Objective-C has no namespaces; it's much like C, everything is within one global namespace. Common practice is to prefix classes with initials, e.g. if you are working at IBM, you could prefix them with "IBM"; if you work for Microsoft, you could use "MS"; and so on. Sometimes the initials refer to the project, e.g. Adium prefixes classes with "AI" (as there is no company behind it of that you could take the initials). Apple prefixes classes with NS and says this prefix is reserved for Apple only.
So far so well. But appending 2 to 4 letters to a class name in front is a very, very limited namespace. E.g. MS or AI could have an entirely different meanings (AI could be Artificial Intelligence for example) and some other developer might decide to use them and create an equally named class. Bang, namespace collision.
Okay, if this is a collision between one of your own classes and one of an external framework you are using, you can easily change the naming of your class, no big deal. But what if you use two external frameworks, both frameworks that you don't have the source to and that you can't change? Your application links with both of them and you get name conflicts. How would you go about solving these? What is the best way to work around them in such a way that you can still use both classes?
In C you can work around these by not linking directly to the library, instead you load the library at runtime, using dlopen(), then find the symbol you are looking for using dlsym() and assign it to a global symbol (that you can name any way you like) and then access it through this global symbol. E.g. if you have a conflict because some C library has a function named open(), you could define a variable named myOpen and have it point to the open() function of the library, thus when you want to use the system open(), you just use open() and when you want to use the other one, you access it via the myOpen identifier.
Is something similar possible in Objective-C and if not, is there any other clever, tricky solution you can use resolve namespace conflicts? Any ideas?
Update:
Just to clarify this: answers that suggest how to avoid namespace collisions in advance or how to create a better namespace are certainly welcome; however, I will not accept them as the answer since they don't solve my problem. I have two libraries and their class names collide. I can't change them; I don't have the source of either one. The collision is already there and tips on how it could have been avoided in advance won't help anymore. I can forward them to the developers of these frameworks and hope they choose a better namespace in the future, but for the time being I'm searching a solution to work with the frameworks right now within a single application. Any solutions to make this possible?
Prefixing your classes with a unique prefix is fundamentally the only option but there are several ways to make this less onerous and ugly. There is a long discussion of options here. My favorite is the #compatibility_alias Objective-C compiler directive (described here). You can use #compatibility_alias to "rename" a class, allowing you to name your class using FQDN or some such prefix:
#interface COM_WHATEVER_ClassName : NSObject
#end
#compatibility_alias ClassName COM_WHATEVER_ClassName
// now ClassName is an alias for COM_WHATEVER_ClassName
#implementation ClassName //OK
//blah
#end
ClassName *myClass; //OK
As part of a complete strategy, you could prefix all your classes with a unique prefix such as the FQDN and then create a header with all the #compatibility_alias (I would imagine you could auto-generate said header).
The downside of prefixing like this is that you have to enter the true class name (e.g. COM_WHATEVER_ClassName above) in anything that needs the class name from a string besides the compiler. Notably, #compatibility_alias is a compiler directive, not a runtime function so NSClassFromString(ClassName) will fail (return nil)--you'll have to use NSClassFromString(COM_WHATERVER_ClassName). You can use ibtool via build phase to modify class names in an Interface Builder nib/xib so that you don't have to write the full COM_WHATEVER_... in Interface Builder.
Final caveat: because this is a compiler directive (and an obscure one at that), it may not be portable across compilers. In particular, I don't know if it works with the Clang frontend from the LLVM project, though it should work with LLVM-GCC (LLVM using the GCC frontend).
If you do not need to use classes from both frameworks at the same time, and you are targeting platforms which support NSBundle unloading (OS X 10.4 or later, no GNUStep support), and performance really isn't an issue for you, I believe that you could load one framework every time you need to use a class from it, and then unload it and load the other one when you need to use the other framework.
My initial idea was to use NSBundle to load one of the frameworks, then copy or rename the classes inside that framework, and then load the other framework. There are two problems with this. First, I couldn't find a function to copy the data pointed to rename or copy a class, and any other classes in that first framework which reference the renamed class would now reference the class from the other framework.
You wouldn't need to copy or rename a class if there were a way to copy the data pointed to by an IMP. You could create a new class and then copy over ivars, methods, properties and categories. Much more work, but it is possible. However, you would still have a problem with the other classes in the framework referencing the wrong class.
