Marshal array of struct into Ptr

Question

I am calling a C library from a C# code. The function I am calling take as parameter a struct containing arrays of struct :

    struct Example1Struct
    {
        char* a;
        uint16_t b;
        AnotherStruct* c; 
    }

c here is an array of pointer to AnotherStruct.

the struct in my C# code look like this

public struct Example1Struct
    {
        public IntPtr StationName;//is char*
        public UInt16 IdCode;
        public IntPtr AnotherStruct; //array of struct  AnotherStruct
    }




public static IntPtr MarshalToPointer(object data)
    {
        Type valueType = data.GetType();
        IntPtr buf = IntPtr.Zero;

        if (valueType.IsArray)
        {
            if (data is char[])
            {
                var d = data as char[];
                buf = Marshal.AllocHGlobal(Marshal.SizeOf(d.GetType().GetElementType()) * d.Length);
            }
            else if (data is char[,])
            {
                var d = data as char[,];
                buf = Marshal.AllocHGlobal(Marshal.SizeOf(d.GetType().GetElementType()) * d.Length);
            }
            else
            {
                buf = Marshal.AllocHGlobal(Marshal.SizeOf(data.GetType().GetElementType()) * count);

                long LongPtr = buf.ToInt64(); // Must work both on x86 and x64
                for (int I = 0; I < data.Lenght; I++)
                {
                    IntPtr RectPtr = new IntPtr(LongPtr);
                    Marshal.StructureToPtr(data[I], RectPtr, false); // You do not need to erase struct in this case
                    LongPtr += Marshal.SizeOf(typeof(Rect));
                }

            }

            return buf;
        }
        else
            buf = Marshal.AllocHGlobal(Marshal.SizeOf(data));
        Marshal.StructureToPtr(data, buf, false);
        return buf;
    }

my problem here is that I cannot cast data (who is an array of AnotherStruct) to object[] , neither in IEnumerable. So I cannot access to data[I] and don't have data.Lenght

Any idea ?

Answer 1

Usually I'd recommend using the MarshalAs attribute rather than writing manual marshalling code. It looks like:

public struct Example1Struct
{
    public IntPtr StationName;//is char*
    public UInt16 IdCode;
    public IntPtr AnotherStruct; //array of struct  AnotherStruct
}

Could be:

public struct Example1Struct
{
    [MarshalAs(UnmanagedType.LPStr)]
    public string StationName;
    public UInt16 IdCode;
    [MarshalAs(UnmanagedType.LPArray)]
    public AnotherStruct[] OtherStructs;
}

And the marshaller should do the right thing for you when you pass it to unmanaged code.

Answer 2

You can get the length of the array like this:

if (data is Array a) 
    Console.WriteLine(a.Length);

Arrays in c# always derive from Array, so you can cast it to that.

But if possible in your real code, I'd recommend Damien's answer