DynamicProxy does not currently support those because by-ref-like values cannot be boxed to be put in the object[] IInvocation.Arguments array.

_{(削除) I haven't yet been able to think of a general way how all by-ref-like types (including user-defined ones) could be supported, ecause of their various limitations. If anyone has ideas, I'd be interested in hearing them! (削除ここまで)}

_{(削除) However, it might be fairly easy to at least add support for Span<T> and ReadOnlySpan<T> specifically. Those types are becoming more and more common in the .NET FCL, so while not an ideal solution, it might still be sufficient for most use cases: (削除ここまで)}

_{(削除) We could introduce a new type in DynamicProxy's namespace: (削除ここまで)}

public interface ISpanMarshaller // NOTE: outdated API proposal!
{
 object BoxSpan<T>(Span<T> span);
 object BoxReadOnlySpan<T>(ReadOnlySpan<T> readOnlySpan);
 ReadOnlySpan<T> UnboxReadOnlySpan<T>(object boxedReadOnlySpan);
 Span<T> UnboxSpan<T>(object boxedSpan);
}

_{(削除) Then we could introduce a new property to ProxyGenerationOptions, ISpanMarshaller SpanMarshaller { get; set; }, which would get injected into generated proxies so they could use it to transfer spans into and out of the object[] IInvocation.Arguments array. (削除ここまで)}

_{(削除) I don't really like an addition that deals with two very specific types, but they are becoming more and more common, and I haven't yet been able to come up with a more general mechanism. (削除ここまで)}

Update

See the PR linked further down for an updated proposal that supports arbitrary by-ref-like types, including user-defined ones.

Opinions, or alternate ideas, anyone?