通过COM将对象从C++传递到C#

Question

您好在C#中有一个COM可见API,如下所示:

public void DoSomething(string par1, string par2, object additionalParameter)

我的想法是,基于字符串参数的值,我希望将不同的类作为第三个参数,并在实现中适当地投射它(我知道这个设计不是最优的,但我在这里没有太大的灵活性).

假设对于字符串参数的某些组合,附加参数的类型如下:

[ComVisible(true)]
[Serializable]
public class TestObject    
{
    public string String{ get; set; }

    public long Long { get; set; }
}

我需要从一些非托管代码中获取我的API方法; 但是我在创建variant第三个参数所需的合适时遇到了困难.

我正在使用CComVariant(...)传递IDispatch指向我刚刚构建的TestObject.

假设这pTestObject是一个指向我的TestObject的IDispatch指针,我有以下内容:

CComVariant pObjectVariant(pTestObject);
DoSomething(BSTR(L"first"), BSTR(L"second"), pObjectVariant);

但是,当最终调用C#函数时,我看到该对象具有类型bool而不是TestObject我期望的类型.

任何的想法？

斯特凡诺

Answer 1

我有几个建议.首先,为你在COM中触摸的任何东西创建一个接口,即使它只是一个没有方法和属性的沼泽标准DTO.COM喜欢界面.它非常爱它们,你在COM中触摸的一切都是一个界面.

另一个建议是你在你接触COM的任何东西上放置一个GuidAttribute.这将确保在使用COM注册托管程序集时,您的注册表不会被删除.COM喜欢GUID,而不是喜欢接口,如果接口注册到多个GUID,它会很容易混淆,如果你没有在代码中修改接口GUID,就会发生这种情况.具体类还需要GUIDAttribute.

我知道这很糟糕,但这就是为什么MS努力让人们远离使用COM.

话虽这么说,你可能想要C#这样:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace ClassLibrary1
{

    [ComVisible(true)]
    [Guid("1CEB24F2-BF13-417F-B5BC-DB8E82A56EAE")]
    public interface ITestEntity1 //This is how TestEntity1 is visible to the COM code...so it needs a Guid.
    {
        bool Thing { get; set; }
    }


    [ComVisible(true)]
    [Guid("C8B5A7C2-F67C-4271-A762-3642754F2233")]
    public class TestEntity1 : ITestEntity1  //Created by the COM runtime...needs a Guid.
    {
        public bool Thing { get; set; }
    }

    [ComVisible(true)]
    [Guid("8904A7EC-D865-4533-91EC-1F68524651D0")]
    public interface ITestEntity2
    {
        string Description { get; set; }
    }

    [ComVisible(true)]
    [Guid("668EE2E8-5A60-468B-8689-D9327090AA44")]
    public class TestEntity2 : ITestEntity2
    {
        public string Description { get; set; }
    }

    [ComVisible(true)]
    [Guid("2791082F-F505-49C4-8952-80C174E4FE96")]
    public interface ITestGateway
    {
        //MarshalAsAttribute is somewhat important, it tells the tlbexp.exe tool to mark
        // the comInputValue parameter as IUnknown* in the COM interface.
        //This is good because VARIANTS kinda suck...You'll see what I mean in the C++
        // side.  It also keeps some jack-wagon from passing a VARIANT_BOOL in
        // on your object parameter.
        void DoSomething(string a, [MarshalAs(UnmanagedType.Interface)]object comInputValue);
    }

    [ComVisible(true)]
    [Guid("C3D079F3-7869-4B3E-A742-263775C6EA63")]
    public class TestGateway : ITestGateway
    {
        public void DoSomething(string a, object comInputValue)
        {
            if (a == "yes")
            {
                var entity = (TestEntity1)comInputValue;
            }
            else
            {
                var entity = (TestEntity2) comInputValue;
            }
            //OR

            if(comInputValue is TestEntity1)
            {
                //Do whatever here, and you don't need to test
                // a string input value.
            }
            else if(comInputValue is TestEntity2)
            {
                //Other stuff is done here.
            }
            else
            {
                //Error condition??
            }
        }
    }
}

这可以通过以下C++调用:

// ComClient.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#import <mscorlib.tlb> raw_interfaces_only

//This creates the CLSID_ and IID_ constants, and
// some strongly-typed interfaces.
#import "..\Debug\ClassLibrary1.tlb" no_namespace named_guids


int _tmain(int argc, _TCHAR* argv[])
{
    ITestGateway* test = NULL;

    char buffer[50];
    gets(buffer);  //Just a pause to attach the debugger in Managed + Native mode...hit enter in the console.

    CoInitialize(NULL);

    HRESULT hr = CoCreateInstance(CLSID_TestGateway,
        NULL,
        CLSCTX_INPROC_SERVER,
        IID_ITestGateway,
        reinterpret_cast<void**>(&test));

    if(FAILED(hr)) {
        printf("Couldn't create the instance!... 0x%x\n", hr);
        gets(buffer);
    } else {
        _bstr_t someString("yes");

        //Instead of fooling with CComVariant,
        // just directly create a TestEntity1...which COM will return
        // as an ITestEntity1.
        ITestEntity1* testEntity1 = NULL;
        HRESULT hr = CoCreateInstance(CLSID_TestEntity1,
            NULL,
            CLSCTX_INPROC_SERVER,
            IID_ITestEntity1,
            reinterpret_cast<void**>(&testEntity1));

        if(FAILED(hr)) {
            printf("Entity was not created!... 0x%x\n", hr);
            gets(buffer);
            return 0;
        }

        //Set some kind of property just for show.
        testEntity1->PutThing(VARIANT_FALSE);



        //If you attached your debugger with Managed code & Native code,
        // you should be able to hit a C# break point during this call.
        //Also, notice that there is no cast for testEntity1.  All interfaces
        // in COM derive from IUnknown, so you can just pass it.
        //IDispatch also derives from IUnknown, so if that's what you already have,
        // you can just pass it as well, with no cast.
        test->DoSomething(someString, testEntity1);

        printf("Something was done.");

        testEntity1->Release(); //Release anything you make through CoCreateInstance()
    }

    test->Release();

    return 0;
}