We need to "protect" a class from having static methods for security purposes. We don't want newbie devs following suggestions of coding tools to make a member static as we need to use a constructor with a password to control access to this class.
Is there any way to protect a .NET class in C# to prevent it from have any static members?
Would such a feature, if not available, be worthwhile for a future version of .NET?
Thank you.
we need to use a constructor with a password to control access to this class.
This sounds like a monumentally bad idea, but of course I know nothing about the security problem you're trying to solve. This is the far more interesting question than your question: What is the threat you are attempting to protect against? There is probably a better way to do it.
Is there any way to protect a .NET class in C# to prevent it from have any static members?
Have senior developers review the checkins of junior developers. Which you should be doing anyway, but particularly if the class has some kind of security semantics.
Would such a feature, if not available, be worthwhile for a future version of .NET?
That's unlikely in the extreme.
Thank you.
You're welcome!
The easiest thing to do would be to set-up FxCop on your build server and write a custom FxCop rule to check for static members.
This question has details on how to write a custom FxCop rule.
Alternatively, as SimpleCoder pointed out, you can use StyleCop to enforce the rule on the source code.
This page describes how to set-up StyleCop with msbuild.
You could probably use StyleCop to enforce a custom rule. It would have to be implemented as a build action, though.
Alternatively, you can use FxCop to analyze the binaries.
You can run a check using reflection at run time, but other than that, there is no language feature preventing static members.
I can think of no good use for such a feature. This is a problem of communication rather than one of implementation.
In your case, you could place your protected functionality in an abstract base class, and run a check to see if the user is authorized before performing any protected function.
I would think a sealed class would suffice, though static members are permitted in the original class declaration. I think derived classes would not allow further static members.
I have to admit I've never heard of such a security scheme. I think there's good reason why I haven't heard of it - are you sure it's as protective as you want it to be?
By requiring the constructor to contain a password, you're trying to protect yourself from people who use your class. But those people can decompile and recompile your class to remove the password check - so what are you protected against?
Suggestions
Training
More Training
Code Reviews
Others suggested stylecop or fxcop - both good options
If #4 fails - more training - this time tie it to performance reviews. :)
I pretty much agree with Eric Lippert. But in those cases I want to do something like this (sometimes just to stop myself to make mistakes) I try to write a unit test for it. Like this:
[Test]
public void Class_should_not_have_static_methods()
{
var staticMethods = typeof (Foo).GetMethods().Where(x => x.IsStatic);
Assert.That(staticMethods.Count(), Is.EqualTo(0), "Static methods: " + string.Join(", ", staticMethods.Select(x => x.Name)));
}
Related
For class members the private modifier is redundant.
For classes the internal modifier is redundant.
Is it possible to get Resharper to show these as code errors?
In short: no.
We have only 'code cleanup' functionality able to remove redundant private/internal modifiers, but no code inspection like this. This easily can be done via plugin or issue request :)
There is no default out-of-the-box rule that enforces this. And I must say that I haven't seen a coding guideline in the last years that ever suggested not specifying the access level on anything. Usually specifying the access level is seen as a good thing, since it shows that there has been at least some thought into the desired level.
Of course some people set everything to public, but then again, that's something your rule won't see anyway.
You'll need to create a custom StyleCop rule (with the StyleCop plugin for Resharper) or a Resharper plugin to detect and flag these issues. It shouldn't be too hard, since the "error" is not too complex and the list acceptance criteria would be short and simple.
I don't mean to troll but I really don't get it. Why would language designers allow private methods instead of some naming convention (see __ in Python) ?
I searched for the answer and usual arguments are:
a) To make the implementation cleaner/avoid long vertical list of methods in IDE autocompletion
b) To announce to the world which methods are public interface and which may change and are just for implementation purpose
c) Readability
Ok so now, all of those could be achieved by naming all private methods with __ prefix or by "private" keyword which doesn't have any implications other than be information for IDE (don't put those in autocompletion) and other programers (don't use it unless you really must). Hell, one could even require unsafe-like keyword to access private methods to really discourage this.
I am asking this because I work with some c# code and I keep changing private methods to public for test purposes as many in-between private methods (like string generators for xml serialization) are very useful for debugging purposes (like writing some part of string to log file etc.).
So my question is:
Is there anything which is achieved by access restriction but couldn't be achieved by naming conventions without restricting the access ?
There are a couple questions/issues that you are raising, so I'll handle each one separately.
How do I test private methods?
Beyond the debate/discussion of if you should test private methods, there are a few ways you can do this.
Refactor
A broad general answer is that you can refactor the behaviour into a separate testable class which the original class leverages. This is debatable and not always applicable depending on your design or privileges to do so.
