How to return array of struct from C++ dll to C#

Question

I need to call a function in dll and return an array of structures. I do not know the size of the array in advance. How can this be done? The error can not marshal 'returns value' invalid managed / unmanaged

Code in C#:

[DllImport("CppDll"]
public static extern ResultOfStrategy[] MyCppFunc(int countO, Data[] dataO, int countF, Data[] dataF);

in C++:

extern "C" _declspec(dllexport) ResultOfStrategy* WINAPI MyCppFunc(int countO, MYDATA * dataO, int countF, MYDATA * dataF)
{
    return Optimization(countO, dataO, countF, dataF);
}

Return array of struct :

struct ResultOfStrategy
{
bool isGood;
double allProfit;
double CAGR;
double DD;
int countDeals;
double allProfitF;
double CAGRF;
double DDF;
int countDealsF;
Param Fast;
Param Slow;
Param Stop;
Param Tp;
newStop stloss;
};

Answer 1

I'll give you two responses. The first one is a method quite basic. The second one is quite advanced.

Given:

C-side:

struct ResultOfStrategy
{
    //bool isGood;
    double allProfit;
    double CAGR;
    double DD;
    int countDeals;
    double allProfitF;
    double CAGRF;
    double DDF;
    int countDealsF;
    ResultOfStrategy *ptr;
};

C#-side:

public struct ResultOfStrategy
{
    //[MarshalAs(UnmanagedType.I1)]
    //public bool isGood;
    public double allProfit;
    public double CAGR;
    public double DD;
    public int countDeals;
    public double allProfitF;
    public double CAGRF;
    public double DDF;
    public int countDealsF;
    public IntPtr ptr;
}

Note that I've removed the bool, because it has some problems with the case 2 (but it works with case 1)... Now...

Case 1 is very basic, and it will cause the .NET marshaler to copy the array built in C to a C# array.

Case 2 as I've written is quite advanced and it tries to bypass this marshal-by-copy and make so that the C and the .NET can share the same memory.

To check the difference I've written a method:

static void CheckIfMarshaled(ResultOfStrategy[] ros)
{
    GCHandle h = default(GCHandle);

    try
    {
        try
        {
        }
        finally
        {
            h = GCHandle.Alloc(ros, GCHandleType.Pinned);
        }

        Console.WriteLine("ros was {0}", ros[0].ptr == h.AddrOfPinnedObject() ? "marshaled in place" : "marshaled by copy");
    }
    finally
    {
        if (h.IsAllocated)
        {
            h.Free();
        }
    }
}

And I've added a ptr field to the struct that contains the original address of the struct (C-side), to see if it has been copied or if it is the original struct.

Case 1:

C-side:

__declspec(dllexport) void MyCppFunc(ResultOfStrategy** ros, int* length)
{
    *ros = (ResultOfStrategy*)::CoTaskMemAlloc(sizeof(ResultOfStrategy) * 2);
    ::memset(*ros, 0, sizeof(ResultOfStrategy) * 2);
    (*ros)[0].ptr = *ros;
    (*ros)[0].allProfit = 100;
    (*ros)[1].ptr = *ros + 1;
    (*ros)[1].allProfit = 200;
    *length = 2;
}

and C#-side:

public static extern void MyCppFunc(
    [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.Struct, SizeParamIndex = 1)] out ResultOfStrategy[] ros, 
    out int length
);

and then:

ResultOfStrategy[] ros;
int length;
MyCppFunc(out ros, out length);

Console.Write("Case 1: ");
CheckIfMarshaled(ros);

ResultOfStrategy[] ros2;

The .NET marshaler knows (because we gave it the information) that the second parameter is the length of the out ResultOfStrategy[] ros (see the SizeParamIndex?), so it can create a .NET array and copy from the C-allocated array the data. Note that in the C code I've used the ::CoTaskMemAlloc to allocate the memory. The .NET wants the memory to be allocated with that allocator, because it then frees it. If you use malloc/new/??? to allocate the ResultOfStrategy[] memory, bad things will happen.

Case 2:

C-Side:

__declspec(dllexport) void MyCppFunc2(ResultOfStrategy* (*allocator)(size_t length))
{
    ResultOfStrategy *ros = allocator(2);
    ros[0].ptr = ros;
    ros[1].ptr = ros + 1;
    ros[0].allProfit = 100;
    ros[1].allProfit = 200;
}

C#-side:

// Allocator of T[] that pins the memory (and handles unpinning)
public sealed class PinnedArray<T> : IDisposable where T : struct
{
    private GCHandle handle;

    public T[] Array { get; private set; }

    public IntPtr CreateArray(int length)
    {
        FreeHandle();

        Array = new T[length];

