Update `GetPinnableReference()` usage in multi-step custom type marshalling (#61539)

* Update GetPinnableReference() usage in multi-step custom tyep marshalling

Change GetPinnableReference() on a marshaller type to be used as a side effect when marshalling in all cases when a fixed() statement is usable.

Use the Value property getter to get the value to pass to native in all cases.

Update our marshallers and tests to follow this design update.

* Apply suggestions from code review

Co-authored-by: Aaron Robinson <arobins@microsoft.com>

* Don't emit the assingment to the Value property in the Unmarshal stage when using [Out]

* Fix the unmarshalling conditions

* Fix Utf16StringMarshaller to correctly handle the "null pointer" case now that we're using spans internally for storage in all cases.

Co-authored-by: Aaron Robinson <arobins@microsoft.com>

1 files changed, 9 insertions, 19 deletions

diff --git a/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/SpanMarshallers.cs b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/SpanMarshallers.cs
index 453259feeda..d5543df3c88 100644
--- a/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/SpanMarshallers.cs
+++ b/src/libraries/System.Runtime.InteropServices/tests/Ancillary.Interop/SpanMarshallers.cs
@@ -20,20 +20,8 @@ namespace System.Runtime.InteropServices.GeneratedMarshalling
         }
 
         public ReadOnlySpanMarshaller(ReadOnlySpan<T> managed, int sizeOfNativeElement)
+            :this(managed, Span<byte>.Empty, sizeOfNativeElement)
         {
-            _allocatedMemory = default;
-            _sizeOfNativeElement = sizeOfNativeElement;
-            if (managed.Length == 0)
-            {
-                _managedSpan = default;
-                NativeValueStorage = default;
-                return;
-            }
-            _managedSpan = managed;
-            _sizeOfNativeElement = sizeOfNativeElement;
-            int spaceToAllocate = managed.Length * sizeOfNativeElement;
-            _allocatedMemory = Marshal.AllocCoTaskMem(spaceToAllocate);
-            NativeValueStorage = new Span<byte>((void*)_allocatedMemory, spaceToAllocate);
         }
 
         public ReadOnlySpanMarshaller(ReadOnlySpan<T> managed, Span<byte> stackSpace, int sizeOfNativeElement)
@@ -81,8 +69,7 @@ namespace System.Runtime.InteropServices.GeneratedMarshalling
         {
             get
             {
-                Debug.Assert(_managedSpan.IsEmpty || _allocatedMemory != IntPtr.Zero);
-                return (byte*)_allocatedMemory;
+                return (byte*)Unsafe.AsPointer(ref GetPinnableReference());
             }
             set
             {
@@ -222,7 +209,7 @@ namespace System.Runtime.InteropServices.GeneratedMarshalling
             {
                 if (_inner.ManagedValues.Length == 0)
                 {
-                    return (byte*)0x1;
+                    return (byte*)0xa5a5a5a5;
                 }
                 return _inner.Value;
             }
@@ -293,7 +280,7 @@ namespace System.Runtime.InteropServices.GeneratedMarshalling
             {
                 if (_inner.ManagedValues.Length == 0)
                 {
-                    return (byte*)0x1;
+                    return (byte*)0xa5a5a5a5;
                 }
                 return _inner.Value;
             }
@@ -369,8 +356,11 @@ namespace System.Runtime.InteropServices.GeneratedMarshalling
         {
             get
             {
-                Debug.Assert(_data.IsEmpty || _allocatedMemory != null);
-                return _allocatedMemory;
+                if (_allocatedMemory  != null)
+                {
+                    return _allocatedMemory;
+                }
+                return (T*)Unsafe.AsPointer(ref GetPinnableReference());
             }
             set
             {