EDIT: The fundamental difference between the C and Objective-C runtimes is, as I understand it, when libraries are loaded, the functions in those libraries contain pointers to any symbols they reference, whereas in Objective-C, they contain string representations of the names of thsoe symbols. Thus, in your example, you can use dlsym to get the symbol's address in memory and attach it to another symbol. The other code in the library still works because you're not changing the address of the original symbol. Objective-C uses a lookup table to map class names to addresses, and it's a 1-1 mapping, so you can't have two classes with the same name. Thus, to load both classes, one of them must have their name changed. However, when other classes need to access one of the classes with that name, they will ask the lookup table for its address, and the lookup table will never return the address of the renamed class given the original class's name.
Several people have already shared some tricky and clever code that might help solve the problem. Some of the suggestions may work, but all of them are less than ideal, and some of them are downright nasty to implement. (Sometimes ugly hacks are unavoidable, but I try to avoid them whenever I can.) From a practical standpoint, here are my suggestions.
In any case, inform the developers of both frameworks of the conflict, and make it clear that their failure to avoid and/or deal with it is causing you real business problems, which could translate into lost business revenue if unresolved. Emphasize that while resolving existing conflicts on a per-class basis is a less intrusive fix, changing their prefix entirely (or using one if they're not currently, and shame on them!) is the best way to ensure that they won't see the same problem again.
If the naming conflicts are limited to a reasonably small set of classes, see if you can work around just those classes, especially if one of the conflicting classes isn't being used by your code, directly or indirectly. If so, see whether the vendor will provide a custom version of the framework that doesn't include the conflicting classes. If not, be frank about the fact that their inflexibility is reducing your ROI from using their framework. Don't feel bad about being pushy within reason — the customer is always right. ;-)
If one framework is more "dispensable", you might consider replacing it with another framework (or combination of code), either third-party or homebrew. (The latter is the undesirable worst-case, since it will certainly incur additional business costs, both for development and maintenance.) If you do, inform the vendor of that framework exactly why you decided to not use their framework.
If both frameworks are deemed equally indispensable to your application, explore ways to factor out usage of one of them to one or more separate processes, perhaps communicating via DO as Louis Gerbarg suggested. Depending on the degree of communication, this may not be as bad as you might expect. Several programs (including QuickTime, I believe) use this approach to provide more granular security provided by using Seatbelt sandbox profiles in Leopard, such that only a specific subset of your code is permitted to perform critical or sensitive operations. Performance will be a tradeoff, but may be your only option
I'm guessing that licensing fees, terms, and durations may prevent instant action on any of these points. Hopefully you'll be able to resolve the conflict as soon as possible. Good luck!
This is gross, but you could use distributed objects in order to keep one of the classes only in a subordinate programs address and RPC to it. That will get messy if you are passing a ton of stuff back and forth (and may not be possible if both class are directly manipulating views, etc).
There are other potential solutions, but a lot of them depend on the exact situation. In particular, are you using the modern or legacy runtimes, are you fat or single architecture, 32 or 64 bit, what OS releases are you targeting, are you dynamically linking, statically linking, or do you have a choice, and is it potentially okay to do something that might require maintenance for new software updates.
If you are really desperate, what you could do is:
Not link against one of the libraries directly
Implement an alternate version of the objc runtime routines that changes the name at load time (checkout the objc4 project, what exactly you need to do depends on a number of the questions I asked above, but it should be possible no matter what the answers are).
Use something like mach_override to inject your new implementation
Load the new library using normal methods, it will go through the patched linker routine and get its className changed
The above is going to be pretty labor intensive, and if you need to implement it against multiple archs and different runtime versions it will be very unpleasant, but it can definitely be made to work.
Have you considered using the runtime functions (/usr/include/objc/runtime.h) to clone one of the conflicting classes to a non-colliding class, and then loading the colliding class framework? (this would require the colliding frameworks to be loaded at different times to work.)
You can inspect the classes ivars, methods (with names and implementation addresses) and names with the runtime, and create your own as well dynamically to have the same ivar layout, methods names/implementation addresses, and only differ by name (to avoid the collision)
Desperate situations call for desperate measures. Have you considered hacking the object code (or library file) of one of the libraries, changing the colliding symbol to an alternative name - of the same length but a different spelling (but, recommendation, the same length of name)? Inherently nasty.
