如何通过方法将返回的接口变量转换为对象？

Question

I am using ac# wrapper, in the c++ library, the called function returns a pointer to the class object. In the c# wrapper, if I call that method it returns an interface variable. That interface variable is null, so I am unable gets the values. How should I handle that interface variable in order to get values. Anyone please help me.

In the below code we have ROOTNET.Interface.NTH1F it is an interface, where ROOTNET.NTH1F is a class

using ROOTNET.Utility;
using ROOTNET.Interface;

NTH1F g = new ROOTNET.NTH1F("g", "background removal", doubleArrayX.Length - 1,    
    doubleArrayX);

g.SetContent(doubleArrayY);
g.GetXaxis().SetRange(xmin, xmax);

ROOTNET.NTH1F bkg = new ROOTNET.NTH1F(g);
bkg.Reset();

bkg.Add(g.ShowBackground(80, ""));

In the above Im expecting the backgroung removed values to be saved in bkg but bkg contains all zeros, can you please help me in getting background removed values of g into bkg.

Where as the code of ShowBackground(int niter, string option) method is

public unsafe virtual NTH1 ShowBackground (int niter, string option)
{
    NetStringToConstCPP netStringToConstCPP = null;
    NetStringToConstCPP netStringToConstCPP2 = new NetStringToConstCPP (option);
    NTH1 bestObject;
    try
    {
        netStringToConstCPP = netStringToConstCPP2;
        int num = *(int*)this._instance + 912;
        bestObject = ROOTObjectServices.GetBestObject<NTH1> (calli ((), this._instance, niter, netStringToConstCPP.op_Implicit (), *num));
    }
    catch
    {
        ((IDisposable)netStringToConstCPP).Dispose ();
        throw;
    }
    ((IDisposable)netStringToConstCPP).Dispose ();
    return bestObject;
}

Answer 1

You cannot treat a pointer value returned from C++ as an interface (unless it's a COM interface, I guess). C++ and C# classes and interfaces may (and mostly probably do) have different low-level structures, so you cannot simply cast one onto another.

The only way is to write another wrapper around C++ class returned by your library. It should look more less like that:

C++/DLL:

__declspec(dllexport) void * ReturnInstance()
{
    return new MyClass();
}

__declspec(dllexport) void MyClass_CallMethod(MyClass * instance)
{
    instance->Method();
}

C#:

[DllImport("MyDll.dll")]
private static extern IntPtr ReturnInstance();

class MyClassWrapper
{
     private IntPtr instance;

     [DllImport("MyDll.dll")]
     private static extern void MyClass_CallMethod(IntPtr instance);

     public MyClassWrapper(IntPtr newInstance)
     {
         instance = newInstance;
     }

     public void Method()
     {
         MyClass_CallMethod(instance);
     }
}

// (...)

IntPtr myClassInstance = ReturnInstance();
MyClassWrapper wrapper = new MyClassWrapper(myClassInstance);
wrapper.Method();

Hope this helps.