We have developed a s/w architecture consisting of set of objects developed in C#. They make extensive use of events to notify the client of changes in status, etc.
The intention originally was to allow legacy code to use these managed objects through COM interop services. That was easy to write in the design specification but, I'm seeing that actually implementing it is more problematic. I've searched for many hours looking for a good sample of event handling using this method. Before we march down that path, I want to make sure that COM interop is the best way to allow legacy code to call our new code.
It appears there are several different options: 1) COM interop, 2) write unmanaged wrapper classes 3) use the /clr compiler switch to enable calling of managed objects, 4) use some sort of reverse pInvoke call. Am I missing any?
Each option will have its benefits & drawbacks and I'm wondering what the best way to go is. Here are specific questions/comments for each
COM INTEROP - It appears event handling is a hurdle. We use events that have variable types as parameters. An event parameter may have an event ID and an object. Based on the event ID, the object will be of a certain type. Can this be handled with COM interop? Many of the objects that are exposed have properties. You can't declare properties in an interface so all properties will need a corresponding get/set method.
WRITE UNMANAGED WRAPPER - I assume this means creating a DLL using the /clr option to allow creating and calling managed objects and exposing unmanaged objects. Would the client of these unmanaged. I haven't done this before. What are benefits/drawbacks of this?
USE THE /CLR SWITCH - I understand this means to add support for managed objects. What are the drawbacks of this approach? Does this option support events as described above? Can we say, "here's the managed library. Use the /clr compiler option with your legacy code and have at it?" I don't know the ramifications of this. Is there a good sample of how this works around? (I'm sure there is, I just haven't found it)
USE A REVERSE PINVOKE - I'm not sure exactly how this would work but, from what I've been able to find, this is not a likely valid solution.
So, what does the decision tree look like to find the correct direction? Any help is appreciated.
DP
I think your initial solution is the best one. COM interop is stable and reasonably well documented. All you need to do is ensure that all the different event objects that might pop out of a given event handler implement the same COM-visible base event object interface (that has the event type id, etc). From there, individual objects can implement whatever other interfaces they want, and your unmanaged code can QI for the right "detail" interface based on whatever criteria you want to define. It's really not that hard. Have a look at this CodeProject article for an end-to-end sample including unmanaged event handlers.
Related
Given Object1 and Object2, are there any techniques for determining if they both implement a common interface? No problem if the interface is known at compile time (use typeof ... is [known interface]), but what about if interface isn't specified at compile time?
Specific use case is implementing a strongly typed collection object. I only want to add Object2 if it shares a common interface as Object1. Typename doesn't work since it returns the underlying object type and I may have two distinct objects each implementing ISomeInterface but on different underlying classes.
An example that doesn't quite work can be found here (as it relies on typename but that doesn't allow for interface comparisons)
Specifically, expanding the IsTypeSafe function found here on CodeReview but adapted so that if an object supports an interface common to all previously added items, it can be added to the list.
Specific question: is there a way to determine if two objects both implement a common interface that is unspecified at compile time?
I got really confused with your "unspecified at compile time" wording, but the crux of your question is here:
if an object supports an interface common to all previously added items, it can be added to the list.
In other words, you're asking if there's a way to do this in VBA (pseudo-mish-mash of VBA/C#):
isOk = item.Type.Interfaces.Any(i => other.Type.Interfaces.Contains(i))
In order to be able to inspect an object variable's implemented interfaces, you'd need to be able to inspect its type at run-time. This ability is called "reflection"... and VBA can't do that.
Rubberduck (disclaimer: I manage this OSS VBIDE add-in project) has a COM API that might eventually grow to support exactly that though (it's open-source, implement it - we are very happy to take pull requests!), but in order to work its magic it needs to literally parse and resolve the entire project and all its references, which means using reflection for what you'd like to use it for, would be a massive performance hit.
A "type-safe" List class in VBA is basically smokes & mirrors. Sorry!
Objective-C’s objects are pretty flexible when compared to similar languages like C++ and can be extended at runtime via Categories or through runtime functions.
