i have a property attribute that can be defined once per class, and an empty interface called ISql which i just use to mark my objects that are allowed to use my custom buillt ORM.
is there a way to force the class that implements ISql to have that attribute at least once?
No, attributes aren't part of the contract of the interface, in terms of what implementations must provide.
For this sort of thing, I usually just add a unit test which uses reflection to find all implementations and validates it that way. It's not as nice as a compile-time check, but it's the best that's available in this case.
For this kind of code validation I like to use Nitriq. You can write your own rule very easy using Linq.
For personal use you can just call Nitriq to validate the code.
In large projects I used to put a step on continuous integration to run Nitriq console to validate the rules against the code
Related
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.
I want to force the usage of an attribute, if another attribute is used.
If a special 3rd party attribute is attached to a property, this attribute also needs to be given to a property.
Is there any possibility of doing this?
For example:
[Some3rdPartyAttribute("...")]
[RequiredAttribute("...)]
public bool Example{get; set;}
should bring no compile error,
[Some3rdPartyAttribute("...")]
public bool Example{get; set;}
should bring a compile error or warning.
The attribute itself is definded like the example from http://msdn.microsoft.com/en-US/library/z919e8tw(v=vs.80).aspx itself . But how to force the usage of the attribute if another attribute is used?
Unfortunately you cannot generate custom compiler warnings from attributes. Some attributes like System.ObsoleteAttribute will generate a warning or error, but this is hard-coded into the C# compiler. You should find another solution to your problem, maybe letting Some3rdPartyAttribute inherit from RequiredAttribute?
Otherwise you have to change the compiler.
Another option is using some AOP techniques. Like for example:
PostSharp.
Using it you can at compilation analyze yur code and emit a error if some condition does not sutisfies your requirements.
For concrete example on attributes, can have a look on :
PostSharp 2.1: Reflecting Custom Attributes
You can make a console app, that will iterate trough all types in your assembly trough reflection, check if the rule is satisfied and return 0 if it is, and some other error code and output error if the rule is broken.
Then make this console app run as post-build task.
As far as I know, there is no way to check for attributes at compile time.
I recently needed to enforce something similar (all classes derived from a certain base class need certain attributes). I ended up putting a manual check (with [Conditional("DEBUG")]) using reflection into the constructor of the base class. This way, whenever someone creates an instance of a class with missing attributes, they get an exception. But this might not be applicable in your case, if your classes do not all derive from the same class.
You could write some code that runs on application start which uses reflection and would then throw runtime exceptions if an attribute was used without the proper match but I believe that's as far as you can go and personally I wouldn't consider that a good approach as you would need to run the application once to make sure it complies with your rules.
Also, take a look at PostSharp which may help you.
How about using #warning + Unit testing? In this way, whenever you run Unit tests, an warning will be generated (or you could just use Debug.Fail instead of #warning)
I was wondering how to go about writing a custom attribute which can access both the input and output parameters of a function.
Below is an example of what I wish to achieve
[CustomAttribute("Creating Foo")]
public Foo CreateFoo(Foo newFoo)
{
//do logic
return newlyCreatedFoo;
}
From this, the CustomAttribute will create an entry in a DB with the "Creating Foo" tag, which is easy enough to do, but I want to be able to access both newFoo and newlyCreatedFoo as well. Is this possible?
Custom attributes can't do that in general: their purpose is to add metadata to a method to view during reflection.
That being said, there are some cases where attributes are leveraged to do that. For example, ASP.Net MVC uses custom attributes that implement IAuthorizationFilter to provide security for some web pages. This works because ASP is using reflection to launch the methods in the first place. Once it gets the method it checks to see if any attributes are IAuthorizationFilters, and does some extra work when they are. See this link for some more info.
Another way to think about this is to consider aspect-oriented programming. I think AOP frameworks for c# tend to make compile time decorations to methods based on attributes that implement a certain interface, but I have not used one.
My favorite way to deal with this is the good old Proxy pattern. Create a logging proxy.
I am afraid that this is not possible with custom attributes.
This could be achieved with post compilation processing. Tools like PostSharp would allow you to achieve this functionality.
Perhaps you should rephrase your question to "How can I intercept calls to specific methods and log the arguments"?
A custom attribute is just a decoration on a method. It does not intercept or in any way affect the execution of a method. It doesn't even know what member it is attached to.
If you want to intercept the calls you can use a dependency injection framework, or any other AOP framework for .NET to do just that. Some of them actually use attributes to mark their targets, but that isn't a requirement.
PostSharp is just one AOP framework for .NET.
Another option, is to use an IoC/Dependency Injection library like MEF to wrap your class in a proxy object that will intercept all calls and only log the values of the methods decorated with a special attribute.
I am developing a set of classes that implement a common interface. A consumer of my library shall expect each of these classes to implement a certain set of static functions. Is there anyway that I can decorate these class so that the compiler will catch the case where one of the functions is not implemented.
I know it will eventually be caught when building the consuming code. And I also know how to get around this problem using a kind of factory class.
Just curious to know if there is any syntax/attributes out there for requiring static functions on a class.
Ed Removed the word 'interface' to avoid confusion.
No, there is no language support for this in C#. There are two workarounds that I can think of immediately:
use reflection at runtime; crossed fingers and hope...
use a singleton / default-instance / similar to implement an interface that declares the methods
(update)
Actually, as long as you have unit-testing, the first option isn't actually as bad as you might think if (like me) you come from a strict "static typing" background. The fact is; it works fine in dynamic languages. And indeed, this is exactly how my generic operators code works - it hopes you have the static operators. At runtime, if you don't, it will laugh at you in a suitably mocking tone... but it can't check at compile-time.
