I want to know if there is any way to limit usage of custom attribute to the specific class.
I read about it and somebody wrote that it's probably impossible in C#. But I have just tried to use AttributeUsage attribute on a class that doesn't derrive from Attribute and Visual Studio has thrown a following error:
Attribute 'AttributeUsage' is only valid on classes derived from System.Attribute
So now I think that there is a pretty smart way to make it. Have you any ideas?
The only way I can think of is by making the attribute class an inner class of the target. I have a hard time coming up with a legitimate reason to use this though.
This will compile:
[InnerAttribute]
public class A
{
private class InnerAttribute : Attribute
{
}
}
But adding this won't:
[InnerAttribute]
public class B
{
}
Related
The problem I'm facing might be stupid, but I've never encountered it, so I guess I need some help. I'm learning how to use a Debug Visualizer.
I've created one: DebuggerSide.cs located in CarGarageVisualizer namespace.
I wanted the type of CarGarage<T> to be seen in this visualizer when debugging instance of this, so I've put following attributes to the class:
[DebuggerVisualizer(typeof(CarGarageVisualizer.DebuggerSide))]
[Serializable]
public class CarGarage<T>:IEnumerable<T>
where T : Car,new()
{
...
}
Now, to add first attribute I needed to add reference to CarGarageVisualizer that contains the DebuggerSide class. That's OK. But now, in my DebuggerSide's overriden method Show() I wanted to explicitly cast object gained from the objectProvider argument to the type of CarGarage<T>. But to be able to do this I would need to reference the CarGarageLibrary that contains the definition of this. And as I said I can't do that, because I get the error about recursive reference.
From other post on this subject, I know it's a bad practice. But, I don't want to copy the CarGarage<T> class to my Visualizer namespace (that would solve the problem, but I'm not sure if it's the right thing to do) unless there's not a better option.
Can anybody help me with this?
You should put the CarGarageVisualizer.DebuggerSide in a separate library that will and can be referenced by both.
Didn't get it correctly, I think.
What about, putting the CarGarage<T> in a separate library.
library CarGarage:
[Serializable]
public class CarGarage<T>:IEnumerable<T>
where T : Car,new()
{
...
}
library DebugVis: (uses CarGarage)
DebuggerSide....
library app: (uses CarGarage)
[DebuggerVisualizer(typeof(CarGarageVisualizer.DebuggerSide))]
public class CarGarageImpl<T> : CarGarage<T> { }
You can use the DebuggerVisualizerAttribute constructor overload that takes string containing the fully qualified type name of the visualizer like this
public static class MyVisualizers
{
public const string AssemblyRef = #"MyVisualizers, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null";
}
[DebuggerVisualizer("MyVisualizersNamespace.CarGarageVisualizer+DebuggerSide, " + MyVisualizers.AssemblyRef)]
[Serializable]
public class CarGarage<T>:IEnumerable<T>
where T : Car,new()
{
...
}
I need to create some custom attributes, to be used for my reflection functions.
Here is the usecase, as I see it:
the user creates some class and marks it with my special attribute ([ImportantAttribute] for example)
then the user does something with functions from my library. Those functions find classes with [ImportantAttribute] and do something with them
The main problem is that functions in my library expects, that classes wich was marked with [ImportantAttribute] inherit my interface (IMyInterface for example)
Is there any way to let user know if he mark his class with [ImportantAttribute] and forget to inherit IMyInterface during compilation, not in run time. Some way to specify that this attribute is only for classes that inherit IMyInterface.
Same with attributes for properties and fields.
Is there any way to let user know if he mark his class with
[ImportantAttribute] and forget to inherit IMyInterface during
compilation, not in run time
Simple answer: no, this is not possible. Not at compile-time. You can check this at runtime though using reflection.
The best you could do with attributes at compile-time (except some special system attributes such as Obsolete but which are directly incorporated into the compiler) is specify their usage with the [AttributeUsage] attribute.
I've used the strategy you mention in a couple of the frameworks I've built with good success. One such example is for providing metadata to a plug-in infrastructure:
[AttributeUsage(AttributeTargets.Class, AllowMultiple=false, Inherited=false)]
public class PluginAttribute : Attribute
{
public string DisplayName { get; set; }
public string Description { get; set; }
public string Version { get; set; }
}
public interface IPlug
{
void Run(IWork work);
}
[Plugin(DisplayName="Sample Plugin", Description="Some Sample Plugin")]
public class SamplePlug : IPlug
{
public void Run(IWork work) { ... }
}
Doing so allows me to figure out information about plug-ins without having to instantiate them and read metadata properties.
In my experience in doing so, the only way I've found to enforce that both requirements are met is to perform runtime checks and make sure it is bold and <blink>blinking</blink> in the documentation. It is far from optimal but it is the best that can be done (that I've found). Then again I'm sure there is a better way to go about handling this but so far this has been pretty solid for me.
So a little confession, I've never written an attribute class. I understand they serve the purpose of decorating classes with flags or extra functionality possibly.
