C++ dll Pinvoke function with function pointer struct in parameter

Question

I'm working on a C# code which uses a C++ dll
One function of the C++ dll takes a struct of 3 function pointers in parameter, I dont know how to manage it any tips or topics ?
I tried this but this doesnt work :

[StructLayout(LayoutKind.Sequential)]
public class DisplayCallBacks
{
    [MarshalAs(UnmanagedType.FunctionPtr)] initProgressBar ();                              
    [MarshalAs(UnmanagedType.FunctionPtr)] logMessage (int msgType, [MarshalAs(UnmanagedType.LPCWStr)] String str);
    [MarshalAs(UnmanagedType.FunctionPtr)] loadBar (int x, int n);                          
}

[DllImport("CubeProgrammer_API.dll", CallingConvention = CallingConvention.Cdecl, EntryPoint = "setDisplayCallbacks")]
internal static extern void SetDisplayCallbacks(DisplayCallBacks c);

C++ prototypes :

typedef struct displayCallBacks
{
    void (*initProgressBar)();                             
    void (*logMessage)(int msgType,  const wchar_t* str); 
    void (*loadBar)(int x, int n);                        
} displayCallBacks;

void setDisplayCallbacks(displayCallBacks c);

Can't tell without seeing the c++ prototype. You should us a structure instead of a class to make the same as c++. — jdweng
– jdweng, Commented Nov 6, 2020 at 14:16
One issue is that none of your methods in DisplayCallbacks have any return types — Chris Akridge
– Chris Akridge, Commented Nov 6, 2020 at 14:21
You need to define the three types ProgressBar, logMessage, Bar and allocate the correct amount of memory for each type. The c++ code structure is 12 bytes since each pointer is 4 bytes. So you need 12 bytes for the structure in unmanaged memory as well as the unmanaged memory for the 3 objects. — jdweng
– jdweng, Commented Nov 6, 2020 at 14:26
What do you mean when you say define the three types ? declare the function ? — GabrielT
– GabrielT, Commented Nov 6, 2020 at 14:32

edtheprogrammerguy · Accepted Answer · 2020-11-06 14:33:32Z

1

Use IntPtr as the type for the function pointers. Then you can declare a delegate. Here is an example from code I wrote:

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
delegate void PrintLogDelegate(string msg);

PrintLogDelegate pld;
System.Runtime.InteropServices.GCHandle callbackPrintLog;
IntPtr ipCBPrintLog = IntPtr.Zero;

pld = new PrintLogDelegate(PrintLogFunc);
callbackPrintLog = System.Runtime.InteropServices.GCHandle.Alloc(pld);
ipCBPrintLog = System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate(pld);


void PrintLogFunc(string msg)
{

}

So ipCBPrintLog is what you pass to C++ as a callback.

Then in your Dispose you have to clean up: callbackPrintLog.Free();

answered Nov 6, 2020 at 14:33

edtheprogrammerguy

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