How to pass struct with unsigned char* from C# to C++?

Question

I have some C++ dll with struct description and some methods:

struct MSG_STRUCT {  
    unsigned long dataSize;
    unsigned char* data;
}

And function for example:

unsigned long ReadMsg( unsigned long msgId, MSG_STRUCT* readMsg)
{
    readMsg->dataSize = someDataSize;
    readMsg->data = someData;
}

So I want to call this function from C#:

[StructLayout(LayoutKind.Sequential)]
struct MSG_STRUCT 
{
    UInt32 dataSize;
    byte[] data;
}

[DllImport("mydll.dll")]
public static Int32 ReadMsg( UInt32 msgId, ref MSG_STRUCT readMsg);

So I tried to call C# function like:

var readMsg = new MSG_STRUCT();
readMsg.data = new byte[4128];
Int32 res = ReadMsg( someMsgId, ref readMsg);

But I didn't get smth normal in data. I also tried to call ReadMsg with IntPrt type parameter, but Marshal.PtrToStructure gave me AccessViolationException sometimes.

I don't have any ideas how to pass a pointer to MSG_STRUCT from C# to C++ and receive the result as filled MSG_STRUCT.data

The final solutionthat worked for me: I used a part of solution offered by xanatos: I set CallingConvention = CallingConvention.Cdecl for my DllImport function. I found out that I also need to change:

[MarshalAs(UnmanagedType.ByValArray, SizeConst = 4128)]
public byte[] Data;

Thanks everyone for your help

"But i didn't get smth normal in data." What did you get, what were you expecting? Is data supposed to be a string or a byte array? — Ian Kemp - SO dead by AI greed
– Ian Kemp - SO dead by AI greed, Commented Mar 18, 2016 at 8:49

xanatos · Accepted Answer · 2016-03-18 09:07:19Z

3

You could try with:

[StructLayout(LayoutKind.Sequential)]
public struct MSG_STRUCT 
{
    int dataSize;
    IntPtr data;

    public byte[] GetData() 
    {
        var bytes = new byte[dataSize];
        Marshal.Copy(data, bytes, 0, dataSize);
        return bytes;
    }
}

[DllImport("NativeLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
public static extern uint ReadMsg(uint msgId, ref MSG_STRUCT readMsg);

and then:

MSG_STRUCT msg = new MSG_STRUCT();
uint res = ReadMsg(123, ref msg);
byte[] bytes = msg.GetData();

Your C function is reassigning the data pointer, so you have to marshal it back to C#. The easiest way (for me) is to simply pass a IntPtr and do some explicit Marshal.Copy(...).

An alternative is to have data a byte[], but then in C-side you have to memcpy(readMsg->data, someData, someDataSize) instead of simply assigning readMsg->data = someData.

edited Mar 18, 2016 at 9:07

answered Mar 18, 2016 at 8:58

xanatos

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Comments

Roman · Accepted Answer · 2016-03-18 12:34:32Z

2

Try to change attribute from

[StructLayout(LayoutKind.Sequential)]

to

[StructLayout(LayoutKind.Sequential, Pack=X)]

Where X is 1,2,4,8 ..

Default packing in c++ is 8, so try to set Pack = 8

edited Mar 18, 2016 at 12:34

answered Mar 18, 2016 at 8:54

Roman

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1 Comment

Roman Over a year ago

I have edited my answer. It may depend on different packing of structures in C++ and C#.

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