CODEWITHSUNDEEP
c#struct
chandrakanth
chandrakanth

Reputation: 49

Marshall.SizeOf reports unexpected size of an array of structures in C#

I dont know if previously someone asked the same. When i tried to search in the net, i couldn't find it. Please help me in solving this?

   using System;
   using System.Collections.Generic;
   using System.Linq;
   using System.Runtime.InteropServices;
   using System.Text;
   using System.Threading.Tasks;

   namespace ConsoleApplication1
   {
class Program
{
        [StructLayout(LayoutKind.Sequential)]
    public class Details
    {
        public uint ID;
        public uint state;
        public uint country;
        public uint place;
    }
    [StructLayout(LayoutKind.Sequential)]
    public class UserDetails
    {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst=8)]
        public Details[] userDetails;
    }
    static void Main(string[] args)
    {

        UserDetails u = new UserDetails();
        int sizeofDetails = Marshal.SizeOf(u);
    }
}
}

When i executed the code, i am expecting the sizeofDetails should be 128. But i am getting 64.

Is there any problem in declaration of the array. Can someone please help?

Upvotes: 2

Views: 89

Answers (1)

user2864740
user2864740

Reputation: 62005

Change class Details to struct Details (and repeat for UserDetails). With the changes the output should be 128 as expected.

In the original code Details is a Reference/Class type, and Details[] is an array of 8 "references to" Detail instances and not an array of 8 Detail struct values. Since it takes 8 bytes for each "reference to", which makes sense on a 64-bit platform, then 8x8 = 64 (which is the observed output).

I'm a bit surprised there was no warning for applying StructLayout to a class, even though a class is a valid target. Maybe it is added by ReSharper? I'm fairly sure I've seen it somewhere..

Upvotes: 5

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