How to use interface pointer exported by C++ DLL in C#

问题

I have a DLL written in C++, which exports a function CreateRisk. The function returns an interface pointer, as follows:

extern "C"
{
    __declspec(dllexport) IRisk* __stdcall CreateRisk()
    {
        return new  Risk();


    }
}

IRisk is derived from IUnknown and has a custom method Calculate:

class IRisk: public IUnknown                               
{
public:
    virtual int __stdcall Calculate(int i,double s) = 0;  
};

the class Risk implements IRisk interfaces (the implementation is omitted here).

What I want is to call the function CreateRisk in c# and obtain a reference to IRisk.

I defined a wrapper interface in c#

    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]        
    public interface IRisk
    {
        [PreserveSig]                                            
        int Calculate(int i,double s);
    }

and I added a dll entry in C#

    [DllImport("Risk.dll")]
    extern static IntPtr CreateRisk();

I can call CreateRisk in C# and obtain a value type of IntPtr. Is there any way to marshal the IntPtr to the c# IRisk interface so that I can call the Calculate method in C#?

I have tried Marshal.GetIUnknownForObjectInContext,Marshal.GetObjectForIUnknown, but to no avail.

I know creating a COM component can do it. However, COM component needs to be registered with the system. I want to avoid those troubles and let C# use the interface exported by the C++ dll directly.

PS:

The following is my Risk class implementation:

class IRisk : public IUnknown                               
{
public:
    virtual int __stdcall Calculate(int i,double y ) = 0;  
};

class Risk:public IRisk
{
    int count;
public:
    Risk();
    virtual int __stdcall Calculate( int i ,double y);
    virtual ULONG __stdcall AddRef();
    ULONG __stdcall Release();
    HRESULT __stdcall QueryInterface(const IID& riid,void **ppvObject);
};

Risk::Risk(){
    count=0;
}

ULONG __stdcall Risk::AddRef()
{
    return ++count;
}
ULONG __stdcall Risk::Release()
{
    return --count;
}
HRESULT __stdcall Risk::QueryInterface(const IID& riid,void **ppvObject) {
    *ppvObject=this;
    AddRef();
    return S_OK;
}
int __stdcall Risk::Calculate(int i ,double y) {
    return (int)(i+y);
}

回答1:

The following will work:

[DllImport("Risk.dll")]
extern static IRiskAssessment CreateRisk();

You will need to add a GUID to the interface:

[Guid("your GUID goes here")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]        
public interface IRiskAssessment
{
    [PreserveSig]                                            
    int Calculate(int i,double s);
}

Obviously you need to use the same GUID in the C# code as was used in the C++ code!

Update

Looking at your C++ COM object implementation, the obvious fault is in QueryInterface. You must not return S_OK for all GUIDs. Only return S_OK for the interfaces you implement. Return E_NOINTERFACE for the others.

回答2:

If I were you I would use SWIG to generate a C# interface for your C++ code, saves you a lot of troubles. Also, I think your assembly may have to be compiled in mixed-mode, or as a managed assembly.

来源：https://stackoverflow.com/questions/13741373/how-to-use-interface-pointer-exported-by-c-dll-in-c-sharp

标签

c++

pinvoke