Attaching an event handler to a Com event: InvalidOperationException (S7-PLCSIM)

Question

I'm trying to attach an Event Handler to an Event of a COM Object. But I get an InvalidOperationException.

using S7PROSIMLib;

private S7ProSim ps = new S7ProSim();

ps.Connect();
ps.SetScanMode(ScanModeConstants.SingleScan);
ps.BeginScanNotify();

try {
    ps.ScanFinished += Ps_ScanFinished;
    //IS7ProSimEvents_ScanFinishedEventHandler scanFinishedDelegate = new IS7ProSimEvents_ScanFinishedEventHandler(Ps_ScanFinished);
    //ps.ScanFinished += scanFinishedDelegate;
}
catch (InvalidOperationException ex)
{
    Console.WriteLine(ex.Source);
    Console.WriteLine(ex.InnerException);
    Console.WriteLine(ex.Message);
    Console.WriteLine(ex.StackTrace);
}

private void Ps_ScanFinished(object ScanInfo)
{
    Console.WriteLine("Event fired");
} 

The Output is:

event invocation for COM objects requires event to be attributed with DispIdAttribute
   at System.Runtime.InteropServices.ComAwareEventInfo.GetDataForComInvocation(EventInfo eventInfo, Guid& sourceIid, Int32& dispid)
   at System.Runtime.InteropServices.ComAwareEventInfo.AddEventHandler(Object target, Delegate handler)
   at PiPLCSimBridge.Form1.Form1_Load(Object sender, EventArgs e) in C:\PiPLCSimBridge\Form1.cs:Line 72.

I have also tried to use the commented out code, but got the same Exception.

Attaching the events like this should work, a popular Tool using the same COM interface is doing it this way.

What is wrong with my Code?

Answer 1

As luck would have it, I recently encountered this exact same error with this exact same COM library.

The problem is caused by the Embed Interop Types setting in the prosim reference.

I discovered this fact via the discussion here.

So, in the project references section, check the properties for the prosim reference. Make sure Embed Interop Types is not set.