No. Basically it sounds like you're after a sort of "static polymorphism". That doesn't exist in C#, although I've suggested a sort of "static interface" notion which could be useful in terms of generics.
One thing you could do is write a simple unit test to verify that all of the types in a particular assembly obey your rules. If other developers will also be implementing the interface, you could put that test code into some common place so that everyone implementing the interface can easily test their own assemblies.
This is a great question and one that I've encountered in my projects.
Some people hold that interfaces and abstract classes exist for polymorphism only, not for forcing types to implement certain methods. Personally, I consider polymorphism a primary use case, and forced implementation a secondary. I do use the forced implementation technique fairly often. Typically, it appears in framework code implementing a template pattern. The base/template class encapsulates some complex idea, and subclasses provide numerous variations by implementing the abstract methods. One pragmatic benefit is that the abstract methods provide guidance to other developers implementing the subclasses. Visual Studio even has the ability to stub the methods out for you. This is especially helpful when a maintenance developer needs to add a new subclass months or years later.
The downside is that there is no specific support for some of these template scenarios in C#. Static methods are one. Another one is constructors; ideally, ISerializable should force the developer to implement the protected serialization constructor.
The easiest approach probably is (as suggested earlier) to use an automated test to check that the static method is implemented on the desired types. Another viable idea already mentioned is to implement a static analysis rule.
A third option is to use an Aspect-Oriented Programming framework such as PostSharp. PostSharp supports compile-time validation of aspects. You can write .NET code that reflects over the assembly at compile time, generating arbitrary warnings and errors. Usually, you do this to validate that an aspect usage is appropriate, but I don't see why you couldn't use it for validating template rules as well.
Unfortunately, no, there's nothing like this built into the language.
While there is no language support for this, you could use a static analysis tool to enforce it. For example, you could write a custom rule for FxCop that detects an attribute or interface implementation on a class and then checks for the existence of certain static methods.
The singleton pattern does not help in all cases. My example is from an actual project of mine. It is not contrived.
I have a class (let's call it "Widget") that inherits from a class in a third-party ORM. If I instantiate a Widget object (therefore creating a row in the db) just to make sure my static methods are declared, I'm making a bigger mess than the one I'm trying to clean up.
If I create this extra object in the data store, I've got to hide it from users, calculations, etc.
I use interfaces in C# to make sure that I implement common features in a set of classes.
Some of the methods that implement these features require instance data to run. I code these methods as instance methods, and use a C# interface to make sure they exist in the class.
Some of these methods do not require instance data, so they are static methods. If I could declare interfaces with static methods, the compiler could check whether or not these methods exist in the class that says it implements the interface.
No, there would be no point in this feature. Interfaces are basically a scaled down form of multiple inheritance. They tell the compiler how to set up the virtual function table so that non-static virtual methods can be called properly in descendant classes. Static methods can't be virtual, hence, there's no point in using interfaces for them.
The approach that gets you closer to what you need is a singleton, as Marc Gravell suggested.
Interfaces, among other things, let you provide some level of abstraction to your classes so you can use a given API regardless of the type that implements it. However, since you DO need to know the type of a static class in order to use it, why would you want to enforce that class to implement a set of functions?
Maybe you could use a custom attribute like [ImplementsXXXInterface] and provide some run time checking to ensure that classes with this attribute actually implement the interface you need?
If you're just after getting those compiler errors, consider this setup:
Define the methods in an interface.
Declare the methods with abstract.
Implement the public static methods, and have the abstract method overrides simply call the static methods.
It's a little bit of extra code, but you'll know when someone isn't implementing a required method.
I'm aware of writing custom attributes that decorate a method and get evaluated before the method executes, but is there a way to modify it so the attribute gets evaluated after the method gets executed?
Theoretically (in pseudo-code):
public void MyMethod()
{
Console.WriteLine("Hello World");
}
[AttributeToExecuteAfter]
Am I misusing the concept of an attribute? If there's a technical reason this shouldn't be possible, what is it?
Are you referring to ActionFilters in .net MVC?
If you are, you can override the OnActionExecuted method by extending the abstract ActionFilterAttribute class. These only apply to mvc action methods though.
I think you are misunderstanding, yes. An attribute isn't code which runs before or after your method is run. An attribute is a piece of metadata attached to the method (or class, etc). Using reflection, code can be written to read that metadata.
Here's the best example I can think of:
The business object framework I use has the concept of a DataPortal. In an n-tier environment, the DataPortal sits on the server and executes my business objects' CRUD methods. The DataPortal was written in such a way that before calling my any of my CRUD methods, it first looks to see if the method has a RunMeInATransaction attribute on it. If it does, it creates a transaction, runs the method and then commits the transaction. Conceptually, it's a lot like methods having properties; my methods can have RunMeInATransaction property.
Using PostSharp you can create custom handlers for entering and exiting decorated method.
As Ken said, an attribute is just metadata attached to a method / property / field / class, useful to "tag" elements and then be able to recognize it with your code using reflection.
I.e, attributes are used in Linq2Sql as a way of mapping classes with database tables.
Your misunderstanding probably came from the fact that the C# compiler actually uses some special attributes to perform concrete operations. But that kind of functionality is not available for us, human developers :)