Passing Structure from C# to C++

Question

I have the following structure in C++:

extern "C" __declspec(dllexport) struct SnapRoundingOption
{
    double PixelSize;
    bool IsISR;
    bool IsOutputInteger;
    int KdTrees;
};

And this is my function declaration in C++:

extern "C" __declspec(dllexport) void FaceGenerationDummy(SnapRoundingOption snapOption);

This is the corresponding C# code:

// I also tried not specifying Pack, but the same error occurred.
[StructLayout(LayoutKind.Sequential, Pack = 1)]
public struct SnapRoundingOption
{
    public  double PixelSize;
    public bool IsISR;
    public bool IsOutputInteger;
    public int KdTrees;

    public SnapRoundingOption(double pixelSize, bool isISR, bool isOutputInt, int kdTrees)
    {
        PixelSize = pixelSize;
        IsISR = isISR;
        IsOutputInteger = isOutputInt;
        KdTrees = kdTrees;
    }
}

[DllImport("Face.dll")]
public static extern void FaceGenerationDummy(SnapRoundingOption snapRoundingOption);

However, when I call the FaceGenerationDummy with this test:

[Test]
public void DummyTest()
{
    SimpleInterop.FaceGenerationDummy(new SnapRoundingOption(10, true, false, 1));
}

I found that KdTrees is 0 in C++, instead of 1 as passed in.

What am I doing wrong?

Edit 1: I am using Visual Studio 2008 on Windows 7 32-bit.

Edit 2: Both sizeof(SnapRoundingOption) return the same number – 16.

I tried both, specify Pack and don't specify Pack, none which worked. — Graviton
– Graviton, Commented Mar 8, 2011 at 8:45
@Graviton: Also, does your C++ compiler consider int to be 32 bits wide? — user541686
– user541686, Commented Mar 8, 2011 at 8:45
@Graviton: Ooooh... where's the word "struct" for your struct? Is that a typo or is it actually what you wrote?! — user541686
– user541686, Commented Mar 8, 2011 at 8:47
Check out the memory representation of your structure in C++ and use the proper Pack and MarshalAs attributes/parameters. — Jaroslav Jandek
– Jaroslav Jandek, Commented Mar 8, 2011 at 8:48

David Heffernan · Accepted Answer · 2012-12-03 16:26:40Z

20

The problem here is how you are marshalling the bool fields. These are single bytes in C++ and so need to be marshalled so:

[StructLayout(LayoutKind.Sequential)]
public struct SnapRoundingOption
{
    public double PixelSize;
    [MarshalAs(UnmanagedType.U1)]
    public bool IsISR;
    [MarshalAs(UnmanagedType.U1)]
    public bool IsOutputInteger;
    public int KdTrees;
}

Match this up on the C++ side:

struct SnapRoundingOption
{
    double PixelSize;
    bool IsISR;
    bool IsOutputInteger;
    int KdTrees;
};

I removed the packing settings so that the structures will have alignment natural to the platform.

You should also make sure that your calling conventions agree. As it stands it looks like the C++ code uses cdecl, and the C# code uses stdcall. For example

[DllImport("Face.dll", CallingConvention=CallingConvention.Cdecl)]

would align the two sides of the interface.

edited Dec 3, 2012 at 16:26

answered Mar 8, 2011 at 9:16

David Heffernan

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10 Comments

David Heffernan Over a year ago

@mehrdad Thanks, I like these P/Invoke questions!

user541686 Over a year ago

@David: Ditto... they make me think harder. :) (I especially like the COM interface questions... those can get really tricky!)

David Heffernan Over a year ago

@Graviton Why didn't you do the C++ part as well? It matters! Anyway, I wrote a test for this and successfully passed this struct in both directions.

Jaroslav Jandek Over a year ago

@David: That's what I suggested 40 minutes ago in my comment :-P. @Graviton: packing alignment should be the same for you in both C++ and C#. It should work with Pack=0 or without it.

Jaroslav Jandek Over a year ago

@David: are you sure about WinAPI using packing alignment 1? MS recommends using the default packing (8) so datatypes are aligned according to their size. Also I have only seen MS using pack=1 for arrays (internally generated)... @Graviton: msdn.microsoft.com/en-us/library/2e70t5y1.aspx

|

user541686 · Accepted Answer · 2011-03-08 09:17:27Z

5

bool is NOT blittable! Its default marshaling is the Win32 BOOL (which is 4 bytes), not bool (which is 1 byte)!

answered Mar 8, 2011 at 9:17

user541686

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1 Comment

mmmmmmmm Over a year ago

+1: Since you stated what bool is marshaled to (A point David Heffernan forgot in his answer :-) )

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