Cannot cast generic 'Group' to 'NoMDEntriesGroup'

[loganm]

Hi all,

I'm trying to get a Group out of a message, but the generic group won't cast into the specific group variable.

The following code is based on the example shown on the website at http://quickfixn.org/tutorial/repeating-groups

public void OnMessage(QuickFix.FIX42.MarketDataIncrementalRefresh message, SessionID sessionID)
{
    MDReqID mdReqID = message.MDReqID;
    var noMDEntriesGroup = new QuickFix.FIX42.MarketDataIncrementalRefresh.NoMDEntriesGroup();
    for (int grpIndex = 1; grpIndex <= message.GetInt(Tags.NoMDEntries); grpIndex += 1)
    {
        noMDEntriesGroup = message.GetGroup(grpIndex, Tags.NoMDEntries);
        // code omitted here
    }
}

It generates the following error in Visual Studio

Error 1 Cannot implicitly convert type 'QuickFix.Group' to 'QuickFix.FIX42.MarketDataIncrementalRefresh.NoMDEntriesGroup'. An explicit conversion exists (are you missing a cast?)

According to the error message, I tried an explicit conversion to no avail (code follows). Doing this got rid of the Visual Studio error message, but resulted in an InvalidCastException at runtime.

noMDEntriesGroup = (QuickFix.FIX42.MarketDataIncrementalRefresh.NoMDEntriesGroup)message.GetGroup(grpIndex, Tags.NoMDEntries);

Thanks, Logan

[gbirchmeier]

Mail list thread: http://lists.quickfixn.com/pipermail/quickfixn-quickfixn.com/2012q2/000124.html

[gbirchmeier]

Has anyone looked at how Java does it?

[judwhite]

A bit heavy handed, but would this work?

public void OnMessage(QuickFix.FIX42.MarketDataIncrementalRefresh message, SessionID sessionID)
{
    MDReqID mdReqID = message.MDReqID;

    int count = message.GetInt(Tags.NoMDEntries);
    var list = new List<QuickFix.Group>();
    for (int grpIndex = 1; grpIndex <= count; grpIndex++)
    {
        list.Add(message.GetGroup(grpIndex, Tags.NoMDEntries));        
    }

    foreach (var noMDEntriesGroup in list.OfType<QuickFix.FIX42.MarketDataIncrementalRefresh.NoMDEntriesGroup>())
    {
      // code omitted here
    }
}

If you're expecting them to all be NoMDEntriesGroup you might replace OfType with Cast.

[judwhite]

Sorry I missed the part where you already tried the explicit cast. The above code probably won't be of any use.

[loganm]

Unfortunately, bigger hammers aren't the solution here. I've tried :).

So as far as a solution in code, what does everyone else think is the best method?

I'm a bit pressed for time but am willing to do some work on this. It'll make my life easier in the long run so my boss shouldn't mind.

[gbirchmeier]

We'd really like to maintain some semblance of similarity between QF/n and QF for C++/Java.

I'd really like to see how QF/J handles this. They might have a design we can steal. I'll have a look later if I can find the time.

[blackhill]

I had the same problem with a MarketDataSnapshotFullRefresh.

A workaround was :

       QuickFix.Group mdGroup;

       for(int i = 1; i < message.GetInt(Tags.NoMDEntries);i++)
       {
           mdGroup = message.GetGroup(i, Tags.NoMDEntries);

           Console.WriteLine(mdGroup.GetString(Tags.MDEntryPx));
           Console.WriteLine(mdGroup.GetString(Tags.MDEntrySize));
       }

Hope this help !

S

[loganm]

That's how I ended up having to do it. Thanks Steven.

This is still a bug, so how should we go about fixing it? I don't mind doing some of the work, but I wouldn't want to attempt anything without guidance. I'm new to contributing to open source projects.

[gbirchmeier]

Notes on methods of other QF versions:

QF: msg.getGroup(1,group_object) (this is a member of Message, not FieldMap) QF/j: msg.getGroup(1,group_object) and group_object=msg.getGroup(1,tag_int) (each are from FieldMap) QF/n: group_object = msg.GetGroup(1, tag_int)

Java: http://quickfixj.org/quickfixj/usermanual/1.5.1/usage/repeating_groups.html C++: http://www.quickfixengine.org/quickfix/doc/html/repeating_groups.html

[gbirchmeier]

Unit tests: https://github.com/gbirchmeier/quickfixn/tree/56_group

[gbirchmeier]

I see the root cause now. When Message::FromString() creates a Message object from a string, it constructs Group objects instead the type-specific subclasses of Group. Therefore, the objects retrieved by GetGroup() are also Group objects. You can't cast a base class to a subclass if it was never a subclass-type object in the first place.

So I need to see if we have some kind of Group factory to create the typed Group objects. If we don't, then I guess I need to generate one.

[NickT1977]

Each of the following classes implements IMessageFactory that has a method called 'Create' that returns a Typed Group object based on the arguments passed in, it has the following signiture: public Group Create(string beginString, string msgType, int correspondingFieldID)

The classes are: QuickFix.FIX40.MessageFactory(); QuickFix.FIX41.MessageFactory(); QuickFix.FIX42.MessageFactory(); QuickFix.FIX43.MessageFactory(); QuickFix.FIX44.MessageFactory();

[gbirchmeier]

Thanks, Nick. I figured it was in there somewhere, but couldn't remember where. With this, I will probably be able to knock this out this week.

[gbirchmeier]

I think I solved it on my branch. Will integrate tomorrow. https://github.com/gbirchmeier/quickfixn/tree/56_group

[gbirchmeier]

Damn, still failing AT 14i in the FIX50x sets.

Note to self: cd AcceptanceTest runat.bat release 5006 definitions\server\fix50\14i_RepeatingGroupCountNotEqual.def cfg\at_50.cfg runat.bat release 5007 definitions\server\fix50sp1\14i_RepeatingGroupCountNotEqual.def cfg\at_50_sp1.cfg runat.bat release 5008 definitions\server\fix50sp2\14i_RepeatingGroupCountNotEqual.def cfg\at_50_sp2.cfg