How to call C# from C to pass array of structs having arrays as members?

Question

I have a native struct:

typedef struct
{
    char Message[STR_MAX];
    char Params[10][STR_MAX];
    int GetParamStr[10];
    int ParamCount;
} FormattedMessage_t;

and callback type:

typedef void(*FormatMsgCB_t)(FormattedMessage_t *FormatMsgs, int FormatMsgCount);

static array:

static FormattedMessage_t gFormattedMessages[10];

callback set function:

extern "C" __declspec(dllexport) void DllGuiSetFormatMsgCB(FormatMsgCB_t pCB)
{
    gFormatMsgCB = pCB;
}

call from native to managed:

void DllGuiSetFormatMessage()
{
    gFormatMsgCB(gFormattedMessages, gFormattedMsgIndex);
}

In Managed:

    [StructLayout(LayoutKind.Sequential)]
    public struct FormattedMessage_t
    {
        [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MaxStrLength)]
        public string Message;

        public string[] ParamStrings;
        public int[] GetParamStrs;
        public int ParamCount;

        public const int MaxStrLength = StrMax;
    }

    public static T[] GetArray<T>(IntPtr aTblPtr, int nRows)
    {
        var entrySize = Marshal.SizeOf(typeof(T));
        IntPtr oneRowPtr = new IntPtr(aTblPtr.ToInt64());
        T[] array = new T[nRows];

        for (int i = 0; i < nRows; i++)
        {
            array[i] = (T)Marshal.PtrToStructure(oneRowPtr, typeof(T));
            oneRowPtr = new IntPtr(oneRowPtr.ToInt64() + entrySize);
        }

        return array;
    }

    private void OnSetFormatMsg(IntPtr formatMsg, int nFormatMsg)
    {
        var array = GetArray<FormattedMessage_t>(formatMsg, nFormatMsg);
        foreach (var msg in array)
        {
            var str = msg.Message;
            // and so on
        }
    }

This GetArray works for simple types as struct members. This is way beyond my P/Invoke and Native Interop skills.

This is probably wrong in many ways. Any hints how this should be done (changing both structs is no problem) would be highly appreciated.

I don't remember the details from the top of my mind, but iirc you can create either a custom marshaller or add the MarshalAs flags to ParamStrings etc. (These seem to be missing.) — atlaste
– atlaste, Commented Mar 2, 2016 at 12:22

David Heffernan · Accepted Answer · 2016-03-02 12:40:25Z

3

You'll need to declare the C# struct like so:

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct FormattedMessage_t
{
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MaxStrLength)]
    public string Message;

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10*MaxStrLength)]
    public byte[] ParamStrings;

    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 10)]
    public int[] GetParamStrs;

    public int ParamCount;

    public const int MaxStrLength = StrMax;
}

Then you'll need to pick out each item from ParamStrings by performing the indexing manually. The i^th value runs from indices i*MaxStrLength to (i+1)*MaxStrLength-1. You'll need to pick that out, find the null terminator, and use Encoding.Default.GetString.

edited Mar 2, 2016 at 12:40

answered Mar 2, 2016 at 12:24

David Heffernan

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5 Comments

char m Over a year ago

Thanks! I realized myself after asking that MarshalAs-decorations were missing, but wasn't sure how to write them. Unfortunately I didn't understand the explanation about the ParamStrings.

David Heffernan Over a year ago

The thing is, that char Params[10][STR_MAX] is a two dimensional array, and the C# marshaller cannot handle that automatically. It's laid out in memory in the same way as a one dimensional array of length 10*STR_MAX. So that's what my MarshalAs does. But now you need to find the 10 strings contained within that flattened one dimensional array, and convert them to C# strings.

char m Over a year ago

Thanks. I understood what you mean. But does this indicate that I can't call Marshal.PtrToStructure at all? That I got to marshal all members separaterly? Marshal.PtrToStructure results in System.AccessViolationException so I guess that I have to marshal all separately somehow.

David Heffernan Over a year ago

There should be no problem with PtrToStructure. You'll just end up with a byte[] array for ParamStrings that will take a little extra work to decode.

char m Over a year ago

Thanks a lot! Now it works. I sort this byte[] to string[] out and I'm done.

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