Add SetEntryAssembly() API to System.Reflection

src/libraries/System.Configuration.ConfigurationManager/tests/System/Configuration/CustomHostTests.cs

@@ -36,7 +36,7 @@ public void FilePathIsPopulatedCorrectly()
         private static void MakeAssemblyGetEntryAssemblyReturnNull()
         {
             typeof(Assembly)
-                .GetField("s_forceNullEntryPoint", BindingFlags.NonPublic | BindingFlags.Static)
+                .GetField("s_overriddenEntryAssembly", BindingFlags.NonPublic | BindingFlags.Static)
                 .SetValue(null, true);
 
             Assert.Null(Assembly.GetEntryAssembly());

src/libraries/System.Diagnostics.TraceSource/tests/System.Diagnostics.TraceSource.Tests/DefaultTraceListenerClassTests.cs

@@ -174,7 +174,7 @@ public void EntryAssemblyName_Null_NotIncludedInTrace()
         private static void MakeAssemblyGetEntryAssemblyReturnNull()
         {
             typeof(Assembly)
-                .GetField("s_forceNullEntryPoint", BindingFlags.NonPublic | BindingFlags.Static)
+                .GetField("s_overriddenEntryAssembly", BindingFlags.NonPublic | BindingFlags.Static)
                 .SetValue(null, true);
 
             Assert.Null(Assembly.GetEntryAssembly());

src/libraries/System.Private.CoreLib/src/System/Reflection/Assembly.cs

@@ -217,13 +217,31 @@ public override string ToString()
             return type.Module?.Assembly;
         }
 
-        // internal test hook
-        private static bool s_forceNullEntryPoint;
+        private static Assembly? s_overriddenEntryAssembly;
+
+        /// <summary>
+        /// Sets the application's entry assembly to the provided assembly object
+        /// as argument.
+        /// </summary>
+        /// <param name="assembly">
+        /// Assembly object that represents the application's new entry assembly.
+        /// </param>
+        /// <remarks>
+        /// It is important to mention that the assembly passed to this function
+        /// has to be a RuntimeAssembly type. Otherwise, an exception will be thrown.
+        /// </remarks>
+        public static void SetEntryAssembly(Assembly? assembly)
+        {
+            if (assembly is not RuntimeAssembly)
+                throw new ArgumentException(SR.Argument_MustBeRuntimeAssembly);
+
+            s_overriddenEntryAssembly = assembly;
+        }
 
         public static Assembly? GetEntryAssembly()
         {
-            if (s_forceNullEntryPoint)
-                return null;
+            if (s_overriddenEntryAssembly is not null)
+                return s_overriddenEntryAssembly;
 
             return GetEntryAssemblyInternal();
         }

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -11157,6 +11157,7 @@ public virtual void GetObjectData(System.Runtime.Serialization.SerializationInfo
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("Types and members the loaded assembly depends on might be removed")]
         [System.ObsoleteAttribute("ReflectionOnly loading is not supported and throws PlatformNotSupportedException.", DiagnosticId="SYSLIB0018", UrlFormat="https://aka.ms/dotnet-warnings/{0}")]
         public static System.Reflection.Assembly ReflectionOnlyLoadFrom(string assemblyFile) { throw null; }
+        public static void SetEntryAssembly(System.Reflection.Assembly? assembly) { throw null; }
         public override string ToString() { throw null; }
         [System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute("Types and members the loaded assembly depends on might be removed")]
         public static System.Reflection.Assembly UnsafeLoadFrom(string assemblyFile) { throw null; }