How to call a function in struct from C++ dll by C#

Question

My C# project have to refer a C++ dll,

SetConfig() in struct is my target, the head look like

extern "C"
{
    int HSAPI GetSDKVersion();
    IHsFutuComm* HSAPI NewFuCommObj(void* lpReserved = NULL);

    struct IHsFutuComm:public IHSKnown
    {
        virtual int HSAPI SetConfig(const char* szSection,const char* szName,const char* szVal) = 0;
        ...
    }
}

And C# look like

class Program
{
    [DllImport("kernel32.dll")]
    private static extern IntPtr LoadLibrary(string dllToLoad);

    [DllImport("kernel32.dll")]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

    public delegate Int32 GetSDKVersion();
    public delegate IntPtr NewFuCommObj(IntPtr lpReserved);

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)]
    public struct IHsFutuComm
    {
        [MarshalAs(UnmanagedType.FunctionPtr)]
        public SetConfig pSetConfig;
    }

    [UnmanagedFunctionPointer(CallingConvention.StdCall)]
    public delegate Int32 SetConfig(string szSection, string szName, string szVal);

    static void Main(string[] args)
    {
        var pDll = LoadLibrary("HsFutuSDK.dll");
        var pGetSDKVersion = GetProcAddress(pDll, "GetSDKVersion");
        var dGetSDKVersion = (GetSDKVersion)Marshal.GetDelegateForFunctionPointer(pGetSDKVersion, typeof(GetSDKVersion));
        var sdkVersion = dGetSDKVersion();
        if (sdkVersion == 0x10000019)
        {
            var pNewFuCommObj = GetProcAddress(pDll, "NewFuCommObj");
            var dNewFuCommObj = Marshal.GetDelegateForFunctionPointer(pNewFuCommObj, typeof(NewFuCommObj)) as NewFuCommObj;
            var pIHsFutuComm = dNewFuCommObj(IntPtr.Zero);
            var IHsFutuComm1 = (IHsFutuComm)Marshal.PtrToStructure(pIHsFutuComm, typeof(IHsFutuComm));

            //error here
            var re = IHsFutuComm1.pSetConfig("futu", "server", "127.0.0.1:2800");
        }
    }
}

The error at last line is "Attempted to read or write protected memory. This is often an indication that other memory is corrupt."

How to call SetConfig() by C# ?

A struct in C++ is indistinguishable from a class, it merely has members that are public by default. You cannot pinvoke instance methods of a C++ class, or struct, you can't get the required this pointer correct. You must write a wrapper in the C++/CLI language. — Hans Passant
– Hans Passant, Commented Jun 6, 2013 at 13:27
You are right, finally I write a C++ wrapper. This is a truly outdated technology — user2458323
– user2458323, Commented Jun 13, 2013 at 0:28

Pruyque · Accepted Answer · 2013-06-06 11:38:13Z

1

In this SetConfig is a (pure) virtual memberfunction of the struct, so the calling convention should be a thiscall, and the function pointer is not held inside the struct like in your C# code, but in it's vtable.

The declaration of the struct in the c++ code is probably more like an interface in C# than a struct in C#.

I'm sorry I can't give a complete answer, but I hope this will point you in the right direction.

answered Jun 6, 2013 at 11:38

Pruyque

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