Is it possible to have something like AttributeUsage to restrict the use of an attribute to a specific class (not just AttributeTargets.Class - that would be any class)?
One way to accomplish this, if you have access to the specific class, is detailed by Marc Gravel here: http://marcgravell.blogspot.com/2009/06/restricting-attribute-usage.html. Basically you implement the attribute as a protected class of the specific type. Then it can only be used by that type.
No. There is nothing in the framework that would do this.
However, the code that uses the attribute in question could always check to make sure that the class's type is the specific class (or one of its subclasses).
Attributes, by themselves, do nothing - so this should have the same effect.
Make all data in the Attribute accessible only by a public static method which takes the class you want in question and checks to see if it has the given attribute.
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Can you get merged attributes for a class in C#?
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I have a certain collection of built-in attributes (like System.Runtime.Serialization.SerializableAttribute) that I want to apply to a certain collection of classes
Is it possible to unite those attributes into one? I don't want to apply all of them to all of my classes explicitly (the attibute collection might change during the development process)
What I want is one attribute, e.g.
public class MyClassAttribute: System.Attribute { ... }
which I could apply easily to my classes
[MyClass]
public class SampleClass { ... }
and that would cause SampleClass to have Serializable attribute and others. Thanks
No, it is not, basically. Actually, [Serializable] is particularly note-worthy because that has different treatment in the compiler - it is not written as an attribute, but as a raw flag (the runtime simply lies if you ask "does it have the [Serializable] attribute" - it checks the flag against the type, and returns what you expect to see, even though that isn't the truth).
I don't think it's possible out of the box, but you could use Mono.Cecil to modify the types in your assembly in a post-build step, removing your collection-attribute and adding the others.
Interesting question. I think a lot of the built-in attributes are sealed so this might not be possible.
I find myself writing a lot of this kind of stuff:
[SameAttribute]
ClassA fieldA;
[SameAttribute]
ClassB fieldB;
[SameAttribute]
ClassC fieldC;
...
Is there a syntax in C# that would allow me to mark several fields with the same attribute at once? May be there are coding conventions about this situation that would make this code less verbose and more readable?
Edit: Just to clarify, I don't want every field of the class to have this attribute, there's just a lot of them.
No. You will have to apply the [SameAttribute] to each field individually.
If you want SomeAttribute to apply to all fields in a class, it might be possible to apply the attribute to the entire class. However, even if SomeAttribute is allowed to target classes, its exact behavior when doing so is dependent on the implementation of SomeAttribute. Otherwise no, you have to apply the attribute to each field individually.
In addition to the other answers above, PostSharp, which allows "aspect-oriented programming" lets you define attributes that will apply to each member in a class. You can use it to make a custom attribute that would apply your desired attribute to all of the members.
There is nothing out of the box, that I know of, but you could use a Visual Studio add-in like ReSharper to create a live template to automatically add the Attribute you wish to use when you use a certain template
http://www.jetbrains.com/resharper/features/code_templates.html
The other day i was looking at C# Boolean struct metadata.
Boolean implements the interface IConvertible. But looking at Boolean's members i could not see most of the IConvertible members.
I've done some tests with some colleagues, including creating our own classes, and came to the conclusion that IConvertible must be implemented explicitly for Boolean.
The question is, why are they not visible? I understand it might be a 'by design decision' but i understand that it would add greater value if they were visible to anyone inspecting the metadata.
The tests were done in VS2010 .NET4.0
The reason is that those methods are there just to implement the I-interface and not to augment the class' public interface.
What I mean is that if you have the following:
public class MyClass : IConvertible
{
// implementation
}
You might want MyClass to be convertible, indeed, so you can pass references of it to methods that expect IConvertible:
public void DoSomethingWithConvertible(IConvertible conv)
But you might not want variables of type MyClass to expose the Convert methods. You simply don't want MyClass's public interface to have that method, then you implement the interface explicitly. That's the whole idea of the approach. This means the following is not allowed:
MyClass a = new MyClass();
a.Convert();
However, the following is still be allowed:
MyClass a = new MyClass();
((IConvertible)a).Convert();
The whole idea behind this is that even though we're using the exact same instance, a as MyClass doesn't have the method. A as IConvertible does have the method. Think of it as if you're allowing the instance to have split personality.
Usually I end implementing every interface implicitly. However, there are very specific situations where I'd implementing them explicitly exactly for the reasons outlined above.
BTW, thanks for the great question!
Because explicit interface implementation actually hides the implementation.
The metadata does indeed show the explicitly implemented. Do you mean intellisense and not metadata?
I'd say that's by design and help the developer of say Boolean to restrict the interface to a subset. By restricting what's suggested to use it also becomes visible what's considered abnormal usage. E.g. it's generally not advised to view a Boolean value as a specific numeric value but in certain cases it's handy to be able to do that anyways.
IDictinary<T,K> is another example. It implements IEnumerable<KeyValuePair<T,K>> making it possible to iterate over all the pairs in the collection and ICollation<KeyValuePair<T,K>>. So you can call Add on the dictionary given a KeyValuePair but usually you should use Add(K, key, T Value)
Try inspecting the class with a tool that provides read access to metadata. ILDASM for one and you can indeed find metadata of the explicitly implemented methods.
They are explicitly implemented. You can find all implemented convertables here: http://msdn.microsoft.com/en-us/library/system.boolean.aspx
I can use TypeDescriptor.AddAttributes to add an attribute to a type in runtime. How do I do the same for a method and parameter? (maybe 2 separate questions...)
TypeDescriptor.AddAttributes only affects a very specific use-case; i.e. from within System.ComponentModel. For the rest of reflection, it knows nothing about the extra attribute. And indeed, System.ComponentModel doesn't really apply to methods or parameters.
So in short; you can't. You will need to store this information somewhere else (bespoke), or add it at compile-time.
As I see from analyzing the TypeDescriptor class in Reflector, the .AddAttributes method internally calls the .AddProvider method. The TypeDescriptionProvider instance passed to it is actually responsible for providing meta-data. You could try adding the [TypeDescriptionProviderAttribute] attribute to your class and implement your own provider by deriving from the TypeDescriptionProvider class. As the documentation says, by overriding TypeDescriptionProvider.CreateInstance, you could provide a substitute object whose type has all necessary attributes. I suspect that the attributes applied to methods inside the substitution type will also take effect. However, I haven't tried that myself, so feel free to experiment...
I would like to define a constrait on my custom (PostSharp) attribute. My goal is to get error or warning while compile time, if class X dont implements Y interface but it has my attribute.
So this should work:
[MyAttributeOnlyForY]
public class X : Y { ... }
but this should break the compile process:
[MyAttributeOnlyForY]
public class X { ... }
How is it possible?
The reason
This attribute works like an aspect (this is PostSharp attribute), and I want to be sure that the weaved class provides all needed information for this attribute.
I want to avoid null result on
(eventArgs.Instance as ILoggerServiceOwner)
and I think complie time checking is a good practice.
Solution
I've found a perfect start here: Validating attribute usage with PostSharp Aspects
You could use the PostSharp method CompileTimeValidate and use reflection to check if type has a derived type. However, it may be computationally expensive do look for all types in the assembly.
I think this is not possible.
A better solution might be to use the Obsolete attribute on your custom attribute constructor to warn that the target class should implement the interface Y.