Any idea what this sentence means? I am relatively new to Objective-C
While technically true, it may be confusing to the reader to call category extension "at runtime." As Justin Meiners explains, categories allow you to add additional methods to an existing class without requiring access to the existing class's source code. The use of categories is fairly common in Objective-C, though there are some dangers. If two different categories add the same method to the same class, then the behavior is undefined. Since you cannot know whether some other part of the system (perhaps even a system library) adds a category method, you typically must add a prefix to prevent collisions (for example rather than swappedString, a better name would likely be something like rnc_swappedString if this were part of RNCryptor for instance.)
As I said, it is technically true that categories are added at runtime, but from the programmer's point of view, categories are written as though just part of the class, so most people think of them as being a compile-time choice. It is very rare to decide at runtime whether to add a category method or not.
As a beginner, you should be aware of categories, but slow to create new ones. Creating categories is a somewhat intermediate-level skill. It's not something to avoid, but not something you'll use every day. It's very easy to overuse them. See Justin's link for more information.
On the other hand, "runtime functions" really do add new functionality to existing classes or even specific objects at runtime, and are completely under the control of code. You can, at runtime, modify a class such that it responds to a method it didn't previously respond to. You can even generate entirely new classes at runtime that did not exist when the program was compiled, and you can change the class of existing objects. (This is exactly how Key-Value Observation is implemented.)
Modifying classes and objects using the runtime is an advanced skill. You should not even consider using these techniques in production code until you have significant experience. And when you have that experience, it will tell you that you very seldom what to do this anyway. You will know the runtime functions because they are C-based, with names like method_exchangeImplmentations. You won't mistake them for normal ObjC (and you generally have to import objc/runtime.h to get to them.)
There is a middle-ground that bleeds into runtime manipulation called message forwarding and dynamic message resolution. This is often used for proxy objects, and is implemented with -forwardingTargetForSelector, +resolveInstanceMethod, and some similar methods. These are tools that allow classes to modify themselves at runtime, and is much less dangerous than modifying other classes (i.e. "swizzling").
It's also important to consider how all of this translates to Swift. In general, Swift has discouraged and restricted the use of runtime class manipulation, but it embraces (and improves) category-like extensions. By the time you're experienced enough to dig into the runtime, you will likely find it an even more obscure skill than it is today. But you will use extensions (Swift's version of categories) in every program.
A category allows you to add functionality to an existing class that you do not have access to source code for (System frameworks, 3rd party APIs etc). This functionality is possible by adding methods to a class at runtime.
For example lets say I wanted to add a method to NSString that swapped uppercase and lowercase letters called -swappedString. In static languages (such as C++), extending classes like this is more difficult. I would have to create a subclass of NSString (or a helper function). While my own code could take advantage of my subclass, any instance created in a library would not use my subclass and would not have my method.
Using categories I can extend any class, such as adding a -swappedString method and use it on any instance of the class, such asNSString transparently [anyString swappedString];.
You can learn more details from Apple's Docs
I feel like I would benefit greatly from understanding the differences in how these functions work so that I could better understand when to use each one.
I'm having a very difficult time working with two different interops (Excel, and EPDM) which have both made extensive use of weak typed parameters. I keep running into problems using returned objects and casting them to the proper type (per the documentation). After wasting a ton of time, I've found that using TypeName, GetType, and a TypeOf operator with COM objects can yield different results, and in different circumstances each one can be more or less reliable than the next.
Now, in most cases TypeName() seems to be the most reliable for determining type with COM objects. However, avoiding the other two functions entirely seems quite cargo cultish to me, and besides that today I ran into an interesting problem where I can't seem to cast an object to the type reported by TypeName(). An interesting notion was brought up in the comments on that problem that objects which implement IDispatch may actually return the dispatched interface typename, which could partially explain the differences.
I'd really like to better understand how these functions actually work, but I get kind of lost running through the .NET ReferenceSource, so I'm offering a bounty on this question in hopes someone can explain how these different functions work and in what context each should be used.
Here is a code excerpt from working with the Excel interop.