It isn't clear if your code is directly calling the two functions with the same name but different implementations or whether the conflict is indirect (nor is it clear whether it makes any difference). However, there's at least an outside chance that renaming would work. It might be an idea, too, to minimize the difference in the spellings, so that if the symbols are in a sorted order in a table, the renaming doesn't move things out of order. Things like binary search get upset if the array they're searching isn't in sorted order as expected.
#compatibility_alias will be able to solve class namespace conflicts, e.g.
#compatibility_alias NewAliasClass OriginalClass;
However, this will not resolve any of the enums, typedefs, or protocol namespace collisions. Furthermore, it does not play well with #class forward decls of the original class. Since most frameworks will come with these non-class things like typedefs, you would likely not be able to fix the namespacing problem with just compatibility_alias.
I looked at a similar problem to yours, but I had access to source and was building the frameworks.
The best solution I found for this was using #compatibility_alias conditionally with #defines to support the enums/typedefs/protocols/etc. You can do this conditionally on the compile unit for the header in question to minimize risk of expanding stuff in the other colliding framework.
It seems that the issue is that you can't reference headers files from both systems in the same translation unit (source file). If you create objective-c wrappers around the libraries (making them more usable in the process), and only #include the headers for each library in the implementation of the wrapper classes, that would effectively separate name collisions.
I don't have enough experience with this in objective-c (just getting started), but I believe that is what I would do in C.
Prefixing the files is the simplest solution I am aware of.
Cocoadev has a namespace page which is a community effort to avoid namespace collisions.
Feel free to add your own to this list, I believe that is what it is for.
http://www.cocoadev.com/index.pl?ChooseYourOwnPrefix
If you have a collision, I would suggest you think hard about how you might refactor one of the frameworks out of your application. Having a collision suggests that the two are doing similar things as it is, and you likely could get around using an extra framework simply by refactoring your application. Not only would this solve your namespace problem, but it would make your code more robust, easier to maintain, and more efficient.
Over a more technical solution, if I were in your position this would be my choice.
If the collision is only at the static link level then you can choose which library is used to resolve symbols:
cc foo.o -ldog bar.o -lcat
If foo.o and bar.o both reference the symbol rat then libdog will resolve foo.o's rat and libcat will resolve bar.o's rat.
Just a thought.. not tested or proven and could be way of the mark but in have you considered writing an adapter for the class's you use from the simpler of the frameworks.. or at least their interfaces?
If you were to write a wrapper around the simpler of the frameworks (or the one who's interfaces you access the least) would it not be possible to compile that wrapper into a library. Given the library is precompiled and only its headers need be distributed, You'd be effectively hiding the underlying framework and would be free to combine it with the second framework with clashing.
I appreciate of course that there are likely to be times when you need to use class's from both frameworks at the same time however, you could provide factories for further class adapters of that framework. On the back of that point I guess you'd need a bit of refactoring to extract out the interfaces you are using from both frameworks which should provide a nice starting point for you to build your wrapper.
You could build upon the library as you and when you need further functionality from the wrapped library, and simply recompile when you it changes.
Again, in no way proven but felt like adding a perspective. hope it helps :)
If you have two frameworks that have the same function name, you could try dynamically loading the frameworks. It'll be inelegant, but possible. How to do it with Objective-C classes, I don't know. I'm guessing the NSBundle class will have methods that'll load a specific class.
Related
I've been reading about encapsulation and keep seeing comments about how changing the privacy of a class or adding getters and setters where there were none before can 'break the code' of people who use your library. I don't really understand this. I'm very inexperienced in programming, and my understanding is that you download a library onto your computer and it's included in the files of the program you're writing, so if the original author changed something in THEIR COPY of the library, it wouldn't affect your copy. Is this wrong? For example, is a library more like a website that your computer connects to through the internet and the original author can update, so that changes they make to it can affect how your code works?
Software is constantly changing, so we must have a way to keep track of the different versions - hence software versions. When you download a library to use in your own program, you (usually, like with a dependency management tool) end up downloading a very specific version of that library.