InternalsVisibleTo
A common routine is to extract testable logic into a method and mark it as internal. In your assembly properties, you can add an attribute [InternalsVisibleTo("MyUnitTestingProject")]. This will allow you to access the method from your unit testing project while still hiding access to all other assemblies. http://msdn.microsoft.com/en-us/library/system.runtime.compilerservices.internalsvisibletoattribute.aspx
However, given the comments made by you, you are unable to change the structure of source code permanently in your workplace; that you are changing the accessors to test, test, then change them back before committing. In this case there are two options:
Partial testing classes
If you mark the class as partial. Create a second partial class file which will contain your tests (or public wrappers to the private members). Then when it comes time to merge/commit, just remove your partial classes from the project and remove the partial keyword from the main class. In addition, you can wrap the entire testing code file with if DEBUG (or other directive) so it's only available when unit testing and will not affect production/development code.
http://weblogs.asp.net/ralfw/archive/2006/04/14/442836.aspx
public partial class MyClass
{
private string CreateTempString()
{
return "Hello World!";
}
}
#if DEBUG
public partial class MyClass //in file "MyClass_Accessor.cs"
{
public string CreateTempString_Accessor()
{
return CreateTempString();
}
}
#endif
Reflection
You can still access private members via reflection:
public class Test
{
private string PrivateField = "private";
}
Test t = new Test();
var publicFieldInfo = typeof(Test).GetField("PrivateField", BindingFlags.Instance | BindingFlags.NonPublic);
Console.WriteLine(publicFieldInfo.GetValue(t)); //outputs "private"
Your unit tests could pull out private/hidden data in classes this way. In fact, Microsoft provides two classes that do exactly this for you: PrivateObject and PrivateType
Given your in-house development process limitations, this is likely your best bet as you'll be able to manage your own tests outside the main project libraries without having to alter anything in the core code.
Note that Silverlight (and likely other Core-CLR runtimes) strictly enforce non-public access during reflection, so this option is not applicable in those cases.
So, there are a few ways to test private members, and I'm sure there are a few more clever/not-so-clever methods of doing so lurking out there.
Could all of those benefits could be achieved by naming all private methods with __ prefix or by introducing a private-but-accessible access modifier?
The benefits cited by you (citing others) being:
To make the implementation cleaner/avoid long vertical list of
methods in IDE autocompletion
To announce to the world which methods are public interface and
which may change and are just for implementation purpose
Readability
Now you add that these could all be achieved with __ or by a change to the language specification and/or IDEs that would support a private-but-accessible access modifier, possibly with some unsafe-like keyword that would discourage this. I don't think it will be worthwhile going into a debate about changing the current features/behaviours of the language and IDE (and possibly it wouldn't be make sense for StackOverflow), so focusing on what is available:
1) Cleaner implementation and intellisense
The Visual Studio IDE (I can't speak for MonoDevelop) does support hiding members from intellisense when they're marked with the [EditorBrowsableAttribute]. But this only works if the developer enables the option "Hide Advanced Members" in their Visual Studio options. (note that it will not supress members in the intellisense when you're working within the same assembly)
http://msdn.microsoft.com/en-us/library/system.componentmodel.editorbrowsableattribute.aspx
So marking a public member as such makes it behave (intellisense-wise) as internal-ish (no [InternalsVisibleTo] support). So if you're in the same assembly, or if you do not have the Hide Advanced Members enabled, you'll still see a long list of __ members in the intellisense. Even if you have it hidden from intellisense, it's still fully accessible according to its current access modifier.
2) Public usage interface/contract
This assumes that all developers in the C#, and Visual Basic, and F#, and C++.NET and any .NET development world will adopt the same __ naming convention and adhere to it as assemblies are compiled and interchanged between developers. Maybe if you're scripting in IronPython, you can get away with it, or if your company internally adopts this approach. But generally speaking, it's not going to happen and .NET developers may likely be hestitant to leverage libraries adopting this convention as that is not the general .NET culture.
3) Readability
This kind of goes with #2 in that what is "readable" depends on the culture and what developers within that field expect; it is certainly debatable and subjective. I would wager that the majority of the C# developers find the strict/enforced encapsulation to significantly improve code readability and I'm sure a good chunk of them would find __ used often would detract from that. (as a side, I'm sure it's not uncommon for developers to adopt _ or __ prefixes for private fields and still keep them private)
However, readability and encapsulation in C# goes beyond just public/private accessors. In C#, there are private, public, protected internal, protected, and internal (am I missing one?) each has their own use and provide different information for developers. Now I'm not sure how you would go about communicating those accessors only via __. Suggesting single underscore is protected, double underscore is private, that would definitely hamper readability.
