DelegateHelpers.Utils.cs

using System;
using System.Runtime.InteropServices;

namespace DotNext
{
    public static partial class DelegateHelpers
    {
        [StructLayout(LayoutKind.Auto)]
        private readonly struct TargetRewriter : ISupplier<Delegate, object?>
        {
            private readonly object target;

            internal TargetRewriter(object newTarget) => target = newTarget;

            object? ISupplier<Delegate, object?>.Invoke(Delegate d) => target;
        }

        [StructLayout(LayoutKind.Auto)]
        private readonly struct EmptyTargetRewriter : ISupplier<Delegate, object?>
        {
            object? ISupplier<Delegate, object?>.Invoke(Delegate d) => d.Target;
        }

        private static TDelegate ChangeType<TDelegate, TRewriter>(this Delegate d, TRewriter rewriter)
            where TDelegate : Delegate
            where TRewriter : struct, ISupplier<Delegate, object?>
        {
            var list = d.GetInvocationList();
            if (list.LongLength == 1L)
                return ReferenceEquals(list[0], d) ? d.Method.CreateDelegate<TDelegate>(rewriter.Invoke(d)) : ChangeType<TDelegate, TRewriter>(list[0], rewriter);

            // We use untyped CreateDelegate to avoid typecast inside of the loop.
            // Also, it's reasonable to reuse already allocated invocation list to store
            // newly created delegates because Delegate.Combine accepts array only
            var delegateType = typeof(TDelegate);
            foreach (ref var sub in list.AsSpan())
                sub = sub.Method.CreateDelegate(delegateType, rewriter.Invoke(sub));

            return (TDelegate)Delegate.Combine(list)!;
        }
    }
}