CODEWITHSUNDEEP
c++visual-studio-2008com
David
David

Reputation: 5537

c++ application cannot find com dll because compiler generating .tlh file with incorrect guids

I have a com dll that gets used in a c++ project. I have not registered the dll but its in the same solution. My application continuously breaks and I found that it breaks here 

hr = CoCreateInstance(rclsid, pOuter, dwClsContext, __uuidof(IUnknown), reinterpret_cast<void**>(&pIUnknown));

because I don't have a com class with that guid (rclsid). I did more digging and I found that it gets the guid from a .tlh file in a temp directory but the guid differs from my code.

.tlh file

    struct __declspec(uuid("b10a18c8-7109-3f48-94d9-e48b526fc928"))
    DocMapEntry;
    // [ default ] interface _DocMapEntry
    // interface _Object
    // interface IDocMapEntry

c# code (com visable dll)

    [ComVisible(true)]
    [Guid("E98676B9-1965-4248-A9B6-74EC5EE5385A")]
    [InterfaceType(ComInterfaceType.InterfaceIsDual)]
    public interface IDocMapEntry
    {
        int Offset { get; set; }
        int PageOffset { get; set; }
        int PageNumber { get; set; }
    }

I tried changing the value to the correct value but it continuously gets changed and visual studio asks me to reload.

When I look at the class using com explorer I get a different clsid, and when I try to unregister it, it completes successfully but the item is still there and the file location is mscoree.

my c++ application uses the guid in the .tlh file but doesn't find the file.

public class DocMapEntry : IDocMapEntry
 {
   ...
 }

My questions are.

Do I need to register my com dll for this to work? how can I set the guid for the class? Why would it create a temp .tlh file with the wrong guid? (optional) why can't I change this file? (optional)

Thank you.

Upvotes: 1

Views: 937

Answers (3)

Hans Passant
Hans Passant

Reputation: 941585

 public interface IDocMapEntry

COM strongly distinguishes between a CLSID and a IID. The IID, the interface identifier, identifies an interface implemented by a COM object. We can see that one, you specified it with the [Guid] attribute. You'd pass it as the 4th argument of CoCreateInstance(), the one where you now pass the IID of IUnknown.

What you need is the CLSID, the class identifier. We can't see it in your snippet, it is the one for whatever class implements your IDocMapEntry. With some odds that it is named DocMapEntry. And additional odds that you didn't give it a [Guid] so the CLR will auto-generate one for you. Note that it will change when you modify the class, one possible reason for CoCreateObject() to fail.

Forgetting to register the .NET assembly with Regasm.exe /codebase is another reason, you stated as much in your question. This is required or COM will not be able to find the DLL and fails the call with error code 0x80040154, "Class not registered". Having it in the same solution is not sufficient, COM search rules are not anything like those used by .NET.

Also note that you are exposing the internals of the class, avoid that by applying the [ClassInterface(ClassInterfaceType.None)] attribute on the class.

Upvotes: 2

soma
soma

Reputation: 11

Is your COM object in a C# dll ? If yes, did you run regasm previously in your system, which could have registered the COM dll.

Guid("E98676B9-1965-4248-A9B6-74EC5EE5385A"): This is your interface GUID, not your component guid.

You should have a GUID declared for COM object in the implementation class, which should be b10a18c8-7109-3f48-94d9-e48b526fc928.

Why would it create a temp .tlh file with the wrong guid?

Are you sure you have not imported the COM dll using #import keyword.

Upvotes: 1

Captain Obvlious
Captain Obvlious

Reputation: 20063

No you do not have to register the COM DLL but you do need to register an object responsible for creating the COM objects. Use do this in your application by calling CoRegisterClassObject after the COM subsystem has been initialized (by calling CoInitialize or CoInitializeEx).

IClassFactory *factory = new ObjectFactory();

// Register the factory responsible for creating our COM objects
DWORD classToken;
HRESULT hr = CoRegisterClassObject(
    Object_CLSIID,
    factory,
    CLSCTX_LOCAL_SERVER,
    REGCLS_MULTIPLEUSE,
    &classToken);

// Now we can create the objects
Object *obj = nullptr;
hr = CoCreateInstance(
    Object_CLSIID,
    nullptr,
    CLSCTX_LOCAL_SERVER,
    Object_IID,
    reinterpret_cast<void**>(&obj));

Not sure why it's creating a temporary .tlh but I suspect it's something in your build settings - possibly in the build steps.

Upvotes: 1

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