can only be called by the server, and they execute on all connected clients. Useful for events that need to be visible to everyone, like playing a sound effect or displaying a message.

                
using UnrealEngine.Runtime;
using UnrealEngine.Engine;
using UnrealEngine.Core;

public class MyRpcActor : AActor
{
    // Server RPC: Called by a client, executes on the server
    [ServerRpc]
    public void RequestServerAction(string Message)
    {
        // This code runs on the server
        UE_LOG.Info("Server received request: {0}", Message);

        // Now call a client RPC from the server
        NotifyAllClients("Server confirmed action!");
    }

    // Client RPC: Called by the server, executes on all clients
    [ClientRpc]
    public void NotifyAllClients(string NotificationMessage)
    {
        // This code runs on all clients
        UE_LOG.Info("Client received notification: {0}", NotificationMessage);
        PrintToScreen(NotificationMessage); // Example UI update
    }

    // Client-side function to trigger the RPC
    public void TriggerServerRequest(string TextToSend)
    {
        if (!IsServer) // Ensure it's called by a client
        {
            RequestServerAction(TextToSend);
        }
    }

    // Helper function to print to screen (for demonstration)
    private void PrintToScreen(string Message)
    {
        GEngine.AddOnScreenDebugMessage(-1, 5.0f, FColor.Yellow, Message);
    }
}
                
            

Server Authority and Client Prediction

In a robust multiplayer system, the server holds the ultimate authority over game state. Clients often implement prediction to provide a smoother experience, guessing the outcome of actions before the server confirms them. This sample focuses on the core replication and RPCs, but for advanced multiplayer, consider exploring client-side prediction and server reconciliation.

Key Considerations

Network Relevance: Only replicate what's necessary to clients. Unreal Engine has systems to manage this automatically based on distance and importance.
Bandwidth: Be mindful of the data being replicated. Optimize data structures and use efficient serialization.
Lag Compensation: For fast-paced games, implement techniques to compensate for network latency, especially for hit detection.
Security: Never trust client input directly for critical game logic. Always validate actions on the server.

This sample provides a foundational understanding of multiplayer programming in UE4 with C#. For more advanced topics like network-aware physics, complex game state management, and optimized replication strategies, refer to the official Unreal Engine documentation and community resources.