c# - 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，我期望一个不同的类作为第三个参数，并在实现中适本地转换它(我知道这种设计不是最优的，但我在这里没有太多的灵 active )。

假设对于字符串参数的某种组合，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获取我的 API 方法；但是我在创建第三个参数所需的适当 variant 时遇到了困难。

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

假设 pTestObject 是指向我的 TestObject 的 IDispatch 指针，我有如下内容:

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

但是，当最终调用 C# 函数时，我看到该对象的类型为 bool 而不是我期望的 TestObject。

有什么想法吗？

斯特凡诺

Answer 1

我有几个建议。首先，为您在 COM 中接触的任何内容创建一个接口(interface)，即使它只是一个没有方法而只有属性的沼泽标准 DTO。 COM 喜欢接口(interface)。它非常喜欢它们，以至于您在 COM 中触摸的一切都是一个界面。

另一个建议是将 GuidAttribute 放置在您在 COM 中触摸的任何对象上。这将确保您的注册表在您向 COM 注册托管程序集时不会崩溃。 COM 喜欢 GUID 胜过喜欢接口(interface)，如果一个接口(interface)注册到多个 GUID，它很容易混淆，如果您不在代码中硬修复接口(interface) 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 MS 如此努力地让人们远离使用 COM 的原因。

话虽这么说，您可能想要这样的 C#:

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??
            }
        }
    }
}

可以被下面的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;
}