Remove dependency on AppDomain from a hello world app (#95710)

* Remove dependency on AppDomain from a hello world app Saves 4% in size on a hello world. A hello world app will bring the `AppDomain` type with it. `AppDomain`'s `ToString` is quite expensive for what it does. * AppDomain trimming --------- Co-authored-by: Jan Kotas <jkotas@microsoft.com>
dotnet · Dec 8, 2023 · b995d33 · b995d33
1 parent 611934f
commit b995d33
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Showing 2 changed files with 21 additions and 9 deletions.
diff --git a/src/libraries/System.Private.CoreLib/src/System/AppContext.cs b/src/libraries/System.Private.CoreLib/src/System/AppContext.cs
@@ -89,17 +89,14 @@ internal static void OnFirstChanceException(object e)
         }
 #endif
 
-        internal static event EventHandler? ProcessExit;
-
         internal static void OnProcessExit()
         {
             AssemblyLoadContext.OnProcessExit();
             if (EventSource.IsSupported)
             {
                 EventListener.DisposeOnShutdown();
             }
-
-            ProcessExit?.Invoke(AppDomain.CurrentDomain, EventArgs.Empty);
+            AppDomain.OnProcessExit();
         }
 
         /// <summary>

diff --git a/src/libraries/System.Private.CoreLib/src/System/AppDomain.cs b/src/libraries/System.Private.CoreLib/src/System/AppDomain.cs
@@ -19,15 +19,28 @@ namespace System
 {
     public sealed partial class AppDomain : MarshalByRefObject
     {
-        private static readonly AppDomain s_domain = new AppDomain();
+        private static AppDomain? s_domain;
+
         private IPrincipal? _defaultPrincipal;
         private PrincipalPolicy _principalPolicy = PrincipalPolicy.NoPrincipal;
         private Func<IPrincipal>? s_getWindowsPrincipal;
         private Func<IPrincipal>? s_getUnauthenticatedPrincipal;
 
         private AppDomain() { }
 
-        public static AppDomain CurrentDomain => s_domain;
+        public static AppDomain CurrentDomain
+        {
+            get
+            {
+                // Create AppDomain instance only once external code asks for it. AppDomain brings lots of unnecessary
+                // dependencies into minimal trimmed app via ToString method.
+                if (s_domain == null)
+                {
+                    Interlocked.CompareExchange(ref s_domain, new AppDomain(), null);
+                }
+                return s_domain;
+            }
+        }
 
         public string BaseDirectory => AppContext.BaseDirectory;
 
@@ -72,10 +85,12 @@ public event EventHandler<FirstChanceExceptionEventArgs>? FirstChanceException
             remove { AppContext.FirstChanceException -= value; }
         }
 
-        public event EventHandler? ProcessExit
+        public event EventHandler? ProcessExit;
+
+        internal static void OnProcessExit()
         {
-            add { AppContext.ProcessExit += value; }
-            remove { AppContext.ProcessExit -= value; }
+            AppDomain? domain = s_domain;
+            domain?.ProcessExit?.Invoke(domain, EventArgs.Empty);
         }
 
         public string ApplyPolicy(string assemblyName)