Can someone give me a quick example of not just creating and applying an attribute to a class, but rather utilizing the attribute from another block of code. The only code samples I've ever seen to utilize any form of attributes was doing so with reflection, though I've always hoped there's a way of using them without reflection.
Attributes are always used with reflection. They are baked into the metadata of the types during compile time and the only way to read them is through reflection. Attributes are used when you want write a type and you want to associate some metadata with it which could be used by consumers of this type.
The simplest and most elegant way to use an attribute from another block of code is to use a property instead of an attribute.
See http://blogs.msdn.com/b/ericlippert/archive/2009/02/02/properties-vs-attributes.aspx for a discussion of the differences between properties and attributes.
First create your attribute
public class ImportableAttribute : Attribute
{
}
Then a class with a item that uses the Attribute
[ImportableAttribute]
public class ImportClass
{
[ImportableAttribute]
public string Item {get; set;}
}
Then check if that property uses that attribute. Can be done with classes to.. Of course :)
PropertyInfo property = typeof(ImportClass).GetProperty("Item");
if (property.IsDefined(typeof(ImportableAttribute),true))
{
// do something
}
With a class:
typeof(ImportClass).IsDefined(typeof(ImportableAttribute), true);
I need to create a custom attribute that is applicable only for non static class member.
How can I validate this constraint on project compilation or using code analysis tools?
There's no such constraint.
You could always write some post-build event that uses reflection to verify this... Granted, it may not be the most elegant of solutions....
To set this up, you would go into project properties, then the 'Build Events' tab. You would then enter the command line for the reflection based tool you'd write to implement this verification
It's probably not what you're looking for, but it's possible to make such an attribute with PostSharp, you will probably have something like this:
[Serializable]
public sealed class StaticAttribute : OnMethodBoundaryAspect
{
public override bool CompileTimeValidate(System.Reflection.MethodBase method)
{
return method.IsStatic;
}
The OnMethodBoundaryAspect basically wrapps your method inside a try/catch block, and the CompileTimeValidate method determines whether or not the attribute is invoked at runtime.
I hit this problem all the time. Suppose I am making a command line interface (Java or C#, the problem is the same I think, I will show C# here).
I define an interface ICommand
I create an abstract base class CommandBase which implements ICommand, to contain common code.
I create several implementation classes, each extending the base class (and by extension the interface).
Now - suppose that the interface specifies that all commands implement the Name property and the Execute method...
For Name each of my instance classes must return a string that is the name of that command. That string ("HELP", "PRINT" etc) is static to the class concerned. What I would love to be able to do is define:
public abstract static const string Name;
However (sadly) you cannot define static members in an interface.
I have struggled with this issue for years now (pretty much any place I have a family of similar classes) and so will post my own 3 possible solutions below for your votes. However since none of them is ideal I am hoping someone will post a more elegant solution.
UPDATE:
I can't get the code formatting to work properly (Safari/Mac?). Apologies.
The example I am using is trivial. In real life there are sometimes dozens of implementing classes and several fields of this semi-static type (ie static to the implementing class).
I forgot to mention - ideally I want to be able to query this information statically:
string name = CommandHelp.Name;
2 of my 3 proposed solutions require that the class be instantiated before you can find out this static information which is ugly.
You may consider to use attributes instead of fields.
[Command("HELP")]
class HelpCommand : ICommand
{
}
As you mentioned, there is no way to enforce this from the interface level. Since you are using an abstract class, however, what you can do is declare the property as abstract in the base class which will force the inheriting class it override it. In C#, that would look like this:
public abstract class MyBaseClass
{
public abstract string Name { get; protected set; }
}
public class MyClass : MyBaseClass
{
public override string Name
{
get { return "CommandName"; }
protected set { }
}
}
(Note that the protected set prevents outside code changing the name.)
This may not be exactly what you're looking for, but it's as close as I think you can get. By definition, static fields do not vary; you simply can't have a member that is both static and overridable for a given class.
public interface ICommand {
String getName();
}
public class RealCommand implements ICommand {
public String getName() {
return "name";
}
}
Simple as that. Why bother having a static field?
Obs.: Do not use a field in an abstract class that should be initiated in a subclass (like David B suggestion). What if someone extends the abstract class and forget to initiate the field?
just add the name property to the base class and pass it ito the base class's constructor and have the constuctor from the derived class pass in it's command name
What I usually do (in pseudo):
abstract class:
private const string nameConstant = "ABSTRACT";
public string Name
{
get {return this.GetName();}
}
protected virtual string GetName()
{
return MyAbstractClass.nameConstant;
}
----
class ChildClass : MyAbstractClass
{
private const string nameConstant = "ChildClass";
protected override string GetName()
{
return ChildClass.nameConstant;
}
}
Of course, if this is a library that other developers will use, it wouldn't hurt if you add some reflection in the property to verify that the current instance in fact does implement the override or throw an exception "Not Implemented".