If a library author was to change the interface to use it, developers using that library would also have to change how they use it when they download the version with those changes. Otherwise, it would break any code that follows an outdated interface.
As long as a library author follows proper versioning procedures, for instance including breaking changes in a new major version, and the changes improve the clarity of the library's interfaces without sacrificing other properties, then the argument is moot. Developers can either continue using the old version or update their code to be compatible with the new version.
Except for maybe in low resource, embedded systems that can use all optimizations available, like accessing object/structure properties directly rather than through a function.
Libraries:
By definition a collection of non-volatile resources used by computer
programs, often for software development. These may include
configuration data, documentation, help data, message templates,
pre-written code and subroutines, classes, values or type
specifications.
Explanation
Let me spend time on defining in coding aspects: Lets say you have a
create a soccer game, what does that need, field, ball, players,
flags.
All this we encapsulate in Class to make as Object Game which
comprises all above.
Now you start building game and realise you are spending redudant time making repeated player names, shirt design , details filling, etc.
To avoid this you make functions which are business specific like 1. generate tshirts and pass (color, design , cloth type) and it returnes you tshirt object in return.
Similary you get player information by passing country and his ID and all this details are return as Player object which have his name, place, country, contacts ets.
This is how the functions in class behaves.
Your ask
privacy of a class or adding getters and setters, ...an 'break the code' of people who use your library
These are ways how you access the object parameters or set values for them , in some languages the getter and setters are auto generated and not required to explicitly set unless you need custom settings during class object creation.
The best advantage of the getting and setter is it ensures the default class creation can be assigned some values which you dont want to change in defaults and also not allow people to enforce new values to that specific parameter of class.
This is how the control is made in place during defining your class and its functions. getter and setter are functions as well with class variables having factility to get/set values as you define the function logic inside.
You ask
my understanding is that you download a library onto your computer and
it's included in the files of the program you're writing, so if the
original author changed something in THEIR COPY of the library, it
wouldn't affect your copy. Is this wrong?
Yes think it like a CD Copy , I sent you a copy so you can use those info from the copy i made, but once i have new features and things added in CompactDisc(CD) it wouldnt be there in your copy i burnt during that time hence you code uses the old version and may use till there is need to update.
You can only get impacted if you take my new CD copy which is called as upgrading your software with my new library version.
Normally big guys software dont immediately change the library in their systems unless there is thorough analysis done with 1. need, 2. security 3. bugs in old fixed in new. factors to address for a new upgrade.
Happy Coding
Software world is free of your mind to code so dont think what is wrong or right just code.
Take a Maths Library building task in hand use anything python, java, c#, objective C, swift, javascript ...
Create library with modules with Circle, Square, Polygon, Sphere objects
Each object they will have thier respective Classes created with theier paramters (circle sample : radius, center(x,y), etc and functions like setRadius, getCircumference, etc)
Similar way all objects makes thier own classes
Abstrat word you used means some function you make private that only class can internally access but not exposed to outside when you create new Maths Object.
Hope this was helpful, happy coding.
I've added SACalendar custom classes to my Xcode project and now when I run the app I get this log:
objc[3230]: Class SACalendar is implemented in both /System/Library/PrivateFrameworks/SAObjects.framework/SAObjects and /private/var/mobile/Containers/Bundle/Application/XXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXX/MyApp.app/MyApp. One of the two will be used. Which one is undefined.
What is such SAObjects.framework? Would this conflict cause actually a problem?
Thanks
As the message says, it is a private framework (meaning that it contains stuff that does not have a public API). I think this one started shipping with iOS7.
IMO, Apple should do a better job naming their private classes, and prefix them with an underscore or something, but they don't. That framework even has classes with no specific prefix (like AceObject).
Since ObjectiveC does not have namespace support, you can (and will) get name collisions. All you can do is rename your classes that clash with system frameworks... and hope your prefix of choice is not chosen by some other framework shipped with the system.
I would advise you to rename your SACalendar class, and any other classes that cause a conflict.
If you want to know the contents of that framework, just load it and use the objc runtime to discover its properties... or use one of the many tools already available (like this one).
It seems to be this: https://github.com/nopshusang/SACalendar
Yes, it is a problem.
