How to create an array of structures in C# using DLL (written in C)?

Question

I want to fill a structure with data so I can pass it to my DLL functions. The structures are similar in both C# and C implementations. For instance something like (in C):

typedef struct{
    apple[] apples;
    leaf[] leaves;
} tree;

typedef struct{
    int taste;
} apple;

typedef struct{
    int color;
} leaf;

I want to create a tree structure on C#, fill it with apples and leaves and then send it to a function I have on a DLL.

How?

Answer 1

First of all, I think you should change the C struct a little bit:

typedef struct{
    UINT cApples;
    const apple *pApples;
    UINT cLeaves;
    const leaf *pLeaves;
} tree;

On the C# side:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
internal struct Tree
{
    internal uint cApples;
    internal IntPtr pApples;
    internal uint cLeaves;
    internal IntPtr pLeaves;
}

You can define Leaf and Apple similarly. Then you can populate them on C# side as follows:

private Tree CreateTree(Leaf[] leaves, Apple[] apples)
{
    var result = new Tree();
    result.cLeaves = (uint)leaves.Length;
    result.cApples = (uint)apples.Length;
    result.pLeaves = AllocateNativeMemory(leaves);
    result.pApples = AllocateNativeMemory(apples);
    return result;
}

private IntPtr AllocateNativeMemory<T>(T[] elements) where T : struct
{
    int elementSize = Marshal.SizeOf(typeof(T));
    IntPtr result = Marshal.AllocHGlobal(elements.Length * elementSize);

    for (int i = 0; i < elements.Length; i++)
    {
        Marshal.StructureToPtr(
            elements[i], new IntPtr((long)result + i * elementSize), false);
    }

    return result;
}

Now you can pass the result of the CreateTree method to the extern method that calls the C side.

Remark: The allocated memory should be freed; otherwise, your app will leak. If you decide that the C# side is responsible to free the allocated memory, you should do it at the end as follows:

private static void FreeTree(Tree tree)
{
    FreeNativeMemory<Leaf>(tree.pLeaves, tree.cLeaves);
    FreeNativeMemory<Apple>(tree.pApples, tree.cApples);
}

private static void FreeNativeMemory<T>(IntPtr arrayPtr, uint arrayLen) where T : struct
{
    int elementSize = Marshal.SizeOf(typeof(T));
    for (uint i = 0; i < arrayLen; i++)
    {
        Marshal.DestroyStructure(new IntPtr((long)arrayPtr + i * elementSize), typeof(T));
    }

    Marshal.FreeHGlobal(arrayPtr);
}