My answer will relate to Java, as that is what I know. Interfaces describe behavior, and not implementation. Additionally, static fields are tied to the classes, and not instances. If you declared the following:
interface A { abstract static NAME }
class B { NAME = "HELP" }
class C { NAME = "PRINT" }
Then how could this code know which NAME to link to:
void test(A a) {
a.NAME;
}
How I would suggest to implement this, is one of the following ways:
Class name convention, and the base class derives the name from the class name. If you wish to deviate from this, override the interface directly.
The base class has a constructor which takes name
Use annotations and enforce their presence through the base class.
However, a much better solution is proabably to use enums:
public enum Command {
HELP { execute() }, PRINT { execute() };
abstract void execute();
}
This is much cleaner, and allows you to use switch statements, and the NAME will be easily derived. You are however not able to extended the number of options runtime, but from your scenario description that might not be even needed.
[Suggested answer # 3 of 3]
I have not tried this yet and it would not be so nice in Java (I think?) but I could just tag my classes with Attributes:
[CammandAttribute(Name="HELP")]
Then I can use reflection to get that static information. Would need some simple helper methods to make the information easily available to the clients of the class but this could go in the base class.
From a design perspective, I think it is wrong to require a static implementation member... The relative deference between performance and memory usage between static and not for the example string is minimal. That aside, I understand that in implementation the object in question could have a significantly larger foot print...
The essential problem is that by trying to setup a model to support static implementation members that are avaialble at a base or interface level with C# is that our options are limited... Only properties and methods are available at the interface level.
The next design challenge is whether the code will be base or implementation specific. With implementation your model will get some valdiation at compile time at the code of having to include similar logic in all implementations. With base your valdiation will occur at run time but logic would be centralized in one place. Unfortunately, the given example is the perfect show case for implemntation specific code as there is no logic associated with the data.
So for sake of the example, lets assume there is some actual logic associated with the data and that it is extensive nad/or complex enough to provide a showcase for base classing. Setting aside whether the base class logic uses any impelementation details or not, we have the problem of insuring implemtation static initialization. I would recommend using an protected abstract in the base class to force all implementations to created the needed static data that would be valdated at compile time. All IDE's I work with make this very quick any easy. For Visual Studio it only takes a few mouse clicks and then just changing the return value essentially.
Circling back to the very specific nature of the question and ignoring many of the other design problems... If you really must keep this entire to the nature of static data and still enforce it thru the nature confines of the problem... Definately go with a method over properties, as there are way to many side effects to make go use of properties. Use a static member on the base class and use a static constructor on the implementations to set the name. Now keep in mind that you have to valdiate the name at run-time and not compile time. Basically the GetName method on the base class needs to handle what happens when an implementation does not set it's name. It could throw an exception making it brutally apparent that something is worng with an implementation that was hopefulyl cause by testing/QA and not a user. Or you could use reflection to get the implementation name and try to generate a name... The problem with reflection is that it could effect sub classes and set up a code situation that would be difficult for a junior level developer to understand and maintain...
For that matter you could always generate the name from the class name thru reflection... Though in the long term this could be a nightmare to maintain... It would however reduce the amount of code needed on the implementations, which seems more important than any other concerns. Your could also use attributes here as well, but then you are adding code into the implementations that is equivalent in time/effort as a static constructor and still have the problem off what todo when the implementation does not include that information.
What about something like this:
abstract class Command {
abstract CommandInfo getInfo();
}
class CommandInfo {
string Name;
string Description;
Foo Bar;
}
class RunCommand {
static CommandInfo Info = new CommandInfo() { Name = "Run", Foo = new Foo(42) };
override commandInfo getInfo() { return Info; }
}
Now you can access the information statically:
RunCommand.Info.Name;
And from you base class:
getInfo().Name;
[Suggested solution #1 of 3]
Define an abstract property Name in the interface to force all implementing classes to implement the name property.
(in c#) Add this property as abstract in the base class.
In the implementations implement like this:
public string Name
{
get {return COMMAND_NAME;}
}
Where name is a constant defined in that class.
Advantages:
Name itself defined as a constant.
Interface mandates the property be created.
Disadvantages:
Duplication (which I hate). The exact same property accessor code pasted into every one of my implementations. Why cant that go in the base class to avoid the clutter?
[Suggested solution #2 of 3]
Make a private member variable name.
Define an abstract property Name in the interface.
Implement the property in the base class like this:
public string Name
{
get {return Name;}
}
Force all implementations to pass name as a constructor argument when calling the abstract base class constructor:
public abstract class CommandBase(string commandName) : ICommand
{
name = commandName;
}
Now all my implementations set the name in the constructor:
public class CommandHelp : CommandBase(COMMAND_NAME) {}
Advantages:
My accessor code is centralised in the base class.
The name is defined as a constant
Disadvantages
Name is now an instance variable -
every instance of my Command classes
makes a new reference rather than
sharing a static variable.