Add DynamicallyAccessedMembers attribute to CreateSafeHandle API (#373)

DllImportGenerator/Ancillary.Interop/GeneratedDllImportAttribute.cs

@@ -1,7 +1,14 @@
 
 namespace System.Runtime.InteropServices
 {
-    // [TODO] Remove once the attribute has been added to the BCL
+    /// <summary>
+    /// Indicates that method will be generated at compile time and invoke into an unmanaged library entry point
+    /// </summary>
+    /// <remarks>
+    /// IL linker/trimming currently has special handling of P/Invokes (pinvokeimpl):
+    ///   - https://github.com/mono/linker/blob/bfab847356063d21eb15e79f2b6c03df5bd6ef3d/src/linker/Linker.Steps/MarkStep.cs#L2623
+    /// We may want to make the linker aware of this attribute as well.
+    /// </remarks>
     [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
     public sealed class GeneratedDllImportAttribute : Attribute
     {

DllImportGenerator/Ancillary.Interop/MarshalEx.cs

@@ -1,4 +1,5 @@
 
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 
 namespace System.Runtime.InteropServices
@@ -9,7 +10,15 @@ namespace System.Runtime.InteropServices
     /// </summary>
     public static class MarshalEx
     {
-        public static TSafeHandle CreateSafeHandle<TSafeHandle>()
+        /// <summary>
+        /// Create an instance of the given <typeparamref name="TSafeHandle"/>.
+        /// </summary>
+        /// <typeparam name="TSafeHandle">Type of the SafeHandle</typeparam>
+        /// <returns>New instance of <typeparamref name="TSafeHandle"/></returns>
+        /// <remarks>
+        /// The <typeparamref name="TSafeHandle"/> must be non-abstract and have a parameterless constructor.
+        /// </remarks>
+        public static TSafeHandle CreateSafeHandle<[DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicParameterlessConstructor | DynamicallyAccessedMemberTypes.NonPublicConstructors)]TSafeHandle>()
             where TSafeHandle : SafeHandle
         {
             if (typeof(TSafeHandle).IsAbstract || typeof(TSafeHandle).GetConstructor(BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.CreateInstance | BindingFlags.Instance, null, Type.EmptyTypes, null) == null)
@@ -21,6 +30,11 @@ public static TSafeHandle CreateSafeHandle<TSafeHandle>()
             return safeHandle;
         }
 
+        /// <summary>
+        /// Sets the handle of <paramref name="safeHandle"/> to the specified <paramref name="handle"/>.
+        /// </summary>
+        /// <param name="safeHandle"><see cref="SafeHandle"/> instance to update</param>
+        /// <param name="handle">Pre-existing handle</param>
         public static void SetHandle(SafeHandle safeHandle, IntPtr handle)
         {            
             typeof(SafeHandle).GetMethod("SetHandle", BindingFlags.NonPublic | BindingFlags.Instance)!.Invoke(safeHandle, new object[] { handle });