How to declare a union in C#?

Question

Dear ladies and sirs.

Observe the following sample code:

struct DDD
{
  [MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 512, ArraySubType = UnmanagedType.I1)]
  byte[] x;
}

struct BBB
{
  DDD x;
}

struct CCC
{
  DDD x;
  ulong y;
  ulong z;
}

[StructLayout(LayoutKind.Explicit)]
struct AAA
{
  [FieldOffsetAttribute(0)]
  BBB a;
  [FieldOffsetAttribute(0)]
  CCC b;
}

Unfortunately, AAA cannot be loaded, trying to execute new AAA() fails with System.TypeLoadException: Could not load type 'AAA' from assembly 'Shunra.Common, Version=1.0.0.0, Culture=neutral, PublicKeyToken=807fc02bc4ce69db' because it contains an object field at offset 0 that is incorrectly aligned or overlapped by a non-object field.

How do one deal with it?

Thanks.

EDIT:

BTW, This is a stripped down version of MINIDUMP_CALLBACK_INPUT struct interop created by PInvokeTool (the original struct is defined in DbgHelp.h)

Answer 1

It looks to me that you cannot have two FieldOffsetAttribute with an offset of 0.

Make sure you have a look at its definition in MSDN library: http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.fieldoffsetattribute.aspx

Answer 2

I think you need to add a constructor within the Struct in order to instantiate types BBB and CCC, in turn, in each of the other structs BBB and CCC, you also need a constructor to instantiate type DDD.

struct DDD
{
  [MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 512, ArraySubType = UnmanagedType.I1)]
  byte[] x;
}

struct BBB
{
  DDD x;
  public BBB(){
     x = new DDD();
  }
}

struct CCC
{
  DDD x;
  ulong y;
  ulong z;
  public CCC(){
     x = new DDD();
  }
}

[StructLayout(LayoutKind.Explicit)]
struct AAA
{
  [FieldOffsetAttribute(0)]
  BBB a;
  [FieldOffsetAttribute(0)]
  CCC b;
  public AAA(){
     a = new BBB();
     b = new CCC();
  }
}

The only gotcha in this is the instantiation of struct DDD which is unknown, as you have declared a field x which is a byte[] array and the size is unknown, so you have to handle this one based on your own requirements. Maybe pass in the parameter, perhaps an int indicating the size of the array...

Hope this helps you, Best regards, Tom.

Answer 3

The problem is that no matter what you specify for MarshalAsAttribute, an array is an array is a managed object. To make your code work you will have to get rid of the managed array. To do this you have two options:

Option 1:

Convert the array to a fixed size buffer, which means change your definition of DDD to this:

unsafe struct DDD {
    [MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst = 512, ArraySubType = UnmanagedType.I1)]
    fixed byte x[512];
}

(I'm not sure whether the MarshalAsAttribute is needed, but I doubt so.)

Now you are using an unsafe struct, so you have to compile with the /unsafe switch.

Option 2:

Convert the array to 512 bytes of members. The easiest way would be to use 64 longs:

struct DDD {
    long x1;
    long x2;
    long x3;
    ...
}

Edit: Clarified.