Is there anything which is achieved by access restriction that couldn't be achieved by naming conventions without restricting the access?
If you're asking why did the C# design team go this route, well I guess you'd have to ask Mr. Hejlsberg one day. I know they were creating a language gleaning the best parts of C/C++ and to strongly focus on the priciples of object-oriented principles.
As to what is achieved by enforcing access via the access modifiers:
More guaranteed proper access by consumers of the API. If your class utilizes a MakeTempStringForXMLSerialization method which stores the string as a class property for serialization, but for performance reasons forgoes costly checks (because you, as a developer have done unit testing to ensure that all of class's fields will be valid via the public API) then a third party does some lovely garbage-in-garbage-out, they'll blame you and/or the vendor for a shoddy library. Is that fair? Not necessarily; they put the garbage in, but the reality is many will still blame the vendor.
For new developers attempting to understand how your API works, it helps to simplify their experience. Yes, developers should read the documentation, but if the public API is intuitive (as it generally should be) and not exposing a boatload of members that shouldn't be accessed, then it's far less confusing and less likely they'll accidentally feed garbage into the system. It will also lower the overhead to get the developers to consume and leverage your API effectively without hassles. This is especially the case when it comes to any updates you publish of your API in which you wish to change/refactor/improve your internal implementation details.
So from a business perspective, it protects them from liability and bad relations with customers and is more appealing for developers to purchase and invest in it.
Now this can all be the case, as you say, if everyone follows the convention that __ members should not be accessed outside of the class or provide some unsafe marker where you say, "If you do this, and it breaks, it's not my fault!" well then you're not on the same planet as C# .NET development. The accessors provided by C# provide that __ convention but ensure that all developers adhere to it.
One could argue that the restricted access is an illusion as consumers can work around it via reflection (as demonstrated above), and thus there is actually no programmatic difference between the access modifiers and __ (or other) notation. (On Silverlight/Core-CLR, there is, most definitely a programmatic difference though!) But the work developers would go through to access those private fields is the difference between you giving consumers an open door with a sign "don't go in" (that you hope they can read) and a door with a lock that they have to bash down.
So in the end what does it actually provide? Standardized, enforced access to members where as __ provides non-standardized, non-enforced access to members. In addition, __ lacks the range of description that the varieties of available access modifiers supply.
Update (January 2nd, 2013)
I know it's been half a year, but I've been reading through the C# language specification and came across this little gem from section 2.4.2 Identifiers which states:
Identifiers containing two consecutive underscore characters (U+005F)
are reserved for use by the implementation. For example, an
implementation might provide extended keywords that begin with two
underscores.
I imagine nothing necessarily bad will happen, most likely nothing will break catastrophically if you do. But it's just one more aspect that should be considered when thinking about using double underscores in your code that the specification suggests that you do not.
The reason private methods exist are to provide encapsulation.
This allows you to provide public contracts by which you want your object to interact, yet have your internal logic and state be encapsulated in a way that refactoring would not affect consumers.
For example, you could provide public named properties, yet decide to store state in a Dictionary, similar to what typed DataSets do.
It's not really a "Security" feature (since you always have reflection to override it), but a way to keep public APIs separate from internal implementations.
Nobody should "depend" on your internal and private implementation, they should only depend on your public (or protected) implementation.
Now, regarding unit testing, it is usually undesired to test internal implementations.
One common approach though is to declare it internal and give the test assemblies access, through InternalsVisibleToAttribute, as Chris mentioned.
Also, a clear distinction between public, private, and protected are extremely useful with inheritance, in defining what you expect to be overridable and what shouldn't.
In general, there is no real point to marking fields or methods private. It provides an artificial sense of security. All the code is running inside the same process, presumably written by people with a friendly relationship to each other. Why do they need access controls to protect the code from each other? That sounds like a cultural issue to fix.
Documentation provides more information than private/protected/public does, and a good system will have documentation anyway. Use that to guide your development. If you mark a method as "private", and a developer calls it anyway, that's a bad developer, and he will ruin your system in some other way eventually. Fix that developer.
Access controls eventually get in the way of development, and you spend time just making the compiler happy.
Of course, if you are talking strictly about C#, then you should stick to marking methods as private, because other C# developers will expect it.
It hides a lot of internal details, especially for library implementers who may actually want to hide those details.
Keep in mind that there are commercial libraries out there, being sold. They expose only a very limited set of options in their interface to their users and they want all the rest to be well hidden.
If you design a language that doesn't give this option, you're making a language that will be used almost exclusively for open-source projects (or mostly for scripting).