However, you get this problem because you and many, many other developers do not read the Objective-C's and Cocoa's naming conventions. There are typically two mistakes:
Prefixes with two letters are reserved for Apple. You are not allowed to use it. I really do not know, why do not respect this. This is the problem in your case.
Prefixes does not denote a subject, but the origin of code. Therefore extending (subclass, category) a class from Cocoa (NS-Prefix) by a third-party developer does not have the prefix NS. But you see it everywhere.
I really do not know, why it is that difficult to accept that simple rules.
Your own classes should use three letter prefixes. These might relate to a combination of your company name and your app name, or even a specific component within your app. As an example, if your company were called Whispering Oak, and you were developing a game called Zebra Surprise, you might choose WZS or WOZ as your class prefix.
As for what SAObjects.framework actually does, according to https://www.theiphonewiki.com/wiki/Siri, it contains "Resources for Siri objects".
Wondering if you might be able to answer a very basic beginner question for me. I’m working through the Cocoa + Swift tutorial on Lynda and I’m a little confused about classes/objects.
Basically, I want to know why we have to create a new swift file for each new class we create.
As far as I know, you can create a new class within any .swift file in the project. My question is, why do we have to continually keep creating .swift files for each new class.
I’m wondering why there isn’t just one .swift file called AllClasses.swift that you can create all the classes in, for instance:
Within AllClasses.swift is the following code:
Class FirstClass : NSObject
Class SecondClass : NSObject
Class ThirdClass : NSObject
Class FourthClass : NSObject
As Opposed to:
Within FirstClass.swift is the following code:
Class FirstClass : NSObject
Within SecondClass.swift is the following code:
Class SecondClass : NSObject
Within ThirdClass.swift is the following code:
Class ThirdClass : NSObject
Within FourthClass.swift is the following code:
Class FourthClass : NSObject
I just want to know why we need to separate different code into files if it can be called from within any area of the project. In the case of a Mac application, it seems like almost everything could be done from within the AppDelegate.swift file.
This is a moronic question, but another hurdle that may be making object orientation a hard concept for me to fully grasp.
Maybe I can explain it in a somewhat amusing way:
In the beginning there was no concept of files and all code was in a single entity. Code within such entities was referenced by line numbers. Because everything was in one place it was easy to find what you wanted, even though programs were small. It was better than punch tape and so there was much rejoicing. We gotta do something about loading from cassette though.
But then someone discovered you could break up the code into separate parts called modules which was just as well as software was getting bigger. Man my 10MB hard drive is huge. Each module was a specialist and could call other specialists. It made your code easier to navigate. There was much rejoicing.
But then someone discovered object-orientation (OO) and files were cheap. Programs were so large now people were having a hard time finding that class that modelled the airspeed of an African Swallow in that multiple-class-containing file of 10000+ lines that maybe its time to start putting each class in its own file. Needless to say there was much rejoicing.
Then software had become so large that someone discovered source control which was most important when a team of coding scribes all meditated on a piece of software. Madness ensured for the brotherhood whose careless endeavour to write a program in one file of 30,000+ lines (research on African Swallows had grown to include European Swallows) even with OO, only lead to line conflict after line conflict during their attempts to check in changes into the source control system. There was much burning at the stake. Later revelations lead to breaking up the code into many texts or files was the way to avoid a lynching.
In summary, there is no rule to say you must have one file per class but its a good practice to do so mainly in the event your program grows to any reasonable size or complexity that navigation and maintenance of your code would become an issue if you do not.
It becomes more important when working with a team where as the number of authors working concurrently on any given file, the probability of source code commit conflict rises.
I believe the monks are studying their favourite colours and capital cities of countries now.
Some reasons:
Encapsulation / Access Control. It's a bad practice to contain several classes in the same file as you'll be able to access every single variable / method from that source file even if that is marked as private, as stated in Apple documentation:
Private access restricts the use of an entity to its own defining
source file. Use private access to hide the implementation details of
a specific piece of functionality.
Separating your classes in separate files helps the compiler to build faster. When Swift 1.2 compiler was released, incremental builds were introduced to speed up the build times. Files that are not edited are not compiled again on your next build:
Incremental builds — Source files that haven’t changed will no longer
be re-compiled by default, which will significantly improve build
times for most common cases. Larger structural changes to your code
may still require multiple files to be rebuilt.
