Marshalling C# structure to C++ Using StructureToPtr

Question

I have C# class:

namespace Models
{
    [StructLayout(LayoutKind.Explicit, Size = 120, CharSet = CharSet.Unicode)]
    public struct DynamicState
    {
        [FieldOffset(0)]
        public double[] Position;

        [FieldOffset(24)]
        public double[] Velocity;

        [FieldOffset(48)]
        public double[] Acceleration;

        [FieldOffset(72)]
        public double[] Attitude;

        [FieldOffset(96)]
        public double[] AngularVelocity;
    }
}

and C++/CLI method:

Models::DynamicState SomeClassClr::DoSomething(Models::DynamicState ds)
{
    int struct_size = Marshal::SizeOf(ds);
    System::IntPtr ptr = Marshal::AllocHGlobal(struct_size);
    DynamicStateStruct ds_struct;
    struct_size = sizeof(ds_struct);

    Marshal::StructureToPtr(ds, ptr, false);
    ds_struct = *(DynamicStateStruct*)ptr.ToPointer();

    Models::DynamicState returnVal;
    mpSomeClass->doSomething(ds_struct);

    return returnVal;
}

where DynamicStateStruct is a native C++ class:

struct DynamicStateStruct
{
    double mPosition[3];
    double mVelocity[3];
    double mAcceleration[3];
    double mAttitude[3];
    double mAngularVelocity[3];
};

When I recover the struct (ds_struct) in native C++ I am not getting the correct values, any ideas with what I am missing?

Note when I'm testing I'm only assigning one array in the managed DynamicState object. — Seth
– Seth, Commented Oct 5, 2012 at 13:32
Why would the arrays of doubles be marshaled to an array of 3 C++ doubles? What if one of the arrays had 6 items? — ta.speot.is
– ta.speot.is, Commented Oct 5, 2012 at 13:41
assume the initialization of each array is constrained to 3. i.e. as per the length of each fieldoffset attribute. — Seth
– Seth, Commented Oct 5, 2012 at 13:52

Pavel · Accepted Answer · 2012-10-05 13:54:59Z

Try the following variant:

public struct DynamicState
{
    [MarshalAs (UnmanagedType.ByValArray, SizeConst=3)]
    public double[] Position;

    [MarshalAs (UnmanagedType.ByValArray, SizeConst=3)]
    public double[] Velocity;

    [MarshalAs (UnmanagedType.ByValArray, SizeConst=3)]
    public double[] Acceleration;

    [MarshalAs (UnmanagedType.ByValArray, SizeConst=3)]
    public double[] Attitude;

    [MarshalAs (UnmanagedType.ByValArray, SizeConst=3)]
    public double[] AngularVelocity;
}

Another option is to use fixed array available in unsafe code:

public unsafe struct DynamicState
{
    public fixed double Position[3];

    public fixed double Velocity[3];

    public fixed double Acceleration[3];

    public fixed double Attitude[3];

    public fixed double AngularVelocity[3];
}

P.S. A good guide on .Net interop can be found here: http://www.mono-project.com/Interop_with_Native_Libraries

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