diff --git a/Shadowsocks.PAC/PACDaemon.cs b/Shadowsocks.PAC/PACDaemon.cs
index 40a7b7dc1..ce0a8b7c3 100644
--- a/Shadowsocks.PAC/PACDaemon.cs
+++ b/Shadowsocks.PAC/PACDaemon.cs
@@ -16,11 +16,11 @@ public class PACDaemon : IEnableLogger
         public const string USER_RULE_FILE = "user-rule.txt";
         public const string USER_ABP_FILE = "abp.txt";
 
-        FileSystemWatcher PACFileWatcher;
-        FileSystemWatcher UserRuleFileWatcher;
+        FileSystemWatcher? PACFileWatcher;
+        FileSystemWatcher? UserRuleFileWatcher;
 
-        public event EventHandler PACFileChanged;
-        public event EventHandler UserRuleFileChanged;
+        public event EventHandler? PACFileChanged;
+        public event EventHandler? UserRuleFileChanged;
 
         private PACSettings _PACSettings;
         private GeositeUpdater _geositeUpdater;
diff --git a/Shadowsocks.PAC/PACServer.cs b/Shadowsocks.PAC/PACServer.cs
index 79c5b3f73..b77a633eb 100644
--- a/Shadowsocks.PAC/PACServer.cs
+++ b/Shadowsocks.PAC/PACServer.cs
@@ -124,7 +124,7 @@ public override bool Handle(byte[] firstPacket, int length, Socket socket, objec
                     {
                         if (kv[0] == "Host")
                         {
-                            if (kv[1].Trim() == ((IPEndPoint)socket.LocalEndPoint).ToString())
+                            if (kv[1].Trim() == (socket.LocalEndPoint as IPEndPoint)?.ToString())
                             {
                                 hostMatch = true;
                             }
@@ -164,7 +164,7 @@ public void SendResponse(Socket socket, bool useSocks)
         {
             try
             {
-                IPEndPoint localEndPoint = (IPEndPoint)socket.LocalEndPoint;
+                IPEndPoint localEndPoint = socket.LocalEndPoint as IPEndPoint ?? throw new ArgumentException("Invalid socket local endpoint.", nameof(socket));
 
                 string proxy = GetPACAddress(localEndPoint, useSocks);
 
diff --git a/Shadowsocks.WPF/Localization/LocalizationProvider.cs b/Shadowsocks.WPF/Localization/LocalizationProvider.cs
index 41358fedc..41d7f2bc6 100644
--- a/Shadowsocks.WPF/Localization/LocalizationProvider.cs
+++ b/Shadowsocks.WPF/Localization/LocalizationProvider.cs
@@ -5,7 +5,7 @@ namespace Shadowsocks.WPF.Localization
 {
     public class LocalizationProvider
     {
-        private static readonly string CallingAssemblyName = Assembly.GetCallingAssembly().GetName().Name;
+        private static readonly string CallingAssemblyName = Assembly.GetCallingAssembly().GetName().Name ?? "Shadowsocks.WPF";
 
         public static T GetLocalizedValue<T>(string key) => LocExtension.GetLocalizedValue<T>($"{CallingAssemblyName}:Strings:{key}");
     }
diff --git a/Shadowsocks.WPF/Services/PortForwarder.cs b/Shadowsocks.WPF/Services/PortForwarder.cs
index b8c363c6c..046e369e2 100644
--- a/Shadowsocks.WPF/Services/PortForwarder.cs
+++ b/Shadowsocks.WPF/Services/PortForwarder.cs
@@ -6,6 +6,7 @@
 
 namespace Shadowsocks.WPF.Services
 {
+#nullable disable
     public class PortForwarder : StreamService
     {
         private readonly int _targetPort;
diff --git a/Shadowsocks.WPF/Services/Sip003Plugin.cs b/Shadowsocks.WPF/Services/Sip003Plugin.cs
index 2f1369582..c57f61275 100644
--- a/Shadowsocks.WPF/Services/Sip003Plugin.cs
+++ b/Shadowsocks.WPF/Services/Sip003Plugin.cs
@@ -12,7 +12,7 @@ namespace Shadowsocks.WPF.Services
     // https://github.com/shadowsocks/shadowsocks-org/wiki/Plugin
     public sealed class Sip003Plugin : IDisposable
     {
-        public IPEndPoint LocalEndPoint { get; private set; }
+        public IPEndPoint? LocalEndPoint { get; private set; }
         public int ProcessId => _started ? _pluginProcess.Id : 0;
 
         private readonly object _startProcessLock = new object();
@@ -20,7 +20,7 @@ public sealed class Sip003Plugin : IDisposable
         private bool _started;
         private bool _disposed;
 