Writing code well organized. Together with defining correctly the responsabilities of your classes (divide and conquer) that will help you (and your teammates, if any) to understand who does what and where. And, as commented in other answers, to make your source control management easier to track.
You don't have to define just one class per file, but I would suggest doing so. I recently worked on a project for a client where there were several classes in some source files, and where some classes were defined in files who's names didn't match the class names. (This was in Objective-C, so each "file" was really a pair of files, a .h header file and a .m implementation file, but logically they were one.)
It was confusing as h*ll, and I wasted a fair amount of time fumbling around trying to find things.
Defining one class per file and making your filenames and class names match exactly is a good convention. It's like having each school subject in a separate binder. When you need to find a class you know exactly what file to open to find it.
As good practice:
If the classes are unrelated, lengthy or used independently from other unrelated classes then they should be in separate files.
However, if the classes are tightly coupled with one another and are not lengthy then they could be in the same file.
This post also touches on this subject.
As a newbie I really agree it is difficult to get the classes and inheritance concepts.
But believe it is much better to handle code in separate documents, perhaps using MVC concept, rather than having this code in a single massive document.
My own experience, it clears out the clouds of your code.
I just want to add the observation that Swift's fileprivate access modifier actually sometimes requires putting many classes inside a single source file.
The "one class per source file" doesn't necessarily fit the design of Swift. For this reason, when I need to tightly control which properties I expose, I often have one very large source file for a single API.
The only alternative is to make a separate framework for each API and using internal fields.
I'm evaluating the many possibilities for a trial protection system and came up with the following question:
If I use my "trial check" class more than once (scattered several times over the application), will it be compiled just once into the exe?
The reason why I'm asking is that if it's only compiled once into the exe, then patching this single class will invalidate all places where it is used.
If it's compiled just once, are there any viable alternatives to prevent this?
Thanks!
EDIT: I'm actually not trying to roll my own protection system, I'm looking at several existing solutions like OnGuard, mxProtector and TRegWare. It was while looking at the various solutions source-code, that I came up with this question.
Yes, even if you create several instances of the class in different places there is only one copy of the methods (implementation), so if hacker patches the class all instances will be patched.
Do you really want to roll your own protection system? It ain't easy to come up with good system and there are several ready to use solutions around, if youre on budget then perhaps TurboPower OnGuard (which is open source now) will do.
BTW the general wisdom is that if they want to crack your app they will do it, no matter what, so one shouldn't waste too much resources on protection schemes. The only foolproof way is to exclude some of the (key) functionality from trial version, ie
{$IFDEF trial_version}
ShowMessage('Sorry, this function is not available in trial version');
{$ELSE}
// do the thing
{$END}
but of course, if full version gets into wild then it will be cracked...
If you use the inline keyword for functions and methods where possible, the executable code will be "multi-plicated". There are some limitations to the use of inlining, though (see the linked doc).
I agree with Ain and Marco that spending effort on protection schemes may be more bother than benefit, and that it makes more sense to use existing solutions than to roll your own.
Yes. The standard workaround is to put the code in an .inc and include that in multiple units.
But that makes less sense in a security setting. Since if sb has learned to search for a pattern, he can simply repeat the search to find the other occurrences, making it a minor nuisance at best.
This is one of the reasons why DIY protection is often a waste of time, and I agree fully with Ain. (both the onguard thing, and the fact that if functionality IS in the exe, it will be unlocked sooner or later, giving sufficient motivation)
Just for the principle: There is actually a possibility to have the "same" class compiled multiple times. If you declare the class with a generic type and later have several instances with different instance types, the class code is compiled for each type. The generic class don't even have to make use of the generic type. If you spread the generic instances over different units, the code will be separated, too.
type
TDummy<T> = class
public
procedure Dummy1;
end;
procedure TDummy<T>.Dummy1;
begin
...
end;
var
FDummy1: TDummy<Integer>;
FDummy2: TDummy<Byte>;
FDummy3: TDummy<TButton>;
FDummy4: TDummy<TLabel>;
I've been using StructureMap recently and have enjoyed the experience thoroughly. However, I can see how one can easily get carried away with interfacing everything out and end up with classes that take in a boatload of interfaces into their constructors. Even though that really isn't a huge problem when you're using a dependency injection framework, it still feels that there are certain properties that really don't need to be interfaced out just for the sake of interfacing them.
