marshal array of formatted blittable classes

[jonpb]

Hi,
I would like to pass an array of formatted blittable classes through
P/Invoke to a C++ dll, but it is not working. Here are the definitions:

C++

struct Point2Rec
{
double x;
double y;
};


void WINAPI AddPolyToList(Point2Rec* pg1, int numPts);

C#

[StructLayout(LayoutKind.Sequential, Pack = 4)]
public class Point2D
{
private double _x;
private double _y;

public double X { get { return _x; } set { _x = value; } }
public double Y { get { return _y; } set { _y = value; } }
}


[DllImport("ascBool.dll")]
public static extern void AddPolyToList([In, Out] Point2D[] pg1, int
numPts);


If I change "class" to "struct" it works. I am taking my information
from the following documentation:

http://msdn2.microsoft.com/en-us/library/23acw07k(VS.80).aspx

Is there something I'm missing in the definitions of formatted and
blittable, or is it just not possible to pass an array of formatted
blittable classes to a C++ dll.

Thanks

 

[Jeroen Mostert]

I would like to pass an array of formatted blittable classes through
P/Invoke to a C++ dll, but it is not working. Here are the definitions:

C++

struct Point2Rec
{
double x;
double y;
};


void WINAPI AddPolyToList(Point2Rec* pg1, int numPts);

C#

[StructLayout(LayoutKind.Sequential, Pack = 4)]
public class Point2D
{
private double _x;
private double _y;

public double X { get { return _x; } set { _x = value; } }
public double Y { get { return _y; } set { _y = value; } }
}

[DllImport("ascBool.dll")]
public static extern void AddPolyToList([In, Out] Point2D[] pg1, int
numPts);


If I change "class" to "struct" it works.

Quite so. So your problem is...? Do you feel that using a class here gives
you an advantage over a struct?

The reason it doesn't work when you use "class" is because it turns Point2D
into a reference type. An array of reference types is marshaled as an array
of pointers (to COM proxies, in this case).

I am taking my information
from the following documentation:

http://msdn2.microsoft.com/en-us/library/23acw07k(VS.80).aspx

The link to "Blittable and Non-Blittable Types" mentions (indirectly) that
reference types (including most classes) are not blittable and will be
converted in some way.

"Default Marshaling for Classes"
(http://msdn2.microsoft.com/en-us/library/s0968xy8(VS.80).aspx) and "Default
Marshaling vor Value Types"
(http://msdn2.microsoft.com/en-us/library/0t2cwe11(VS.80).aspx) provide more
insight.