sizeof.cs

public static uint SizeOf(object obj)
{
    unsafe
    {
        var objRef = __makeref(obj);
        var objPtr = **(IntPtr**)(&objRef);
        var mtPtr = *((IntPtr*)objPtr);

        var componentSize = *((ushort*)(mtPtr + 0));
        var flags = *((ushort*)(mtPtr + 2));
        var baseSize = *((uint*)(mtPtr + 4));

        // array, check enum_flag_HasComponentSize
        if ((flags & 0x8000) != 0)
        {
            var length = *(int*)(objPtr + sizeof(IntPtr));
            return (uint)(componentSize * length);
        }

        // value type, check enum_flag_Category_ValueType
        if ((flags & 4) != 0)
        {
            var arr = Expression.Lambda<Func<object>>(
                Expression.NewArrayBounds(obj.GetType(), Expression.Constant(1))).Compile()();
            return SizeOf(arr);
        }

        return baseSize;
    }
}

static void Main(string[] args)
{
    Console.WriteLine(SizeOf(1));
    Console.WriteLine(SizeOf(1L));
    Console.WriteLine(SizeOf(new ClassA()));
    Console.WriteLine(SizeOf(new ClassB()));
    Console.WriteLine(SizeOf(new int[100]));
}