I don't know much about Python though, would be interesting to know if there are commercial closed-source libraries written in python.
The reasons you mentioned are good enough too.
If you're publishing a library with public members, it's possible for users to use these members. You can use naming conventions to tell them they shouldn't, but some people will.
If your "internal" members are public, it becomes impossible for you to guarantee compatibility with the old version of the library, unless you keep all of the internal structure the same and make only very minor changes. That means that every program using your library must use the version it was compiled against (and will most likely do this by shipping its own version, rather than using a shared system version).
This becomes a problem when your library has a security update. At best, all programs are using different shared versions, and you have to backport the security update to every version that's in the wild. More likely, every program ships its own version and simply will not get the update unless that program's author does it.
If you design your public API carefully, with private members for the things you might change, you can keep breaking changes to a minimum. Then all programs/libraries can use the latest version of your library, even if they were compiled for an earlier one, and they can benefit from the updates.
This can lead to some interesting debate. A main reason for marking private methods (and member variables) is to enforce information hiding. External code shouldn't know or care about how your class works. You should be free to change your impl without breaking other things. This is well established software practice.
However in real life, private methods can get in the way of testing and unforeseen changes that need to occur. Python has a belief "we're all consenting adults" and discourages private methods. So, sometimes I'll just use naming conventions and/or comment "not intended for public use". It's another one of those OO-theory vs. reality issues.
Although this is a fairly common problem, I am struggling with what the best way to approach it (if it needs approached at all in this case).
I have inherited a website (ASP.NET, C#) part of which contains a class full of static methods (it's a very large class, honestly). One method in particular is for sending e-mails. It has every possible parameter I can think of and it works well enough. However, the internals of that particular method are rather cumbersome to manage and understand due to the fact that everything is shoved inside - particularly when most of the parameters aren't used. In addition, it is somewhat difficult to handle errors, again, due to all the parameters for this one method.
Would it make more sense to actually have an EMail class which is instantiated when you want to send an e-mail? This just "feels" more right to me, though I can't full explain why. What are your thoughts on the way to go in this particular case? How about in general?
Thanks.
What you're describing sounds like an example of the aphorism, "You can write FORTRAN in any language."
A massive class full of static methods is often (not always) a sign that somebody just didn't "get" OOP, was stuck in a procedural-programming mindset and was trying to twist the language to do what he wanted.
As a rule of thumb: If any method, static or instance, takes more than about 5 parameters, it's often a sign that the method is trying to do too many things at once, and is a good candidate for refactoring into one or more classes.
Also, if the static methods are not really related, then they should at least be split up into classes that implement related functionality.
I'm actually wondering why you'd have a "send e-mail" method at all, given that the System.Net.Mail namespace handles just about every case, and is configurable via the app.config/web.config file, so you don't need to pass it a server name or port. Is this perchance a "notification" method - something that individual pages are supposed to call out to in order to send one of several "standard" messages based on templates with various values filled in, and certain headers/footers automatically added? If so, there are a number of designs for this type of interaction that are much easier to work with than what you seem to have inherited. (i.e. MailDefinition)
Update: Now having seen your comment that this is being used for exception handling, I think that the most appropriate solution is an actual exception handler. There are a ton of resources on this. For ASP.NET WebForms, I actually took the one Jeff Atwood wrote years ago, ported it to C# and made a few changes (like ignoring 404 errors). There are a number of good links in this previous question.
My preference these days is just to treat exception handling (and subsequent e-mailing of exception reports) as a subset of logging. log4net has an SmtpAppender that's quite capable, and you can configure it to only be used for "fatal" errors (i.e. unhandled exceptions - in your handler, you just make a LogFatal call).
The important thing, which you'll no doubt pick up from the SO link above and any referenced links, is that there are actually two anti-patterns here - the "miscellaneous" static class, and catching exceptions that you don't know how to handle. This is a poor practice in .NET - in most cases you should only catch application-specific exceptions that you can recover from, and let all other exceptions bubble up, installing a global exception handler if necessary.
Here are the Microsoft guidelines for when to use static types, generally.
Some things I would add, personally:
You must use static types to write extension methods.
Static types can make unit testing hard as they are difficult/impossible to mock.
Static types enforce immutability and referentially transparent functions, which can be a good design. So use them for things which are designed to be immutable and have no external dependencies. E.g., System.Math.
Some argue (e.g.) that the Singleton pattern is a bad idea. In any event, it would be wrong to think of static types as Singletons; they're much more broad than that.
This particular case has side-effects (sending e-mails) and doesn't appear to require extension methods. So it doesn't fit into what I would see as the useful case for static types. On the other hand, using an object would allow mocking the e-mail, which would be helpful for a unit test. So I think you're correct to say that a static type is inappropriate here.
