How can I write a generic container class that implements a given interface in C#?

This isn’t pretty, but it seems to work:

public static class GroupGenerator
{
    public static T Create<T>(IEnumerable<T> items) where T : class
    {
        return (T)Activator.CreateInstance(Cache<T>.Type, items);
    }
    private static class Cache<T> where T : class
    {
        internal static readonly Type Type;
        static Cache()
        {
            if (!typeof(T).IsInterface)
            {
                throw new InvalidOperationException(typeof(T).Name
                    + " is not an interface");
            }
            AssemblyName an = new AssemblyName("tmp_" + typeof(T).Name);
            var asm = AppDomain.CurrentDomain.DefineDynamicAssembly(
                an, AssemblyBuilderAccess.RunAndSave);
            string moduleName = Path.ChangeExtension(an.Name,"dll");
            var module = asm.DefineDynamicModule(moduleName, false);
            string ns = typeof(T).Namespace;
            if (!string.IsNullOrEmpty(ns)) ns += ".";
            var type = module.DefineType(ns + "grp_" + typeof(T).Name,
                TypeAttributes.Class | TypeAttributes.AnsiClass |
                TypeAttributes.Sealed | TypeAttributes.NotPublic);
            type.AddInterfaceImplementation(typeof(T));

            var fld = type.DefineField("items", typeof(IEnumerable<T>),
                FieldAttributes.Private);
            var ctor = type.DefineConstructor(MethodAttributes.Public,
                CallingConventions.HasThis, new Type[] { fld.FieldType });
            var il = ctor.GetILGenerator();
            // store the items
            il.Emit(OpCodes.Ldarg_0);
            il.Emit(OpCodes.Ldarg_1);
            il.Emit(OpCodes.Stfld, fld);
            il.Emit(OpCodes.Ret);

            foreach (var method in typeof(T).GetMethods())
            {
                var args = method.GetParameters();
                var methodImpl = type.DefineMethod(method.Name,
                    MethodAttributes.Private | MethodAttributes.Virtual,
                    method.ReturnType,
                    Array.ConvertAll(args, arg => arg.ParameterType));
                type.DefineMethodOverride(methodImpl, method);
                il = methodImpl.GetILGenerator();
                if (method.ReturnType != typeof(void))
                {
                    il.Emit(OpCodes.Ldstr,
                        "Methods with return values are not supported");
                    il.Emit(OpCodes.Newobj, typeof(NotSupportedException)
                        .GetConstructor(new Type[] {typeof(string)}));
                    il.Emit(OpCodes.Throw);
                    continue;
                }

                // get the iterator
                var iter = il.DeclareLocal(typeof(IEnumerator<T>));
                il.Emit(OpCodes.Ldarg_0);
                il.Emit(OpCodes.Ldfld, fld);
                il.EmitCall(OpCodes.Callvirt, typeof(IEnumerable<T>)
                    .GetMethod("GetEnumerator"), null);
                il.Emit(OpCodes.Stloc, iter);
                Label tryFinally = il.BeginExceptionBlock();

                // jump to "progress the iterator"
                Label loop = il.DefineLabel();
                il.Emit(OpCodes.Br_S, loop);

                // process each item (invoke the paired method)
                Label doItem = il.DefineLabel();
                il.MarkLabel(doItem);
                il.Emit(OpCodes.Ldloc, iter);
                il.EmitCall(OpCodes.Callvirt, typeof(IEnumerator<T>)
                    .GetProperty("Current").GetGetMethod(), null);
                for (int i = 0; i < args.Length; i++)
                { // load the arguments
                    switch (i)
                    {
                        case 0: il.Emit(OpCodes.Ldarg_1); break;
                        case 1: il.Emit(OpCodes.Ldarg_2); break;
                        case 2: il.Emit(OpCodes.Ldarg_3); break;
                        default:
                            il.Emit(i < 255 ? OpCodes.Ldarg_S
                                : OpCodes.Ldarg, i + 1);
                            break;
                    }
                }
                il.EmitCall(OpCodes.Callvirt, method, null);

                // progress the iterator
                il.MarkLabel(loop);
                il.Emit(OpCodes.Ldloc, iter);
                il.EmitCall(OpCodes.Callvirt, typeof(IEnumerator)
                    .GetMethod("MoveNext"), null);
                il.Emit(OpCodes.Brtrue_S, doItem);
                il.Emit(OpCodes.Leave_S, tryFinally);

                // dispose iterator
                il.BeginFinallyBlock();
                Label endFinally = il.DefineLabel();
                il.Emit(OpCodes.Ldloc, iter);
                il.Emit(OpCodes.Brfalse_S, endFinally);
                il.Emit(OpCodes.Ldloc, iter);
                il.EmitCall(OpCodes.Callvirt, typeof(IDisposable)
                    .GetMethod("Dispose"), null);
                il.MarkLabel(endFinally);
                il.EndExceptionBlock();
                il.Emit(OpCodes.Ret);
            }
            Cache<T>.Type = type.CreateType();
#if DEBUG       // for inspection purposes...
            asm.Save(moduleName);
#endif
        }
    }
}