How to convert a C++ struct to C# equivalent?

Question

This is my C++ struct (the docs say that each instance has to be exactly 10 bytes in size):

#pragma pack (1) struct LaserPoint { WORD x; WORD y; byte colors[6]; };

I made the following C# struct:

[StructLayout(LayoutKind.Sequential, Pack=1)] public struct LaserPoint { public UInt16 x; // 2 bytes public UInt16 y; // 2 bytes public byte[] colors; // 6 bytes }

Here's my full test code in my C# project:

using System; using System.Runtime.InteropServices; namespace StructSizeTest { class Program { [StructLayout(LayoutKind.Sequential, Pack=1)] public struct LaserPoint { public UInt16 x; // 2 bytes public UInt16 y; // 2 bytes public byte[] colors; // byte[6] = 6 bytes } static void Main(string[] args) { LaserPoint point = new LaserPoint(); point.x = (UInt16)16384; point.y = (UInt16)32768; point.colors = new byte[6]; point.colors[0] = 255; point.colors[1] = 255; point.colors[2] = 255; point.colors[3] = 255; point.colors[4] = 255; point.colors[5] = 255; Console.WriteLine("LaserPoint.Size: " + Marshal.SizeOf(point)); Console.ReadLine(); } } }

This is the output on the console:

LaserPoint.Size: 8

Why is point 8 bytes in size and not 10 bytes?

UInt16 = 2 bytes UInt16 = 2 bytes byte[6] = 6 bytes Total = 10 bytes ?

What I am missing here?

Because the array is not a compile-time fixed-size array. Think of it more like a pointer in C++. — Some programmer dude
– Some programmer dude, Commented May 2, 2018 at 11:17

adjan · Accepted Answer · 2018-05-02 17:41:20Z

[StructLayout(LayoutKind.Sequential, Pack=1)] public struct LaserPoint { public UInt16 x; // 2 bytes public UInt16 y; // 2 bytes public byte[] colors; // 6 bytes }

byte[] colors is not 6 bytes wide. It is not even an actual array, but a reference to a byte array on the heap. Depending on your platform, it has either 32 bit (4 bytes) or 64 bit (8 bytes) in size (internally, it is a pointer). That is why you are getting a size of 8 bytes as a result (2 byte + 2 byte + 4 byte). On a 64 bit platform you would get a size of 12 bytes (2+2+8).

To have the struct behave like a C++ struct add the MarshalAsAttribute and specify UnmanagedType.ByValArray and SizeConst:

[StructLayout(LayoutKind.Sequential, Pack=1)] public struct LaserPoint { public UInt16 x; public UInt16 y; [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)] public byte[] colors; }

Elvis Dukaj · Accepted Answer · 2018-05-02 11:16:07Z

I think the problem is with the array... Try this:

[StructLayout(LayoutKind.Sequential, Pack=1)] public struct LaserPoint { public UInt16 x; // 2 bytes public UInt16 y; // 2 bytes [MarhalAs(UnmanagedType.ByValArray, SizeConst = 6] public byte[] colors; // 6 bytes }

Thank you very much! I decided to accept Adrian's answer as solution since he added some useful explanation. But I upvoted your's aswell.

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