Add SetEntryAssembly() API to System.Reflection

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

@@ -31,14 +31,12 @@ public void FilePathIsPopulatedCorrectly()
         }
 
         /// <summary>
-        /// Makes Assembly.GetEntryAssembly() return null using private reflection.
+        /// Makes Assembly.GetEntryAssembly() return null by passing the null literal
+        /// to Assembly.SetEntryAssembly().
         /// </summary>
         private static void MakeAssemblyGetEntryAssemblyReturnNull()
         {
-            typeof(Assembly)
-                .GetField("s_forceNullEntryPoint", BindingFlags.NonPublic | BindingFlags.Static)
-                .SetValue(null, true);
-
+            Assembly.SetEntryAssembly(null);
             Assert.Null(Assembly.GetEntryAssembly());
         }
     }

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

@@ -169,14 +169,12 @@ public void EntryAssemblyName_Null_NotIncludedInTrace()
         }
 
         /// <summary>
-        /// Makes Assembly.GetEntryAssembly() return null using private reflection.
+        /// Makes Assembly.GetEntryAssembly() return null by passing the null literal
+        /// to Assembly.SetEntryAssembly().
         /// </summary>
         private static void MakeAssemblyGetEntryAssemblyReturnNull()
         {
-            typeof(Assembly)
-                .GetField("s_forceNullEntryPoint", BindingFlags.NonPublic | BindingFlags.Static)
-                .SetValue(null, true);
-
+            Assembly.SetEntryAssembly(null);
             Assert.Null(Assembly.GetEntryAssembly());
         }
     }

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

@@ -217,13 +217,38 @@ public override string ToString()
             return type.Module?.Assembly;
         }
 
-        // internal test hook
-        private static bool s_forceNullEntryPoint;
+        private static object? 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 runtime defined Assembly type object. Otherwise, an exception
+        /// will be thrown.
+        /// </remarks>
+        public static void SetEntryAssembly(Assembly? assembly)
+        {
+            if (assembly is null)
+            {
+                s_overriddenEntryAssembly = string.Empty;
+                return;
+            }
+
+            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 as Assembly;
 
             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; }

src/libraries/System.Runtime/tests/System.Reflection.Tests/AssemblyTests.cs

@@ -7,10 +7,12 @@
 using System.Globalization;
 using System.IO;
 using System.Linq;
+using System.Reflection.Emit;
 using System.Reflection.Tests;
 using System.Runtime.CompilerServices;
 using System.Security;
 using System.Text;
+using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 
 [assembly:
@@ -181,6 +183,32 @@
             Assert.True(correct, $"Unexpected assembly name {assembly}");
         }
 
+        [Fact]
+        public void SetEntryAssembly()
+        {
+            Assert.NotNull(Assembly.GetEntryAssembly());
+
+            RemoteExecutor.Invoke(() =>
+            {
+                Assembly.SetEntryAssembly(null);
+                Assert.Null(Assembly.GetEntryAssembly());
+
+                Assembly originalAssembly = typeof(AssemblyTests).Assembly;
+
+                Assembly.SetEntryAssembly(originalAssembly);
+                Assert.Equal(Assembly.GetEntryAssembly(), originalAssembly);
+
+                var invalidAssembly = new PersistedAssemblyBuilder(
+                    new AssemblyName("NotaRuntimeAssemblyTest"),
+                    typeof(object).Assembly
+                );
+
+                Assert.Throws<ArgumentException>(
+                    () => Assembly.SetEntryAssembly(invalidAssembly)
+                );
+            }).Dispose();
+        }
+
         [Fact]
         public void GetFile()
         {