class Program
    {
        interface ICountGetter
        {
            int GetCount(object obj);
        }

        class CountGetter<Cls, T> : ICountGetter
        {
            private Func<Cls, int> _getter;

            public CountGetter(Func<Cls, int> getter)
            {
                _getter = getter;
            }

            public int GetCount(object obj)
            {
                return _getter((Cls)obj);
            }
        }

        static void Main(string[] args)
        {
            var obj = new MyClass();
            var type = obj.GetType();
            var declType = type.GetInterfaces().FirstOrDefault(
                t => t.IsGenericType
                && t.GetGenericTypeDefinition() == typeof(ICollection<>));
            if (declType == null)
            {
                // set error and return
            }
            var pi = declType.GetProperty("Count");
            var elemType = declType.GenericTypeArguments[0];
            var getterType = typeof(CountGetter<,>).MakeGenericType(declType, elemType);
            var getterFuncType = typeof(Func<,>).MakeGenericType(declType, typeof(int));
            var getterFunc = Delegate.CreateDelegate(getterFuncType, pi.GetMethod);
            var getter = (ICountGetter)Activator.CreateInstance(getterType, getterFunc);
            // You should cache the ([decltype] = getter), it makes you no reflection costs when you call GetCount next time.
            int count = getter.GetCount(obj);
            Console.WriteLine(count);
        }
    }