Dim DocProps As Object
DocProps = WeeklyReports.CustomDocumentProperties 'WeeklyReports is a Workbook object
Debug.Print(DocProps Is Nothing)
Debug.Print(TypeName(DocProps))
Debug.Print(TypeOf (DocProps) Is DocumentProperties)
Debug.Print(DocProps.GetType.ToString)
The output is:
False
DocumentProperties
False
System.__ComObject
It is a long story and a bit doubtful that English is going to cut it. It does require understanding how COM works and how it was integrated into the Office products.
At breakneck speed, COM is very heavily an interface-based programming paradigm at its core. Interfaces are easy, classes are hard. Something you see back in the .NET design as well, a class can derive from only one single base class but can implement any number of interfaces. To make language interop work smoothly, it is important to take as few dependencies on language implementation details as possible.
There is a lot that COM does not do that you'd be used to in any modern language. It does not support exceptions, only error codes. No notion of generics at all. No Reflection. No support for method overloads. No support for implementation inheritance whatsoever, the notion of a class is completely hidden. It only appears as a number, the CLSID, a guid that identifies a class type. With a factory function implemented in the COM component that creates an object of the class. The COM component retains ownership of that object. The client code then only ever uses interfaces to make calls to use methods and get or set properties. CoCreateInstance() is the primary runtime support function that does this.
This was further whittled down to a subset called OLE Automation, the flavor that you use when you interop with Office. It strictly limits the kind of types you can use for properties and method arguments with prescribed ways to deal with the difficult ones like strings and arrays. It does add some capabilities, it supports late binding through the IDispatch interface, important to scripting languages. And VARIANTs, a data type that can store a value or object reference of an arbitrary type. And supports type libraries, a machine-readable description of the interfaces implemented by the COM server. .NET metadata is the exact analogue.
And important to this question, it limit the number of interfaces that a class can implement to just one. Important to languages that don't support the notion of interfaces at all, like VBA, scripting languages like Javascript and VBScript and early Visual Basic versions. The Office interop object model was very much designed with these limitations in mind.
So from the point of view from a programmer that uses such a language to automate an Office program, it is completely invisible that his language runtime is actually using interfaces. All he ever sees and uses in his program are identifiers that look like class names, not interface names. That DocumentProperties is actually an interface name is something you can see in Object Browser. Just type the name in the search box, it properly annotates "public interface DocumentProperties / Member of Microsoft.Office.Core" in the lower-right panel.
One specific detail of the Office object model matters a great deal here, many properties and method return types are VARIANTs. A OLE Automation type that can store an arbitrary value or object reference, it is mapped to System.Object when you use .NET. The Workbook.CustomDocumentProperties property is like that. Even though the property is documented to actually return a DocumentProperties interface reference. They probably did this to leave elbow room to some day return another kind of interface. Fairly necessary for "custom document properties".
That the property is a VARIANT doesn't matter that much in languages that support dynamic typing, they take them with stride. It is however pretty painful in a strongly typed language. And pretty unfriendly to programming editors that support auto-completion, like VS's IntelliSense. What you normally do is declare your variable to the expected interface type:
Dim DocProps As DocumentProperties
DocProps = CType(WeeklyReports.CustomDocumentProperties, Microsoft.Office.Core.DocumentProperties)
And now everything lights up. You don't need the CType() cast either if you favor programming VB.NET with Option Strict Off in effect. Which turns it into a programming language that supports dynamic typing well.
We're getting there. As long as you declare DocProps as Object then the compiler knows beans about the interface. Nor does the debugger, it isn't helped by the variable declaration and can only see that it is a __System.ComObject from the runtime type. So it isn't Nothing, that's easy enough to understand, the property getter did not fail and the document has properties.
The TypeName() function uses a feature of the IDispatch interface, it exposes type information at runtime. That happens to work in your case, it usually doesn't, the function first calls IDispatch::GetTypeInfo() to get an ITypeInfo interface reference, then calls ITypeLib::GetDocumentation(). That works, you get the interface name back. Otherwise pretty comparable to Reflection in .NET, just not nearly as powerful. Do not rely on it heavily, there are lots of COM components that don't implement this.
