[API Proposal] Create ReadOnlySpanMarshaller<T, TUnmanaged>.ManagedToUnmanagedOut marshaller

[jtschuster]

Background and motivation

To address #96525, we can provide a marshaller for ReadOnlySpan<T> for ManagedToUnmanagedOut direction. We have marshallers for UnmanagedToManagedOut and ManagedToUnmanagedIn. This API wasn't created previously due to the assumption that the managed ReadOnlySpan<T> would need to be written to in generated marshaling code, but that isn't strictly necessary for stateful marshallers. This API would allow LibraryImport methods to return ReadOnlySpan<T> without creating a new marshaller or specifying the marshaller type in the method definition.

API Proposal

  [System.Runtime.InteropServices.Marshalling.ContiguousCollectionMarshallerAttribute]
  [System.Runtime.InteropServices.Marshalling.CustomMarshallerAttribute(typeof(System.ReadOnlySpan<>), System.Runtime.InteropServices.Marshalling.MarshalMode.ManagedToUnmanagedIn, typeof(System.Runtime.InteropServices.Marshalling.ReadOnlySpanMarshaller<,>.ManagedToUnmanagedIn))]
+ [System.Runtime.InteropServices.Marshalling.CustomMarshallerAttribute(typeof(System.ReadOnlySpan<>), System.Runtime.InteropServices.Marshalling.MarshalMode.ManagedToUnmanagedOut, typeof(System.Runtime.InteropServices.Marshalling.ReadOnlySpanMarshaller<,>.ManagedToUnmanagedOut))]
  [System.Runtime.InteropServices.Marshalling.CustomMarshallerAttribute(typeof(System.ReadOnlySpan<>), System.Runtime.InteropServices.Marshalling.MarshalMode.UnmanagedToManagedOut, typeof(System.Runtime.InteropServices.Marshalling.ReadOnlySpanMarshaller<,>.UnmanagedToManagedOut))]
  public static unsafe class ReadOnlySpanMarshaller<T, TUnmanagedElement> where TUnmanagedElement : unmanaged
  {
+     public struct ManagedToUnmanagedOut
+     {
+         public void FromUnmanaged(TUnmanagedElement* unmanaged) { }
+         public ReadOnlySpan<T> ToManaged() { }
+         public ReadOnlySpan<TUnmanagedElement> GetUnmanagedValuesSource(int numElements) { }
+         public Span<T> GetManagedValuesDestination(int numElements) { }
+         public void Free() { }
+     }
}

API Usage

The API would primarily be used in code generated by the LibraryImport generator to unmarshal a native array into a managed ReadOnlySpan<T> when a method returns a ReadOnlySpan<T> or has an out parameter of type ReadOnlySpan<T>.

[LibraryImport("DllName")]
[return: MarshalUsing(CountElementName = nameof(length))]
public static partial ReadOnlySpan<int> GetRange(int start, int end, out int length);

Alternative Designs

A workaround is to create a LibraryImport that returns another built-in collection type and create a wrapper that converts the returned collection into a ReadOnlySpan<T>, or use hand-written marshalling for the method.

Risk

Minimal. We already provide marshallers for ReadOnlySpan<T> to pass a parameter in, and we have marshallers for other collections to return a value from a native method, so this is not introducing a new concept.

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[terrajobst]

Video

• Looks good as proposed

namespace System.Runtime.InteropServices.Marshalling;

[CustomMarshaller(typeof(System.ReadOnlySpan<>), MarshalMode.ManagedToUnmanagedOut, typeof(ReadOnlySpanMarshaller<,>.ManagedToUnmanagedOut))]
public partial class ReadOnlySpanMarshaller<T, TUnmanagedElement> where TUnmanagedElement : unmanaged
{
    public struct ManagedToUnmanagedOut
    {
        public void FromUnmanaged(TUnmanagedElement* unmanaged);
        public ReadOnlySpan<T> ToManaged();
        public ReadOnlySpan<TUnmanagedElement> GetUnmanagedValuesSource(int numElements);
        public Span<T> GetManagedValuesDestination(int numElements);
        public void Free();
    }
}

[jtschuster]

Closed via #101095