EnterContextualReflection handle null (dotnet/coreclr#23953)

* EnterContextualReflection handle null

* Add ContextualReflection MockAssembly test

* Fix ContextualReflection typo

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

1 files changed, 12 insertions, 3 deletions

diff --git a/netcore/System.Private.CoreLib/shared/System/Runtime/Loader/AssemblyLoadContext.cs b/netcore/System.Private.CoreLib/shared/System/Runtime/Loader/AssemblyLoadContext.cs
index 4efd5dec1d5..70d5f10cf3d 100644
--- a/netcore/System.Private.CoreLib/shared/System/Runtime/Loader/AssemblyLoadContext.cs
+++ b/netcore/System.Private.CoreLib/shared/System/Runtime/Loader/AssemblyLoadContext.cs
@@ -509,9 +509,18 @@ namespace System.Runtime.Loader
         /// </remarks>
         public static ContextualReflectionScope EnterContextualReflection(Assembly activating)
         {
-            return activating != null ?
-                GetLoadContext(activating).EnterContextualReflection() :
-                new ContextualReflectionScope(null);
+            if (activating == null)
+                return new ContextualReflectionScope(null);
+
+            AssemblyLoadContext assemblyLoadContext = GetLoadContext(activating);
+
+            if (assemblyLoadContext == null)
+            {
+                // All RuntimeAssemblies & Only RuntimeAssemblies have an AssemblyLoadContext
+                throw new ArgumentException(SR.Arg_MustBeRuntimeAssembly, nameof(activating));
+            }
+
+            return assemblyLoadContext.EnterContextualReflection();
         }
 
         /// <summary>Opaque disposable struct used to restore CurrentContextualReflectionContext</summary>