Oh my gosh yes.
It sounds like its an old Classic ASP app that was ported.
It violates the single responsibility principle. If you can refactor that class. Use overloading for that function.
That is an example of the Utils anti-pattern.
It is always a good idea to separate those methods according on their responsibility. Creating an Email class is definitely a Good Idea™. It will give you a much nicer interface to use, and it allows you to mock out the Email in tests.
See The Little Manual of API Design, which describes the benefits of classes having minimal constructors and lots of getters/setters over the alternative of using constructor/methods having many parameters.
Since most of the parameters of the methods you mention are not used, a better approach is to use simple constructors that assume reasonable default settings for the internal variables. Having setter methods allows you to then set the few parameters (and only those parameters) that require non-default values.
Is it possible to override a private method by using Reflection in .NET 3.5?
Well, it would need to be virtual to be possible to override it (by writing a dynamic type that inherits from the class), and you can't have a private virtual (it makes no sense). You could perhaps override an internal virtual, but I suspect even this may hit security issues. So ultimately, I'd say no.
Not by using Reflection alone. Perhaps the best you could do is to use Reflection, combined with Reflection.Emit or the CodeDom to duplicate the class into a new namespace. When you come across the private method you want to replace, you don't copy it, you emit your replacement.
However, there are many techniques a developer can use that make this technique much, much harder. Breaking the implementation of the class into many private or internal classes is one such.
Note: using the CodeDom you'd have to build the graph in memory, compile it, and then load the resulting assembly.
This is probably a LOT more trouble than it is worth.
The other way to do it would be to use Reflector to disassemble the class, take the code and build your own class from it with the method replace. Again there are significant technical and legal hurdles to overcome. You may learn a lot from the disassembled code though.
Not by using Reflection. You need to use some sort of AOP.
Typemock Isolator is supposed to be able to do this, but does so through the .NET profiler APIs (according to Roy Osherove in The Art of Unit Testing).
Seems like every C# static analyzer wants to complain when it sees a public field. But why? Surely there are cases where a public (or internal) field is enough, and there is no point in having a property with its get_ and set_ methods? What if I know for sure that I won't be redefining the field or adding to it (side effects are bad, right?) - shouldn't a simple field suffice?
Because it breaks encapsulation -- this is why most people use accessors heavily. However, if you think it's the right solution for your task, ignore it (meaning the strict encapsulation complaints) and do what's right for your project. Don't let the OO nazis tell you otherwise.
It's really about future-proofing your code. When you say (emphasis mine):
What if I know for sure that I won't
be redefining the field or adding to
it (side effects are bad, right?) -
shouldn't a simple field suffice?
That's an absolute statement, and as we know (as well as most static analyzers), there are only two absolutes in life.
It's just trying to protect you from that. If it is an issue, you should be able to tell the analyzer to ignore it (through attributes that are dependent on the analysis tool you are using).
Given the fact that current C# 3.0 allows for automatic properties whose syntax is like:
public int Property {get; set;}
the extra work required for using Properties over public fields is almost zero. The thing is you can never be completely sure a field won't be used differently or the accessor won't ever change and given the trade off in work there's no reason not to implement a property.
Anyway, the analyzer complains about things that in a high percentage (in this case like 99.99% of the cases) are bad programming practices... but anyway it is just complaining. Fields can be made public and there are some extreme cases where its direct use may be justified. As ever, use your common sense... but keep in mind the elemental rule for best programming practices ... Is there a really good reason to break the convention? If there's then go ahead, if not or if the answer is "it involves more work" then stick to the practice...
Because changing public fields later to have get/set accessors will break code.
See this answer for more information
In general, it's a good idea to hide fields behind properties, even if you "know for sure" that you won't be redefining the field. All too often, what you "know for sure" today changes tomorrow. And, making a property to refer to a field is just a little bit of trouble.
That said, static analyzers are no substitute for thought. If you're happy with your design and in your judgement the analyzer is wrong, then ignore or (if possible) suppress that warning in that circumstance.
I think the point is that generally you don't know for sure that you won't be redefining the field or adding to it later. The whole point of encapsulating and hiding the data is that you are then free to do these things without changing the public interface and subsequently breaking dependent classes. If your property accessors are just simple get/sets then they'll be compiled down to that anyway, so ther are no performance concerns - given this your question should be is there any good reason not to use them?
One other benefit properties bring to the table is when doing Reflection. When you reflect over your class, you can get all the properties in one shot, rather than having to get the properties AND the fields.
And let's not forget that accessors give you flexibility when working with multiple threads.