COM object event handling problem with late binding

Luca Poalini

Hi all,

I'm developping a C# COM object with event that I must handle in my application.

If I add the COM object in the application's References all work fine but if I use the COM object with late binding I don't receive the event.

The code relative to the interface is:

[Guid("CA2F7751-32F3-4E87-BC88-CC99F4186A09"),

InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]

public interface ER755COM_Events

{

[DispId(5)]

void OnReceiveDataEvent(string args);

}

[Guid("94E7D0FD-2627-4D37-90A5-D8F7A01B4D41"),

ClassInterface(ClassInterfaceType.AutoDispatch),

ComSourceInterfaces(typeof(ER755COM_Events))]

public class ER755 : ER755COM_Interface

{

….

public delegate void OnReceiveDataEventHandler(string args);

public event OnReceiveDataEventHandler OnReceiveDataEvent;

….

if (OnReceiveDataEvent != null)

{

Console.WriteLine("OnDataReceive calling OnReceiveDataEvent");

OnReceiveDataEvent(CardID);

}

….

}

And the code in the application is:

[ComImport, Guid("CA2F7751-32F3-4E87-BC88-CC99F4186A09")]

class ER755

{

}

public delegate void OnReceiveDataEventHandler(string args);

System.Type objType;

dynamic comObject;

…..

objType = System.Type.GetTypeFromProgID("ER755.ER755");

comObject = System.Activator.CreateInstance(objType);

comObject.OnReceiveDataEvent += new OnReceiveDataEventHandler(OnMyReceiveDataEventHandler);

…..

Any suggestions?

Thanks in advance

18 Replies

Know the answer? Post it — somebody with the same question will find it here.

Luca Poalini11y ago
Thanks a lot. I will try it as soon as possible!!
Vulpes11y ago
Just add the ER755COM_Events interface and the ER755COM_Events_SinkHelper class to your project as if you were adding an ordinary interface and class to it.

The only thing that's really special about the ER755COM_Events_SinkHelper class is that it implements the ER755COM_Events interface and so therefore has to have an implementation of the OnReceiveDataEvent method.

We use can then create and use an object of this class as an 'event sink' and link it up to the connection point for the events interface using the IConnectionPoint.Advise() method.

When the event occurs, the OnReceiveDataEvent method of our sink should then get called automatically.

For now, I'd just put some code in this method - a Console/Debug/Trace.WriteLine or MessageBox.Show - to (hopefully) indicate that notifications are being received.
Luca Poalini11y ago
Maybe I didnt understand your suggestion: ER755COM_Events_SinkHelper is a class declared in the COM object, how can I use it on late binding declaring an object of this type?
Vulpes11y ago
I don't understand how there can be error on this line because ER755COM_Events_SinkHelper is a class we've created ourselves within the project:

ER755COM_Events_SinkHelper sink = new ER755COM_Events_SinkHelper();

What is the actual error?

I'd like to try and get this approach to work if we can because it's the only thing I've seen which actually makes sense in the context of events and late binding.
Luca Poalini11y ago
Hi,
now the problem is on this row of code:
ER755COM_Events_SinkHelper sink = new ER755COM_Events_SinkHelper();
Meanwhile I created a new project with same characteristics and in this one I can see a right value on Console.WriteLine(comObject.GetType().ToString()); than now is Smurf.Pants.
Now the question is: how do bind the com event?
The code is in the attached files
Vulpes11y ago
OK I'm back with some code.

After a certain amount of googling I found this CP article which shows how to receive events from late bound COM servers:

http://www.codeproject.com/Articles/10262/Receiving-Events-from-late-bound-COM-servers

I've tried to adapt this code to deal with your event below.