        // try... finally trick to be sure that the code isn't interrupted by asynchronous exceptions
        try
        {
        }
        finally
        {
            handle = GCHandle.Alloc(Array, GCHandleType.Pinned);
        }

        return handle.AddrOfPinnedObject();
    }

    // Some overloads to handle various possible length types
    // Note that normally size_t is IntPtr
    public IntPtr CreateArray(IntPtr length)
    {
        return CreateArray((int)length);
    }

    public IntPtr CreateArray(long length)
    {
        return CreateArray((int)length);
    }

    public void Dispose()
    {
        FreeHandle();
    }

    ~PinnedArray()
    {
        FreeHandle();
    }

    private void FreeHandle()
    {
        if (handle.IsAllocated)
        {
            handle.Free();
        }
    }
}

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
public delegate IntPtr AllocateResultOfStrategyArray(IntPtr length);

[DllImport("CplusPlusSide.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern void MyCppFunc2(
    AllocateResultOfStrategyArray allocator
);

and then

ResultOfStrategy[] ros;

using (var pa = new PinnedArray<ResultOfStrategy>())
{
    MyCppFunc2(pa.CreateArray);
    ros = pa.Array;

    // Don't do anything inside of here! We have a
    // pinned object here, the .NET GC doesn't like
    // to have pinned objects around!

    Console.Write("Case 2: ");
    CheckIfMarshaled(ros);
}

// Do the work with ros here!

Now this one is interesting... The C function receives an allocator from the C#-side (a function pointer). This allocator will allocate length elements and must then remember the address of the allocated memory. The trick here is that C#-side we allocate a ResultOfStrategy[] of the size required by C, that is then used directly C-side. This will break badly if ResultOfStrategy isn't blittable (a term meaning that you can only use some types inside ResultOfStrategy, mainly numerical types, no string, no char, no bool, see here). The code is quite advanced because on top of all of this, it has to use the GCHandle to pin the .NET array, so that it isn't moved around. Handling this GCHandle is quite complex, so I had to create a ResultOfStrategyContainer that is IDisposable. In this class I even save the reference to the created array (the ResultOfStrategy[] ResultOfStrategy). Note the use of the using. That is the correct way to use the class.

bool and case 2

As I've said, while bool work with case 1, they don't work with case 2... But we can cheat:

C-side:

struct ResultOfStrategy
{
    bool isGood;

C#-side:

public struct ResultOfStrategy
{
    private byte isGoodInternal;
    public bool isGood
    {
        get => isGoodInternal != 0;
        set => isGoodInternal = value ? (byte)1 : (byte)0;
    }

this works:

C-side:

extern "C"
{
    struct ResultOfStrategy
    {
        bool isGood;
        double allProfit;
        double CAGR;
        double DD;
        int countDeals;
        double allProfitF;
        double CAGRF;
        double DDF;
        int countDealsF;
        ResultOfStrategy *ptr;
    };

    int num = 0;
    int size = 10;

    __declspec(dllexport) void MyCppFunc2(ResultOfStrategy* (*allocator)(size_t length))
    {
        ResultOfStrategy *ros = allocator(size);

        for (int i = 0; i < size; i++)
        {
            ros[i].isGood = i & 1;
            ros[i].allProfit = num++;
            ros[i].CAGR = num++;
            ros[i].DD = num++;
            ros[i].countDeals = num++;
            ros[i].allProfitF = num++;
            ros[i].CAGRF = num++;
            ros[i].DDF = num++;
            ros[i].countDealsF = num++;
            ros[i].ptr = ros + i;
        }

        size--;
    }
}

C#-side:

[StructLayout(LayoutKind.Sequential)]
public struct ResultOfStrategy
{
    private byte isGoodInternal;
    public bool isGood
    {
        get => isGoodInternal != 0;
        set => isGoodInternal = value ? (byte)1 : (byte)0;
    }
    public double allProfit;
    public double CAGR;
    public double DD;
    public int countDeals;
    public double allProfitF;
    public double CAGRF;
    public double DDF;
    public int countDealsF;
    public IntPtr ptr;
}

and then

ResultOfStrategy[] ros;

for (int i = 0; i < 10; i++)
{
    using (var pa = new PinnedArray<ResultOfStrategy>())
    {
        MyCppFunc2(pa.CreateArray);
        ros = pa.Array;

        // Don't do anything inside of here! We have a
        // pinned object here, the .NET GC doesn't like
        // to have pinned objects around!
    }

    for (int j = 0; j < ros.Length; j++)
    {
        Console.WriteLine($"row {j}: isGood: {ros[j].isGood}, allProfit: {ros[j].allProfit}, CAGR: {ros[j].CAGR}, DD: {ros[j].DD}, countDeals: {ros[j].countDeals}, allProfitF: {ros[j].allProfitF}, CAGRF: {ros[j].CAGRF}, DDF: {ros[j].DDF}, countDealsF: {ros[j].countDealsF}");
    }

    Console.WriteLine();
}