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.NET Forum / .NET Framework / Interop / March 2005Using calling .NET dll from VB6
Hi, We have a legacy application (which is quite complex and large) written in VB6. We are currently upgrading it and enhancing its functionalities. We have also adopted .NET as our new development evironment. As a result of this, the upgraded version of our application contains code which calls .NET dlls. This was quite a challenge for us because we had never used interop abilities from COM->.NET... it was usually the other way around. The application at the moment has this really strange bug that occurs once in a blue moon it's happened about 3 times in the past 2 weeks). A 3rd party component written in C# is being used by the VB6 application to access a keyboard smart card reader. It usually works most of the time, however, on rare occassions, the application is unable to connect to the keyboard by calling the C# dll's Connect() method. It comes up with this exception (of which i cannot remember). We wrote a little test app in C# that uses the 3rd party dll to connect to the keyboard and it works fine, suggesting to us that it is an Interop issue. We're thinking that some resources or objects accessed form the .NET dll may have not been disposed or something....?? As a general question, is it recommended for developers to exploit the advantages of COM Interop during the migration from unmanaged to managed code? Would it be ideal for us to start from scratch and write everything in .NET? I understand that this would depend greatly on the size of the application and the available time, but what would you do? What suggestions do the more experienced developers have for the less experienced? Thanks. Michael--J. From everything I have read and even attempted, hosting controls written in .NET as ActiveX controls is NOT supported by Microsoft, so its interesting to hear that you are using a commercial product that does this. I would ask the developer of that product to document that what they are doing is supported by Microsoft. I have been researching and experimenting with creating a .NET ActiveX control myself which is why I am fairly familiar with it being unsupported. If in your research you find otherwise, I would be very happy to hear about it. I've currently got a control working except that events fired from the control are: 1) not being received in VB6, and 2) causing an NotImplementedException to be thrown in Visual FoxPro (I'm assuming the same thing is happening in VB except that VB handles the exception differently). Hi Mike, If you check this site: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/c allnetfrcom.asp, i believe that it is supported and was intended to allow developers to slowly migrate from COM to .NET. Thanks. > From everything I have read and even attempted, hosting controls written in > ..NET as ActiveX controls is NOT supported by Microsoft, so its interesting [quoted text clipped - 9 lines] > NotImplementedException to be thrown in Visual FoxPro (I'm assuming the same > thing is happening in VB except that VB handles the exception differently). Calling .NET _components_ (akin to ActiveX/COM .DLLs or .EXEs) via COM wrappers is supported through interop. Using visual .NET _user controls_ (akin to ActiveX .OCX) with a COM wrapper is NOT supported, from what I understand. I skimmed that article you linked to and nowhere is there a user control used as the example. The ActiveX Control (an OCX vs just a .DLL or .EXE) seems to be a superset of plain old COM component functionality. The "extra" stuff required to make a visual control work does not seem to be fully supported in .NET interop. The only place it seems to be supported is when hosting the control in Internet Explorer. Here's a few links to support what I'm saying: http://www.ondotnet.com/pub/a/dotnet/2003/01/20/winformshosting.html http://www.c-sharpcorner.com/Code/2003/March/ActiveXInNet.asp http://www.eggheadcafe.com/ng/microsoft.public.dotnet.framework.interop/post2022 8888.asp Here's a killer link on a post from Microsoft (its 2001, but I haven't heard anything different from MS since then): http://discuss.develop.com/archives/wa.exe?A2=ind0107b&L=dotnet&F=&S=&P=16992 I have this same problem. It's probably describing some of the programmability issues Microsoft mentioned in the previous link: http://www.dotnet247.com/247reference/msgs/30/150026.aspx > Hi Mike, > > If you check this site: http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/c allnetfrcom.asp, > i believe that it is supported and was intended to allow developers to slowly > migrate from COM to .NET. Thanks. [quoted text clipped - 12 lines] > > NotImplementedException to be thrown in Visual FoxPro (I'm assuming the same > > thing is happening in VB except that VB handles the exception differently). I see what you mean Mike: User Controls created in .NET cannot be "called" from VB6 using COM. Ok. Fortunately that is not what we are trying to achieve. We are only calling a .NET dll from VB6 using COM. Would you have any idea as to what causes the problem i mentioned in my fisrt post - given that we are calling a .NET dll rather than a .NET User Control? Thanks. Michael--J. > Calling .NET _components_ (akin to ActiveX/COM .DLLs or .EXEs) via COM > wrappers is supported through interop. Using visual .NET _user controls_ [quoted text clipped - 50 lines] > > > thing is happening in VB except that VB handles the exception > differently). haha, how can we help you if you do not remember the exception. It seems that the dll can not get the handle of the KB at that moment or you do not dispose the ref first. In addition, how can you know the test app in C# works in a blue moon night as well :) haha, M$ would like you to do everything in .Net Egghead > Hi, > [quoted text clipped - 33 lines] > > Michael--J. Well the reason why i can't remember the exception is because i'm not the one actually writing the app - one of my colleagues simply asked me to help him out about a particular problem he'd been having, and he could not actually recall the exact exception message, but he described to me what was happening. I told him, next time the exception takes place, make sure you get the exception message. The exception happened again, and this time he only got a portion of the message - the string was apparently incomplete. This was it: "System.InvalidOperationException: Operation is not valid due to the current state of the object. at Subsembly.SmartCard.PcSc.SCardComm.Connect(String sReader, SCardAccessMode nAccessMode, SCardProtocolIdentifiers nPreferredProtocols) at PCSC_O" Apart from the C# test app, we also have another C# app realesed commercially that uses the same .NET dll. We have had no problems with that at all. Thanks. Michael--J. > haha, how can we help you if you do not remember the exception. > It seems that the dll can not get the handle of the KB at that moment or you [quoted text clipped - 41 lines] > > > > Michael--J. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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