通过COM将一个对象从C++传递给C#

Question

你好，我有一个在C#中可见的COM API，如下所示：

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

这个想法是，根据字符串参数的值，我期望第三个参数是不同的类，并在实现中适当地进行转换（我知道这种设计并不理想，但我在这里没有太多的灵活性）。

假设对于某些字符串参数的组合，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调用我的API方法；然而我在创建第三个参数所需的正确的variant时遇到了困难。

我正在使用CComVariant(...)，传递一个指向我刚刚构建的TestObject的IDispatch。

假设pTestObject是指向我的TestObject的IDispatch指针，我有以下代码：

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

然而，当C#函数最终被调用时，我发现对象的类型是bool而不是我期望的TestObject。

有什么想法吗？

Stefano

- Stefano Ricciardi

---

当您在调用之前使用非托管调试器查看pTestObject.vt时，其值是多少？ - Hans Passant

实际上它确实是VT_BOOL类型。不确定为什么。也许这是默认值？ - Stefano Ricciardi

Answer 1

我有几个建议。首先，为您在COM中使用的任何东西创建一个接口，即使它只是一个普通的DTO，没有方法，只有属性。COM喜欢接口。它非常喜欢接口，以至于您在COM中接触到的所有内容都是接口。

另一个建议是，在您在COM中接触到的任何东西上放置GuidAttribute。这将确保在使用COM注册托管程序集时，您的注册表不会出现问题。COM比接口更喜欢GUID，并且如果一个接口注册到多个GUID上，COM很容易感到困惑，如果您不在代码中硬编码接口GUID，则可能会发生这种情况。具体类也需要一个GUIDAttribute。

我知道这很糟糕，但这就是为什么微软正在努力让人们远离使用COM。

话虽如此，您可能希望像这样使用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??
            }
        }
    }
}

以下的C++代码可以实现该功能：

以下的C++代码可以实现该功能：

// 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;
}

- ntcolonel

---

非常感谢，这解决了问题！注意：我实际上有一个TestObject的接口，只是没有在我的问题中提到，以免使问题过于混乱。也许为了更清晰，我应该把它放进去。 - Stefano Ricciardi