将COM接口传递回库的正确方法是什么？

Question

尝试将接口作为参数从.NET传递到DLL时，我收到System.InvalidCastException运行时错误。 代码在.NET端失败，永远不会转移到Delphi端。

在Delphi中构造一个简单的ActiveX库dll。 两个自动化对象： MyContent和MyContainer 。 MyContainer有单个方法Add ，它接受由IMyContent继承的IBase接口。 IBase是一个IDispatch

[
  uuid(E29018FF-F142-4BAE-B7A4-AE0A8847E930),
  version(1.0)

]
library DelphiComLib
{

  importlib("stdole2.tlb");

  interface IMyBase;
  interface IMyContainer;
  coclass MyContainer;
  interface IMyContent;
  coclass MyContent;


  [
    uuid(07D17021-9E7F-4D7A-B861-59A35EC686A0),
    dual,
    oleautomation
  ]
  interface IMyBase: IDispatch
  {
  };

  [
    uuid(A6AB6F7D-8BEB-459F-A2F8-BC06FF81A45D),
    helpstring("Dispatch interface for MyContainer Object"),
    dual,
    oleautomation
  ]
  interface IMyContainer: IDispatch
  {
    [id(0x000000C9)]
    HRESULT _stdcall Add([in] IMyBase* AMyBase);
  };

  [
    uuid(AB82964C-13D7-423B-9B16-A789D5D30421),
    helpstring("Dispatch interface for MyContent Object"),
    dual,
    oleautomation
  ]
  interface IMyContent: IMyBase
  {
  };

  [
    uuid(DDDF77E5-E6A6-4429-BD4A-D9695E9E6CED),
    helpstring("MyContainer Object")
  ]
  coclass MyContainer
  {
    [default] interface IMyContainer;
  };

  [
    uuid(134BF8B2-30C3-4C37-8F74-3F677808300A),
    helpstring("MyContent Object")
  ]
  coclass MyContent
  {
    [default] interface IMyContent;
  };

};

类的实现无关紧要。 为了使样本保持最小， Add的实现可以留空。 这是容器

unit Unit1;

{$WARN SYMBOL_PLATFORM OFF}

interface

uses
  ComObj, ActiveX, DelphiComLib_TLB, StdVcl;

type
  TMyContainer = class(TAutoObject, IMyContainer)
  protected
    procedure Add(const AMyBase: IMyBase); safecall;

  end;

implementation

uses ComServ;

procedure TMyContainer.Add(const AMyBase: IMyBase);
begin

end;

initialization
  TAutoObjectFactory.Create(ComServer, TMyContainer, Class_MyContainer,
    ciMultiInstance, tmApartment);
end.

......和内容

unit Unit2;

{$WARN SYMBOL_PLATFORM OFF}

interface

uses
  ComObj, ActiveX, DelphiComLib_TLB, StdVcl;

type
  TMyContent = class(TAutoObject, IMyContent)
  protected

  end;

implementation

uses ComServ;

initialization
  TAutoObjectFactory.Create(ComServer, TMyContent, Class_MyContent,
    ciMultiInstance, tmApartment);
end.

注册库并添加对控制台应用程序的引用。 以下代码将产生运行时错误

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using DelphiComLib;
namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            IMyContainer container = new MyContainer();
            IMyContent content = new MyContent();
            container.Add(content);


        }
    }
}

如果我在.NET中创建一个实现IBase的类，我可以成功地传回接口。

这是.NET的StackTrace

   at System.StubHelpers.InterfaceMarshaler.ConvertToNative(Object objSrc, IntPtr itfMT, IntPtr classMT, Int32 flags)
   at DelphiComLib.IMyContainer.Add(IMyBase AMyBase)
   at ConsoleApplication2.Program.Main(String[] args) in c:\users\jaspers\documents\visual studio 2015\Projects\ConsoleApplication2\ConsoleApplication2\Program.cs:line 15
   at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
   at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Answer 1

您的问题与接口传递给COM DLL的方式无关。

引发错误System.InvalidCastException只是因为在Delphi实现端从IMyContent为IMyBase失败。 原因是严格来说，您的实现并不直接实现IMyBase接口（即使它实现了派生的IMyContent接口）。 你可以试试：

var
  MyContent: IMyContent;
  MyBase: IMyBase;
begin
  MyContent := CoMyContent.Create;
  MyBase := MyContent as IMyBase; // EIntfCastError is raised
end;

您的实现类必须显式实现IMyBase才能使转换工作：

这将改变您的实现类的声明：

type
  TMyContent = class(TAutoObject, IMyContent, IMyBase)
  ...
  end;

这反过来会使任何COM客户端代码（Delphi，C ++，.NET等）的转换工作