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_overriddenEntryAssembly", 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_overriddenEntryAssembly", BindingFlags.NonPublic | BindingFlags.Static)
-                .SetValue(null, true);
-
+            Assembly.SetEntryAssembly(null);
             Assert.Null(Assembly.GetEntryAssembly());
         }
     }

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

@@ -217,7 +217,7 @@ public override string ToString()
             return type.Module?.Assembly;
         }
 
-        private static Assembly? s_overriddenEntryAssembly;
+        private static object? s_overriddenEntryAssembly;
 
         /// <summary>
         /// Sets the application's entry assembly to the provided assembly object
@@ -228,10 +228,17 @@ public override string ToString()
         /// </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.
+        /// 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 = new object();
+                return;
+            }
+
             if (assembly is not RuntimeAssembly)
                 throw new ArgumentException(SR.Argument_MustBeRuntimeAssembly);
 
@@ -241,7 +248,7 @@ public static void SetEntryAssembly(Assembly? assembly)
         public static Assembly? GetEntryAssembly()
         {
             if (s_overriddenEntryAssembly is not null)
-                return s_overriddenEntryAssembly;
+                return s_overriddenEntryAssembly as Assembly;
 
             return GetEntryAssemblyInternal();
         }

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

@@ -181,6 +181,19 @@ public void GetEntryAssembly()
             Assert.True(correct, $"Unexpected assembly name {assembly}");
         }
 
+        [Fact]
+        public void SetEntryAssembly()
+        {
+            Assembly? originalAssembly = Assembly.GetEntryAssembly();
+            Assert.NotNull(originalAssembly);
+
+            Assembly.SetEntryAssembly(null);
+            Assert.Null(Assembly.GetEntryAssembly());
+
+            Assembly.SetEntryAssembly(originalAssembly);
+            Assert.Equal(Assembly.GetEntryAssembly(), originalAssembly);
+        }
+
         [Fact]
         public void GetFile()
         {