Where do you draw the line on what to interface out vs just adding a property to the class?
The main problem with dependency injection is that, while it gives the appearance of a loosely coupled architecture, it really doesn't.
What you're really doing is moving that coupling from the compile time to the runtime, but still if class A needs some interface B to work, an instance of a class which implements interface B needs still to be provided.
Dependency injection should only be used for the parts of the application that need to be changed dynamically without recompiling the base code.
Uses that I've seen useful for an Inversion of Control pattern:
A plugin architecture. So by making the right entry points you can define the contract for the service that must be provided.
Workflow-like architecture. Where you can connect several components dynamically connecting the output of a component to the input of another one.
Per-client application. Let's say you have various clients which pays for a set of "features" of your project. By using dependency injection you can easily provide just the core components and some "added" components which provide just the features the client have paid.
Translation. Although this is not usually done for translation purposes, you can "inject" different language files as needed by the application. That includes RTL or LTR user interfaces as needed.
Think about your design. DI allows you to change how your code functions via configuration changes. It also allows you to break dependencies between classes so that you can isolate and test objects easier. You have to determine where this makes sense and where it doesn't. There's no pat answer.
A good rule of thumb is that if its too hard to test, you've got some issues with single responsibility and static dependencies. Isolate code that performs a single function into a class and break that static dependency by extracting an interface and using a DI framework to inject the correct instance at runtime. By doing this, you make it trivial to test the two parts separately.
Dependency injection should only be used for the parts of the
application that need to be changed dynamically without recompiling
the base code
DI should be used to isolate your code from external resources (databases, webservices, xml files, plugin architecture). The amount of time it would take to test your logic in code would almost be prohibitive at a lot of companies if you are testing components that DEPEND on a database.
In most applications the database isn't going to change dynamically (although it could) but generally speaking it's almost always good practice to NOT bind your application to a particular external resource. The amount involve in changing resources should be low (data access classes should rarely have a cyclomatic complexity above one in it's methods).
What do you mean by "just adding a property to a class?"
My rule of thumb is to make the class unit testable. If your class relies on the implementation details of another class, that needs to be refactored/abstracted to the point that the classes can be tested in isolation.
EDIT: You mention a boatload of interfaces in the constructor. I would advise using setters/getters instead. I find that it makes things much easier to maintain in the long run.
I do it only when it helps with separation of concerns.
Like maybe cross-project I would provide an interface for implementers in one of my library project and the implementing project would inject whatever specific implementation they want in.
But that's about it... all the other cases it'd just make the system unnecessarily complex
Even with all the facts and processes in the world.. every decision boils down to a judgment call - Forgot where I read that
I think it's more of a experience / flight time call.
Basically if you see the dependency as a candidate object that may be replaced in the near future, use dependency injection. If I see 'classA and its dependencies' as one block for substitution, then I probably won't use DI for A's deps.
The biggest benefit is that it will help you understand or even uncover the architecture of your application. You'll be able to see very clearly how your dependency chains work and be able to make changes to individual parts without requiring you to change things that are unrelated. You'll end up with a loosely coupled application. This will push you into a better design and you'll be surprised when you can keep making improvements because your design will help you keep separating and organizing code going forward. It can also facilitate unit testing because you now have a natural way to substitute implementations of particular interfaces.
There are some applications that are just throwaway but if there's a doubt I would go ahead and create the interfaces. After some practice it's not much of a burden.
Another item I wrestle with is where should I use dependency injection? Where do you take your dependency on StructureMap? Only in the startup application? Does that mean all the implementations have to be handed all the way down from the top-most layer to the bottom-most layer?
I use Castle Windsor/Microkernel, I have no experience with anything else but I like it a lot.
As for how do you decide what to inject? So far the following rule of thumb has served me well: If the class is so simple that it doesn't need unit tests, you can feel free to instantiate it in class, otherwise you probably want to have a dependency through the constructor.
As for whether you should create an interface vs just making your methods and properties virtual I think you should go the interface route either if you either a) can see the class have some level of reusability in a different application (i.e. a logger) or b) if either because of the amount of constructor parameters or because there is a significant amount of logic in the constructor, the class is otherwise difficult to mock.