-        public static Sip003Plugin CreateIfConfigured(Server server, bool showPluginOutput)
+        public static Sip003Plugin? CreateIfConfigured(Server server, bool showPluginOutput)
         {
             if (server == null)
             {
@@ -119,7 +119,7 @@ public bool StartIfNeeded()
             return true;
         }
 
-        public string ExpandEnvironmentVariables(string name, StringDictionary environmentVariables = null)
+        public string ExpandEnvironmentVariables(string name, StringDictionary? environmentVariables = null)
         {
             // Expand the environment variables from the new process itself
             if (environmentVariables != null)
diff --git a/Shadowsocks.WPF/Services/SystemProxy/RAS.cs b/Shadowsocks.WPF/Services/SystemProxy/RAS.cs
index b60b56e92..50dfb5129 100644
--- a/Shadowsocks.WPF/Services/SystemProxy/RAS.cs
+++ b/Shadowsocks.WPF/Services/SystemProxy/RAS.cs
@@ -42,7 +42,7 @@ public class RAS
             // pointer to full path and file name of phone-book file
             string lpszPhonebook,
             // buffer to receive phone-book entries
-            [In, Out] RasEntryName[] lprasentryname,
+            [In, Out] RasEntryName[]? lprasentryname,
             // size in bytes of buffer
             ref int lpcb,
             // number of entries written to buffer
@@ -55,14 +55,14 @@ public static string[] GetAllConnections()
             int entryNameSize = 0;
             int lpSize = 0;
             uint retval = ESuccess;
-            RasEntryName[] names = null;
+            RasEntryName[] names = Array.Empty<RasEntryName>();
 
             entryNameSize = Marshal.SizeOf(typeof(RasEntryName));
 
             // Windows Vista or later:  To determine the required buffer size, call RasEnumEntries
             // with lprasentryname set to NULL. The variable pointed to by lpcb should be set to zero.
             // The function will return the required buffer size in lpcb and an error code of ERROR_BUFFER_TOO_SMALL.
-            retval = RasEnumEntries(null, null, null, ref lpSize, out lpNames);
+            retval = RasEnumEntries("", "", null, ref lpSize, out lpNames);
             if (retval == EBufferTooSmall)
             {
                 names = new RasEntryName[lpNames];
@@ -71,7 +71,7 @@ public static string[] GetAllConnections()
                     names[i].dwSize = entryNameSize;
                 }
 
-                retval = RasEnumEntries(null, null, names, ref lpSize, out lpNames);
+                retval = RasEnumEntries("", "", names, ref lpSize, out lpNames);
             }
 
             if (retval == ESuccess)
diff --git a/Shadowsocks.WPF/Services/SystemProxy/WinINet.cs b/Shadowsocks.WPF/Services/SystemProxy/WinINet.cs
index de90a6f89..2f80e465c 100644
--- a/Shadowsocks.WPF/Services/SystemProxy/WinINet.cs
+++ b/Shadowsocks.WPF/Services/SystemProxy/WinINet.cs
@@ -136,7 +136,7 @@ static WinINet()
         {
             try
             {
-                Record();
+                initialSetting = Query();
             }
             catch (DllNotFoundException)
             {
@@ -153,9 +153,14 @@ static WinINet()
                 }
                 else
                 {
-                    throw we;
+                    throw;
                 }
             }
+            finally
+            {
+                if (initialSetting == null)
+                    initialSetting = new();
+            }
         }
 
         public static void ProxyGlobal(string server, string bypass)
@@ -188,11 +193,6 @@ public static void Direct()
             Exec(options);
         }
 
-        private static void Record()
-        {
-            initialSetting ??= Query();
-        }
-
         public static void Restore()
         {
             Set(initialSetting);
@@ -231,7 +231,7 @@ public static WinINetSetting Query()
                 new InternetPerConnectionOption{dwOption = (int)InternetPerConnectionOptionEnum.AutoConfigUrl},
             };
 
