Set AssemblyName.ProcessorArchitecture for compatibility.

src/coreclr/System.Private.CoreLib/src/System/Reflection/AssemblyName.CoreCLR.cs

@@ -106,46 +106,41 @@ internal AssemblyNameFlags RawFlags
         internal void SetProcArchIndex(PortableExecutableKinds pek, ImageFileMachine ifm)
         {
 #pragma warning disable SYSLIB0037 // AssemblyName.ProcessorArchitecture is obsolete
-            ProcessorArchitecture = CalculateProcArchIndex(pek, ifm, _flags);
+            ProcessorArchitecture = CalculateProcArch(pek, ifm, _flags);
 #pragma warning restore SYSLIB0037
         }
 
-        private static ProcessorArchitecture CalculateProcArchIndex(PortableExecutableKinds pek, ImageFileMachine ifm, AssemblyNameFlags flags)
+        private static ProcessorArchitecture CalculateProcArch(PortableExecutableKinds pek, ImageFileMachine ifm, AssemblyNameFlags aFlags)
         {
-            if (((uint)flags & 0xF0) == 0x70)
+            // 0x70 specifies "reference assembly".
+            // For these, CLR wants to return None as arch so they can be always loaded, regardless of process type.
+            if (((uint)aFlags & 0xF0) == 0x70)
                 return ProcessorArchitecture.None;
 
-            if ((pek & PortableExecutableKinds.PE32Plus) == PortableExecutableKinds.PE32Plus)
+            switch (ifm)
             {
-                switch (ifm)
-                {
-                    case ImageFileMachine.IA64:
-                        return ProcessorArchitecture.IA64;
-                    case ImageFileMachine.AMD64:
-                        return ProcessorArchitecture.Amd64;
-                    case ImageFileMachine.I386:
-                        if ((pek & PortableExecutableKinds.ILOnly) == PortableExecutableKinds.ILOnly)
+                case ImageFileMachine.IA64:
+                    return ProcessorArchitecture.IA64;
+                case ImageFileMachine.ARM:
+                    return ProcessorArchitecture.Arm;
+                case ImageFileMachine.AMD64:
+                    return ProcessorArchitecture.Amd64;
+                case ImageFileMachine.I386:
+                    {
+                        if ((pek & PortableExecutableKinds.ILOnly) != 0 &&
+                            (pek & PortableExecutableKinds.Required32Bit) == 0)
+                        {
+                            // platform neutral.
                             return ProcessorArchitecture.MSIL;
-                        break;
-                }
-            }
-            else
-            {
-                if (ifm == ImageFileMachine.I386)
-                {
-                    if ((pek & PortableExecutableKinds.Required32Bit) == PortableExecutableKinds.Required32Bit)
-                        return ProcessorArchitecture.X86;
+                        }
 
-                    if ((pek & PortableExecutableKinds.ILOnly) == PortableExecutableKinds.ILOnly)
-                        return ProcessorArchitecture.MSIL;
-
-                    return ProcessorArchitecture.X86;
-                }
-                if (ifm == ImageFileMachine.ARM)
-                {
-                    return ProcessorArchitecture.Arm;
-                }
+                        // requires x86
+                        return ProcessorArchitecture.X86;
+                    }
             }
+
+            // ProcessorArchitecture is a legacy API and does not cover other Machine kinds.
+            // For example ARM64 is not expressible
             return ProcessorArchitecture.None;
         }
 

src/libraries/System.Reflection.Metadata/src/System/Reflection/Metadata/MetadataReader.netstandard.cs

@@ -85,6 +85,12 @@ public static unsafe AssemblyName GetAssemblyName(string assemblyFile)
                     peReader = new PEReader((byte*)safeBuffer.DangerousGetHandle(), (int)safeBuffer.ByteLength);
                     MetadataReader mdReader = peReader.GetMetadataReader(MetadataReaderOptions.None);
                     AssemblyName assemblyName = mdReader.GetAssemblyDefinition().GetAssemblyName();
+
+                    AssemblyFlags aFlags = mdReader.AssemblyTable.GetFlags();
+#pragma warning disable SYSLIB0037 // AssemblyName.ProcessorArchitecture is obsolete
+                    assemblyName.ProcessorArchitecture = CalculateProcArch(peReader, aFlags);
+#pragma warning restore SYSLIB0037
+
                     return assemblyName;
                 }
                 finally
@@ -101,6 +107,42 @@ public static unsafe AssemblyName GetAssemblyName(string assemblyFile)
             }
         }
 
+        private static ProcessorArchitecture CalculateProcArch(PEReader peReader, AssemblyFlags aFlags)
+        {
+            // 0x70 specifies "reference assembly".
+            // For these, CLR wants to return None as arch so they can be always loaded, regardless of process type.
+            if (((uint)aFlags & 0xF0) == 0x70)
+                return ProcessorArchitecture.None;
+
+            PEHeaders peHeaders = peReader.PEHeaders;
+            switch (peHeaders.CoffHeader.Machine)
+            {
+                case Machine.IA64:
+                    return ProcessorArchitecture.IA64;
+                case Machine.Arm:
+                    return ProcessorArchitecture.Arm;
+                case Machine.Amd64:
+                    return ProcessorArchitecture.Amd64;
+                case Machine.I386:
+                    {
+                        CorFlags flags = peHeaders.CorHeader!.Flags;
+                        if ((flags & CorFlags.ILOnly) != 0 &&
+                            (flags & CorFlags.Requires32Bit) == 0)
+                        {
+                            // platform neutral.
+                            return ProcessorArchitecture.MSIL;
+                        }
+
+                        // requires x86
+                        return ProcessorArchitecture.X86;
+                    }
+            }
+
+            // ProcessorArchitecture is a legacy API and does not cover other Machine kinds.
+            // For example ARM64 is not expressible
+            return ProcessorArchitecture.None;
+        }
+
         private static AssemblyNameFlags GetAssemblyNameFlags(AssemblyFlags flags)
         {
             AssemblyNameFlags assemblyNameFlags = AssemblyNameFlags.None;

src/libraries/System.Reflection.Metadata/tests/Metadata/MetadataReaderTests.cs

@@ -3091,7 +3091,12 @@ public void GetAssemblyName()
             if (PlatformDetection.HasAssemblyFiles)
             {
                 Assembly a = typeof(MetadataReaderTests).Assembly;
-                Assert.Equal(new AssemblyName(a.FullName).ToString(), MetadataReader.GetAssemblyName(AssemblyPathHelper.GetAssemblyLocation(a)).ToString());
+                AssemblyName name = MetadataReader.GetAssemblyName(AssemblyPathHelper.GetAssemblyLocation(a));
+                Assert.Equal(new AssemblyName(a.FullName).ToString(), name.ToString());
+
+#pragma warning disable SYSLIB0037 // AssemblyName.ProcessorArchitecture is obsolete
+                Assert.Equal(ProcessorArchitecture.MSIL, name.ProcessorArchitecture);
+#pragma warning restore SYSLIB0037
             }
         }
     }