And crucial to your question, TypeOf (DocProps) Is DocumentProperties is a fail whale. Something you'll discover when you try to write the code I proposed earlier. You'll get a nasty runtime exception, System.InvalidCastException:
{"Unable to cast COM object of type 'System.__ComObject' to interface type 'Microsoft.Office.Core.DocumentProperties'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{2DF8D04D-5BFA-101B-BDE5-00AA0044DE52}' failed due to the following error: No such interface supported (Exception from HRESULT: 0x80004002 (E_NOINTERFACE))."}
In other words, the Excel documentation is lying to you. You get an interface back that resembles DocumentProperties, it still has the members that this interface documents, but is no longer identical to the Microsoft.Office.Core.DocumentProperties. It probably once was, many moons ago. A nasty little detail that's buried inside this KB article:
Note The DocumentProperties and the DocumentProperty interfaces are late bound interfaces. To use these interfaces, you must treat them like you would an IDispatch interface.
I am beginner for Visual C++ and currently just learning the concepts of it.
I came to know that there are 2 classes: Managed class & Value class.
Value classes are like normal C++ classes, whose objects can be created which will hold the data.
Managed classes are memory managed by garbage collector.
Questions:
Objects of the managed classes can't be created only handles can be created. Why is this?
Please give me brief idea about an instance? Is it just an object creation or something else?
It is not that you only create handles for managed classes, the instances (or objects) of the managed classes are created on the managed heap and you are given a handle to access that instance.
The full answer is a wiki entry in it's own right, but I'll try give you an idea of what the issues here are;
Managed vs. Value classes are C++/CLI (.Net) type classes. The reference documentation relating to C# and .Net is still valid here and will probably answer some of the more subtle questions here.
A "reference" you speak of is a .Net reference. Some people compare this to a pointer (a smart pointer) and that may help understand some of the code, but it is not a pointer.
Just because it is C++, doesn't mean the usual .Net rules don't apply. The mixed mode is there to allow code to cross the native/.Net boundary, but on either side the native/.Net, respectively, the rules still apply. Speaking from experience, try to keep this "contact" area small and specific helps in dealing with the subtleties and nuances when you try to have one foot in each camp.
Essentially, a managed class must have all it's members managed as well, since the garbage collection of this data is non-deterministic.
In a similar manner, "native data" in a mixed mode application cannot contain managed classes (or references).
gcroot and raw pointers are generally used to mix the two. RAII classes help to manage these elements, but can be specific to your project, so general solutions don't always help.
I have a COM inproc DLL that we are using in our product.
Now if someone finds out which interface and APIs we have exposed from the DLL then those APIs can be called easily.
Is there a way to stop unknown applications from calling my APIs?
Can we add some signature in COM?
The formal way of controlling use of your object is by implementing IClassFactory2 on the class factory that creates your COM objects.
Here's a link at MSDN explaining the interface.
IClassFactory2 at MSDN
The benefit of creating an implementation is that nobody can fetch an instance without clearing the hurdles of registration through IClassFactory2.
The downside is that you'll have to inspect all the locations where you are creating an object, to make sure that they haven't broken. Creating instances becomes more burdensome, although some languages already have facilities to make the process less painful (ex. VB6).
If you are trying to protect an object that has a lot of instantiation activity, you might want to go with Mastermind's method of adding a key parameter, or add an unlock method of some sort to your interfaces that must be called correctly before the component behind it can be used.
You could make your interfaces inheriting directly from IUnknown (without IDispatch) and not include the type library into the DLL. This way only those who have access to the type library will be able to find what interfaces are supported and the only other way to discover the interfaces will be to just guess. If you go this way you might also wish to minimize the number of classes exposed to registry (those that can be created with CoCreateInstance()) and use a set of factory methods of some dedicated registry-exposed class instead.
This implies that only vtable early-binding will work with your component. You will also be unable to use default call marshaling with this component (since no type library is included). And this is not real protection, just a way to hide things.
Nothing prevents you from adding a "key" parameter to the methods which will just return if the key is wrong.
Very simple but will do for starters.
Other than some sort of 'key' param, you can't prevent the curious from discovering your function and then calling it. All it takes is a debugger and some patience. To be totally secure you'd have to require some sort of certificate that authorized code could obtain but all others couldn't but that would mean you're code would have to be able to verify the certificate.