-            (IntPtr unmanagedList, int listSize) = PrepareOptionList(options, null);
+            (IntPtr unmanagedList, int listSize) = PrepareOptionList(options, "");
             bool ok = InternetQueryOption(IntPtr.Zero, (int)InternetOptions.PerConnectionOption, unmanagedList, ref listSize);
             if (!ok)
             {
@@ -260,13 +260,13 @@ public static WinINetSetting Query()
                         proxy.Flags = (InternetPerConnectionFlags)o.Value.dwValue;
                         break;
                     case InternetPerConnectionOptionEnum.AutoConfigUrl:
-                        proxy.AutoConfigUrl = Marshal.PtrToStringAuto(o.Value.pszValue);
+                        proxy.AutoConfigUrl = Marshal.PtrToStringAuto(o.Value.pszValue) ?? "";
                         break;
                     case InternetPerConnectionOptionEnum.ProxyBypass:
-                        proxy.ProxyBypass = Marshal.PtrToStringAuto(o.Value.pszValue);
+                        proxy.ProxyBypass = Marshal.PtrToStringAuto(o.Value.pszValue) ?? "";
                         break;
                     case InternetPerConnectionOptionEnum.ProxyServer:
-                        proxy.ProxyServer = Marshal.PtrToStringAuto(o.Value.pszValue);
+                        proxy.ProxyServer = Marshal.PtrToStringAuto(o.Value.pszValue) ?? "";
                         break;
                     default:
                         break;
@@ -343,7 +343,7 @@ private static void ClearOptionList(IntPtr list)
 
         private static void Exec(List<InternetPerConnectionOption> options)
         {
-            Exec(options, null);
+            Exec(options, "");
             foreach (string conn in RAS.GetAllConnections())
             {
                 Exec(options, conn);
diff --git a/Shadowsocks.WPF/Utils/AutoStartup.cs b/Shadowsocks.WPF/Utils/AutoStartup.cs
index 64ee77da5..b0f242a89 100644
--- a/Shadowsocks.WPF/Utils/AutoStartup.cs
+++ b/Shadowsocks.WPF/Utils/AutoStartup.cs
@@ -15,7 +15,7 @@ public static class AutoStartup
 
         public static bool Set(bool enabled)
         {
-            RegistryKey runKey = null;
+            RegistryKey? runKey = null;
             try
             {
                 runKey = Registry.CurrentUser.CreateSubKey(registryRunKey, RegistryKeyPermissionCheck.ReadWriteSubTree);
@@ -26,7 +26,7 @@ public static bool Set(bool enabled)
                 }
                 if (enabled)
                 {
-                    runKey.SetValue(Key, Process.GetCurrentProcess().MainModule?.FileName);
+                    runKey.SetValue(Key, Utilities.ExecutablePath);
                 }
                 else
                 {
@@ -60,7 +60,7 @@ public static bool Set(bool enabled)
 
         public static bool Check()
         {
-            RegistryKey runKey = null;
+            RegistryKey? runKey = null;
             try
             {
                 runKey = Registry.CurrentUser.CreateSubKey(registryRunKey, RegistryKeyPermissionCheck.ReadWriteSubTree);
@@ -78,7 +78,7 @@ public static bool Check()
                         continue;
                     }
                     // Remove other startup keys with the same executable path. fixes #3011 and also assures compatibility with older versions
-                    if (Utilities.ExecutablePath.Equals(runKey.GetValue(valueName).ToString(), StringComparison.InvariantCultureIgnoreCase)
+                    if (Utilities.ExecutablePath.Equals(runKey.GetValue(valueName)?.ToString(), StringComparison.InvariantCultureIgnoreCase)
                         is bool matchedDuplicate && matchedDuplicate)
                     {
                         runKey.DeleteValue(valueName);
diff --git a/Shadowsocks.WPF/Utils/FileManager.cs b/Shadowsocks.WPF/Utils/FileManager.cs
index 3e69b0fa7..377b405aa 100644
--- a/Shadowsocks.WPF/Utils/FileManager.cs
+++ b/Shadowsocks.WPF/Utils/FileManager.cs
@@ -59,7 +59,7 @@ public static string NonExclusiveReadAllText(string path, Encoding encoding)
             catch (Exception ex)
             {
                 LogHost.Default.Error(ex, "");
-                throw ex;
+                throw;
             }
         }
     }
diff --git a/Shadowsocks.WPF/Utils/IPCService.cs b/Shadowsocks.WPF/Utils/IPCService.cs
index 6e3ad4e6d..42fab6840 100644
--- a/Shadowsocks.WPF/Utils/IPCService.cs
+++ b/Shadowsocks.WPF/Utils/IPCService.cs
@@ -21,7 +21,7 @@ internal class IPCService
         private const int OP_OPEN_URL = 1;
         private static readonly string PIPE_PATH = $"Shadowsocks\\{Utilities.ExecutablePath.GetHashCode()}";
 
