Marshalling a struct that contains a union...

[Tim Overbay]

Here's the C++ definition of a structure I'm trying to marshal for DirectX:

typedef struct _D3DMATRIX {
union {
struct {
float _11, _12, _13, _14;
float _21, _22, _23, _24;
float _31, _32, _33, _34;
float _41, _42, _43, _44;
};
float m[4][4];
};
} D3DMATRIX;

I'm trying to define this in C# and this is what I came up with:

[StructLayout(LayoutKind.Explicit)]
public struct D3DMATRIX
{
[FieldOffset(0)] public float _11;
[FieldOffset(4)] public float _12;
[FieldOffset(8)] public float _13;
[FieldOffset(12)] public float _14;

[FieldOffset(16)] public float _21;
[FieldOffset(20)] public float _22;
[FieldOffset(24)] public float _23;
[FieldOffset(28)] public float _24;

[FieldOffset(32)] public float _31;
[FieldOffset(36)] public float _32;
[FieldOffset(40)] public float _33;
[FieldOffset(44)] public float _34;

[FieldOffset(48)] public float _41;
[FieldOffset(52)] public float _42;
[FieldOffset(56)] public float _43;
[FieldOffset(60)] public float _44;

[FieldOffset(0)]
[MarshalAs(UnmanagedType.LPArray, SizeConst=16)]
public float[,] m;
}

When I try to use D3DMATRIX, I get the following error:

"Could not load type D3DMATRIX from assembly 0
because it contains an object field at offset 0 that is
incorrectly aligned or overlapped by a non-object field."

Can anybody tell me what I'm missing here?

Thanks,

Tim

[Rob Teixeira]

You can't overlap an object field (array is an object) with a value type
field.

My suggestion is to:
1) use one or the other. you rarely ever need both
2) create two seperate structures and use a custom marshaller that can turn
one into the other for the sake of passing it to a method

-Rob

"Tim Overbay" <toverbay@pbsj.com> wrote:
>Here's the C++ definition of a structure I'm trying to marshal for DirectX:
>
> typedef struct _D3DMATRIX {
> union {
> struct {
> float _11, _12, _13, _14;
> float _21, _22, _23, _24;
> float _31, _32, _33, _34;
> float _41, _42, _43, _44;
> };
> float m[4][4];
> };
> } D3DMATRIX;
>
>I'm trying to define this in C# and this is what I came up with:
>
> [StructLayout(LayoutKind.Explicit)]
> public struct D3DMATRIX
> {
> [FieldOffset(0)] public float _11;
> [FieldOffset(4)] public float _12;
> [FieldOffset(8)] public float _13;
> [FieldOffset(12)] public float _14;
>
> [FieldOffset(16)] public float _21;
> [FieldOffset(20)] public float _22;
> [FieldOffset(24)] public float _23;
> [FieldOffset(28)] public float _24;
>
> [FieldOffset(32)] public float _31;
> [FieldOffset(36)] public float _32;
> [FieldOffset(40)] public float _33;
> [FieldOffset(44)] public float _34;
>
> [FieldOffset(48)] public float _41;
> [FieldOffset(52)] public float _42;
> [FieldOffset(56)] public float _43;
> [FieldOffset(60)] public float _44;
>
> [FieldOffset(0)]
> [MarshalAs(UnmanagedType.LPArray, SizeConst=16)]
> public float[,] m;
> }
>
>When I try to use D3DMATRIX, I get the following error:
>
> "Could not load type D3DMATRIX from assembly 0
> because it contains an object field at offset 0 that is
> incorrectly aligned or overlapped by a non-object field."
>
>Can anybody tell me what I'm missing here?
>
>Thanks,
>
>Tim
>
>

[Tim Overbay]

Hrmm... that was such an elegant way of implementing both access methods,
too...

I wouldn't mind just implementing the array. I think the _11, _12 etc.
members are a little hokey, but I'm not sure how DirectX accesses the
matrix. Do the DirectX functions use the array or the _11, _12 members when
dealing with D3DMATRIX structures? Does it even matter?

Thanks,

Tim

"Rob Teixeira" <RobTeixeira@@msn.com> wrote in message
news:3c7d0c87$1@10.1.10.29...
>
>
> You can't overlap an object field (array is an object) with a value type
> field.
>
> My suggestion is to:
> 1) use one or the other. you rarely ever need both
> 2) create two seperate structures and use a custom marshaller that can
turn
> one into the other for the sake of passing it to a method
>
> -Rob

[Tim Overbay]

I kinda got around this issue by implementing the _mn access methods with
properties:
public float _12
{
get {return m[0,1];}
set {m[0,1] = value;}
}
. . . etc . . .

The next problem I encountered was that there didn't seem to be any way to
initialize the matrix array without using parameterized constructors. So I
also converted the struct to a class to get around this. According to the
Interop docs, I should be able to marshal it as a structure. The only
difference I can think of is that I have to explicitly initialize my
D3DMATRIX objects with New now. Not nearly as 'neat' as I had hoped, but I
think this should work..

Thanks,

Tim



"Tim Overbay" <toverbay@pbsj.com> wrote in message
news:3c7d23e2@10.1.10.29...
> Hrmm... that was such an elegant way of implementing both access methods,
> too...
>
> I wouldn't mind just implementing the array. I think the _11, _12 etc.
> members are a little hokey, but I'm not sure how DirectX accesses the
> matrix. Do the DirectX functions use the array or the _11, _12 members
when
> dealing with D3DMATRIX structures? Does it even matter?
>
> Thanks,
>
> Tim
>
> "Rob Teixeira" <RobTeixeira@@msn.com> wrote in message
> news:3c7d0c87$1@10.1.10.29...
> >
> >
> > You can't overlap an object field (array is an object) with a value type
> > field.
> >
> > My suggestion is to:
> > 1) use one or the other. you rarely ever need both
> > 2) create two seperate structures and use a custom marshaller that can
> turn
> > one into the other for the sake of passing it to a method
> >
> > -Rob
>
>

[Rob Teixeira]

Shouldn't matter at all. In fact, off the top of my head, in memory both
signatures look the same. It was defined this way so C++ programmers could
either use array or field access to the structure.

-Rob

"Tim Overbay" <toverbay@pbsj.com> wrote:
>Hrmm... that was such an elegant way of implementing both access methods,
>too...
>
>I wouldn't mind just implementing the array. I think the _11, _12 etc.
>members are a little hokey, but I'm not sure how DirectX accesses the
>matrix. Do the DirectX functions use the array or the _11, _12 members when
>dealing with D3DMATRIX structures? Does it even matter?
>
>Thanks,
>
>Tim