Fix GC hole in string marshaling

src/WinRT.Runtime/ApiCompatBaseline.net5.0.txt

@@ -1 +1,2 @@
-Total Issues: 0
+MembersMustExist : Member 'public WinRT.MarshalString.HStringHeader WinRT.MarshalString.HStringHeader WinRT.MarshalString._header' does not exist in the implementation but it does exist in the contract.
+TypesMustExist : Type 'WinRT.MarshalString.HStringHeader' does not exist in the implementation but it does exist in the contract.

src/WinRT.Runtime/Marshalers.cs

@@ -32,17 +32,15 @@ public static void Dispose(this GCHandle handle)
     // as well as passing marshalers to FromAbi by ref so they can be conditionally disposed.
     public class MarshalString
     {
-        public unsafe struct HStringHeader // sizeof(HSTRING_HEADER)
-        {
-            public fixed byte Reserved[24];
-        };
-        public HStringHeader _header;
+        private IntPtr _header;
         public GCHandle _gchandle;
         public IntPtr _handle;
 
         public void Dispose()
         {
             _gchandle.Dispose();
+            Marshal.FreeHGlobal(_header);
+            _header = IntPtr.Zero;
         }
 
         public static unsafe MarshalString CreateMarshaler(string value)
@@ -54,10 +52,11 @@ public static unsafe MarshalString CreateMarshaler(string value)
             try
             {
                 m._gchandle = GCHandle.Alloc(value, GCHandleType.Pinned);
-                fixed (void* chars = value, header = &m._header, handle = &m._handle)
+                m._header = Marshal.AllocHGlobal(24); // sizeof(HSTRING_HEADER)
+                fixed (void* chars = value, handle = &m._handle)
                 {
                     Marshal.ThrowExceptionForHR(Platform.WindowsCreateStringReference(
-                        (char*)chars, value.Length, (IntPtr*)header, (IntPtr*)handle));
+                        (char*)chars, value.Length, (IntPtr*)m._header, (IntPtr*)handle));
                 };
                 return m;
             }

src/cswinrt/strings/WinRT.cs

@@ -237,18 +237,16 @@ public static unsafe (ObjectReference<IActivationFactoryVftbl> obj, int hr) GetA
         {
             // TODO: "using var" with ref struct and remove the try/catch below
             var m = MarshalString.CreateMarshaler(runtimeClassId);
-            Func<bool> dispose = () => { m.Dispose(); return false; };
             try
             {
                 IntPtr instancePtr;
                 int hr;
                 (instancePtr, hr) = GetActivationFactory(MarshalString.GetAbi(m));
                 return (hr == 0 ? ObjectReference<IActivationFactoryVftbl>.Attach(ref instancePtr) : null, hr);
             }
-            catch (Exception) when (dispose())
+            finally
             {
-                // Will never execute
-                return default;
+                m.Dispose();
             }
         }