-        public event EventHandler<RequestAddUrlEventArgs> OpenUrlRequested;
+        public event EventHandler<RequestAddUrlEventArgs>? OpenUrlRequested;
 
         public async void RunServer()
         {
diff --git a/Shadowsocks.WPF/Utils/ProtocolHandler.cs b/Shadowsocks.WPF/Utils/ProtocolHandler.cs
index 9a0dbd2b4..ca2e35024 100644
--- a/Shadowsocks.WPF/Utils/ProtocolHandler.cs
+++ b/Shadowsocks.WPF/Utils/ProtocolHandler.cs
@@ -10,7 +10,7 @@ static class ProtocolHandler
 
         public static bool Set(bool enabled)
         {
-            RegistryKey ssURLAssociation = null;
+            RegistryKey? ssURLAssociation = null;
 
             try
             {
@@ -59,7 +59,7 @@ public static bool Set(bool enabled)
 
         public static bool Check()
         {
-            RegistryKey ssURLAssociation = null;
+            RegistryKey? ssURLAssociation = null;
             try
             {
                 ssURLAssociation = Registry.CurrentUser.OpenSubKey(ssURLRegKey, true);
@@ -69,8 +69,8 @@ public static bool Check()
                     return false;
                 }
 
-                var shellOpen = ssURLAssociation.OpenSubKey("shell").OpenSubKey("open").OpenSubKey("command");
-                return (string)shellOpen.GetValue("") == $"{Utilities.ExecutablePath} --open-url %1";
+                var shellOpen = ssURLAssociation.OpenSubKey("shell")?.OpenSubKey("open")?.OpenSubKey("command");
+                return shellOpen?.GetValue("") as string == $"{Utilities.ExecutablePath} --open-url %1";
             }
             catch (Exception e)
             {
diff --git a/Shadowsocks.WPF/Views/ServerSharingView.xaml.cs b/Shadowsocks.WPF/Views/ServerSharingView.xaml.cs
index e4288a3a4..bf695e985 100644
--- a/Shadowsocks.WPF/Views/ServerSharingView.xaml.cs
+++ b/Shadowsocks.WPF/Views/ServerSharingView.xaml.cs
@@ -32,7 +32,7 @@ public ServerSharingView()
                     view => view.urlTextBox.Text)
                     .DisposeWith(disposables);
 
-                this.BindCommand(ViewModel,
+                this.BindCommand(ViewModel!,
                     viewModel => viewModel.CopyLink,
                     view => view.copyLinkButton)
                     .DisposeWith(disposables);
diff --git a/Shadowsocks.WPF/Views/VersionUpdatePromptView.xaml.cs b/Shadowsocks.WPF/Views/VersionUpdatePromptView.xaml.cs
index ffaddbcd6..7e5404aa9 100644
--- a/Shadowsocks.WPF/Views/VersionUpdatePromptView.xaml.cs
+++ b/Shadowsocks.WPF/Views/VersionUpdatePromptView.xaml.cs
@@ -21,15 +21,15 @@ public VersionUpdatePromptView()
                     view => releaseNotesMarkdownScrollViewer.Markdown)
                     .DisposeWith(disposables);*/
 
-                this.BindCommand(ViewModel,
+                this.BindCommand(ViewModel!,
                     viewModel => viewModel.Update,
                     view => view.updateButton)
                     .DisposeWith(disposables);
-                this.BindCommand(ViewModel,
+                this.BindCommand(ViewModel!,
                     viewModel => viewModel.SkipVersion,
                     view => view.skipVersionButton)
                     .DisposeWith(disposables);
-                this.BindCommand(ViewModel,
+                this.BindCommand(ViewModel!,
                     viewModel => viewModel.NotNow,
                     view => view.notNowButton)
                     .DisposeWith(disposables);