As this is quite an old article some of the interfaces (those beginning with UCOM) are now obsolete, so I've replaced them with the recommended alternatives:

// using directives needed

using System.Runtime.InteropServices;
using System.Runtime.InteropServices.ComTypes;

// interface and class needed

[Guid("CA2F7751-32F3-4E87-BC88-CC99F4186A09"),
InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface ER755COM_Events
{
[DispId(5)]
void OnReceiveDataEvent(string args);
}

[ClassInterface(ClassInterfaceType.None)]
public sealed class ER755COM_Events_SinkHelper: ER755COM_Events
{
public void OnReceiveDataEvent(string args)
{
// add your code to handle event
}
}

// late binding code

System.Type objType = System.Type.GetTypeFromProgID("ER755.ER755");
dynamic comObject = System.Activator.CreateInstance(objType);

// Get the connection point container.
IConnectionPointContainer connectionPointContainer =
comObject as IConnectionPointContainer;

// The GUID of the connection point to query for.
// This is the same as the GUID of the ER755COM_Events interface.
Guid guid = new Guid("CA2F7751-32F3-4E87-BC88-CC99F4186A09");

// Get the connection point of the given GUID.
IConnectionPoint connectionPoint;
connectionPointContainer.FindConnectionPoint(
ref guid,
out connectionPoint );

// Create the actual object to receive the event
// notifications.
ER755COM_Events_SinkHelper sink =
new ER755COM_Events_SinkHelper();

// Connect the sink.
int sinkCookie;
connectionPoint.Advise(sink, out sinkCookie);

// when done you need to clean up with the following code

// Disconnect the sink.
connectionPoint.Unadvise(sinkCookie);

// Release the com object.
Marshal.ReleaseComObject(connectionPoint);
Luca Poalini11y ago
many thanks
Vulpes11y ago
Hmm, I was afraid that would be the answer but it at least explains why you're not seeing any events.

System.__ComObject is a generic COM type rather than the actual COM type and doesn't have any events itself.

To get at the event, I think you'll therefore need to go through COM's IConnectionPointContainer interface which isn't a particularly straightforward matter.

I'll look into it and get back to you later.
Luca Poalini11y ago
I see: "System.__ComObject"
Vulpes11y ago
Well, that's definitely the correct ProgID :)

If immediately after executing this line:

comObject = System.Activator.CreateInstance(objType);

you execute this one:

MessageBox.Show(comObject.GetType().ToString());

what do you see?
Luca Poalini11y ago

Here you are my registry key:
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\ER755.ER755\CLSID]
@="{94E7D0FD-2627-4D37-90A5-D8F7A01B4D41}"
Vulpes11y ago
Well, you're getting the Type object for the COM interface using this programmatic identifier (ProgID):

"ER755.ER755"

so I was wondering whether you're sure that this is correct - in other words is this the ProgID shown in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Classes\ ?

Presumably, it is producing an object reference otherwise you'd have received a null reference exception earlier in the code.

I don't think it would help here for me to have the whole project as this is a specific problem related to a COM component installed on your machine.
Luca Poalini11y ago
Always the same also with BindingFlags.
Sorry what does you means with ProgID?
Would you like to have the entire project?
Vulpes11y ago
Possibly, though I'd check first that there are no non-public events:

BindingFlags bf = BindingFlags.Static | BindingFlags.Instance | BindingFlags.NonPublic;
EventInfo[] comEvents = objType.GetEvents(bf);
// etc

Presumably you're happy that you're using the correct ProgID here?
Luca Poalini11y ago
Hi Vulpes, unfortunately comEvents is an empty array; maybe there is a problem with event registration... :(
Vulpes11y ago
That's strange as it looks like "OnReceiveDataEvent" is the correct name fpr the event :-/

I'd see if you can get a list of public event names for the COM object by running this code:

EventInfo[] comEvents = objType.GetEvents();
string names = "";

foreach (EventInfo eInfo in comEvents)
{
names += eInfo.Name + "\r\n";
}

MessageBox.Show(names);
Luca Poalini11y ago
Hi Vulpes thanks for your reply, I tried your code but after EventInfo comEvent = objType.GetEvent("OnReceiveDataEvent"); comEvent is null...
Vulpes11y ago
To add an 'already coded' event handler to a late bound object I'd try:

using System.Reflection;

// ...

objType = System.Type.GetTypeFromProgID("ER755.ER755");
comObject = System.Activator.CreateInstance(objType);
EventInfo comEvent = objType.GetEvent("OnReceiveDataEvent");
comEvent.AddEventHandler(comObject, new OnReceiveDataEventHandler(MyReceiveDataEventHandler));

// ...

private void MyReceiveDataEventHandler(string args)
{
// code to handle event
}