diff --git a/Runtime/Core/ShrinkNetworkRuntime.cs b/Runtime/Core/ShrinkNetworkRuntime.cs
index 7c02afe..1ff64c5 100644
--- a/Runtime/Core/ShrinkNetworkRuntime.cs
+++ b/Runtime/Core/ShrinkNetworkRuntime.cs
@@ -1,17 +1,21 @@
+#if UNITY_5_3_OR_NEWER
 using UnityEngine;
+#endif
 
 namespace ShrinkNetwork
 {
     public static class ShrinkNetworkRuntime
     {
-        public static ShrinkNetworkService Default { get; private set; }
+        public static ShrinkNetworkService Default { get; private set; } = null!;
 
         static ShrinkNetworkRuntime()
         {
             RebuildDefault();
         }
 
+#if UNITY_5_3_OR_NEWER
         [RuntimeInitializeOnLoadMethod(RuntimeInitializeLoadType.SubsystemRegistration)]
+#endif
         private static void ResetOnPlayModeEnter()
         {
             RebuildDefault();
diff --git a/Runtime/Transport/Kcp/ShrinkKcpClientTransport.cs b/Runtime/Transport/Kcp/ShrinkKcpClientTransport.cs
index 3653244..f68bc70 100644
--- a/Runtime/Transport/Kcp/ShrinkKcpClientTransport.cs
+++ b/Runtime/Transport/Kcp/ShrinkKcpClientTransport.cs
@@ -1,3 +1,5 @@
+#nullable enable
+
 using System;
 using System.Net;
 using System.Net.Sockets;
@@ -16,14 +18,14 @@ namespace ShrinkNetwork
         private readonly ShrinkKcpTransportOptions _options;
         private readonly object _syncRoot = new();
 
-        private CancellationTokenSource _cts;
-        private UdpClient _udpClient;
-        private ShrinkKcpPeer _peer;
+        private CancellationTokenSource? _cts;
+        private UdpClient? _udpClient;
+        private ShrinkKcpPeer? _peer;
         private long _handshakeNonce;
         private int _started;
         private int _connected;
 
-        public ShrinkKcpClientTransport(string host, int port, ShrinkKcpTransportOptions options = null, long sessionId = 1)
+        public ShrinkKcpClientTransport(string host, int port, ShrinkKcpTransportOptions? options = null, long sessionId = 1)
         {
             if (string.IsNullOrWhiteSpace(host))
                 throw new ArgumentException("Host cannot be empty.", nameof(host));
@@ -39,7 +41,7 @@ namespace ShrinkNetwork
 
         public bool IsStarted => Volatile.Read(ref _started) == 1;
 
-        public event Action<ShrinkNetworkTransportEvent> OnEvent;
+        public event Action<ShrinkNetworkTransportEvent>? OnEvent;
 
         public void Start()
         {
@@ -52,7 +54,7 @@ namespace ShrinkNetwork
             _udpClient.Connect(_host, _port);
 
             _handshakeNonce = CreateHandshakeNonce();
-            ReceiveLoopAsync(_cts.Token).Forget();
+            ReceiveLoopAsync(_udpClient, _cts.Token).Forget();
             HandshakeLoopAsync(_cts.Token).Forget();
             UpdateLoopAsync(_cts.Token).Forget();
         }
@@ -145,7 +147,7 @@ namespace ShrinkNetwork
             }
         }
 
-        private async UniTaskVoid ReceiveLoopAsync(CancellationToken cancellationToken)
+        private async UniTaskVoid ReceiveLoopAsync(UdpClient udpClient, CancellationToken cancellationToken)
         {
             try
             {
@@ -154,7 +156,7 @@ namespace ShrinkNetwork
                     UdpReceiveResult result;
                     try
                     {
-                        result = await _udpClient.ReceiveAsync();
+                        result = await udpClient.ReceiveAsync();
                     }
                     catch (ObjectDisposedException)
                     {
diff --git a/Runtime/Transport/Kcp/ShrinkKcpServerTransport.cs b/Runtime/Transport/Kcp/ShrinkKcpServerTransport.cs
index 0b3abc4..87c482e 100644
--- a/Runtime/Transport/Kcp/ShrinkKcpServerTransport.cs
+++ b/Runtime/Transport/Kcp/ShrinkKcpServerTransport.cs
@@ -37,7 +37,6 @@ namespace ShrinkNetwork
         private CancellationTokenSource? _cts;
         private UdpClient? _udpClient;
         private long _sessionIdGenerator;
-        private int _conversationIdGenerator;
 
         public ShrinkKcpServerTransport(IPAddress listeningAddress, int port, ShrinkKcpTransportOptions? options = null)
         {
diff --git a/package.json b/package.json
index 129ec0d..f43644d 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "com.cneicy.shrink-network",
-  "version": "0.2.1",
+  "version": "0.3.0",
   "displayName": "ShrinkNetwork",
   "description": "面向 Unity 的轻量网络框架，提供会话、消息注册、RPC、权限控制，以及 TCP/KCP 传输抽象。",
   "unity": "2022.3",