In C# there is a attribute called AttributeUsage, if you want to set this attribute to a class it automatically detects if the class is derived from the Attribute class and if not it will throw an error.
How can I create such restrictions?
I want to create an attribute which should only be available/settable on a specific class.
What you are trying to do is not possible via attributes AttributeUsage alone, but can be achieved in other way.
AttributeUsage has ValidOn property, that is of AttributeTarget type. It lets you specify that it's valid only on class, but not type of this class.
So, why AttributeUsage only works on classes that derive from Attribute? It's a compiler rule CS0641
'attribute' : attribute is only valid on classes derived from System.Attribute
An attribute was used that can only be used on a class that derives from System.Attribute.
It's not part of AttributeUsage itself.
If you want to achieve something like this, you will need to write your own Roslyn analyzer, that will check it for you. There are few tutorials on how to do write one, i.e. official one or this), I also suggest to look at roslyn-analyzers code, CS0641 is not there, since it's a compiler error, not an analyzer, but you can get quite a lot of references here.
I hope it helps you, wish you good luck!
Related
My company has a base database model class that is subclassed by particular instances of our product. The class represents primary keys in a database. The base class has a field, which we'll call AlwaysPresent, which is common to all instances of the product and is not used in querying.
abstract class BaseClass
{
private string AlwaysPresent
}
But it is a requirement that subclasses add at least one more field, as we will use reflection later to treat those other fields as database column names for a query. If there are no other fields, we can't query.
So, my question: is it possible to use C#'s reflection capabilities to force a non-abstract subclass to define new fields without specifying their names?
I am a Python programmer by trade, and I know exactly how to solve this kind of problem in Python using metaclasses. To my knowledge, C# does not have metaclasses. And I cannot raise an exception in the base class constructor, because (for various reasons) we don't use constructors for these classes (just initializers), and even if we did the base class constructor could be overridden.
Reflection cannot be used to force something. At least not at compile time. Via reflection you can read how a type is. In your case you can probably check its fields and throw an exception if required at run time.
In any case usually it is much better to use properties instead of fields. Properties are more extensible and better to hide the internal structure of a class.
A common way to enforce a specific design (properties or methods definition) of a class is to use interfaces.You can have also a class that implement more than one interface.
If properties names or fields are not know when designing the interface you cannot enforce your requirements at compile time but only at run time.
Another common c# technique is to decorate properties or fields with attributes. Maybe you can create a custom attribute and at run time check for fields with that attribute (always with reflection).
This can be done with aspects, specifically PostSharp. It allows you to execute custom code during compilation (in fact, it hooks on postcompile action) in the CompileTimeValidate:
http://www.postsharp.net/blog/post/Architectural-Validation
You can of course replace PostSharp with any custom code triggered on postcompile at build-time.
Turns out this is not a feature in C#, but you can write it like this to force people to implement it
abstract class BaseClass
{
private abstract string GetAlwaysPresent();
}
I recently read about attributes and reflection and I thought it would be a good method to include metadata in my program. I have this abstract class and I wanted all classes inheriting from it to declare with the class some attribute, since I wanted custom components(those derived classes) to be created for my program and wanted to read the metadata of these classes on runtime. However, the derived classes all have to explicitly declare the attribute in which I store metadata. So how to I force an attribute declaration in the derived classes? Thanks.
Define your attribute class to itself have an AttributeUsageAttribute attribute where the Inherited property is true.
Or don't, since that's the default...
Derived targets (that is, classes if the attribute is on a class, methods if it is on a method, etc.) will then inherit the attribute without explicit declaration.
If by "force", you mean "compile time enforcement": You can't.
As Daniel said, you cannot enforce attributes at compile time.
But if you want to read the data at runtime, why bother with attributes and reflection at all? You can create an abstract method in your abstract class:
abstract class Base
{
public abstract string Metadata();
}
class Derived1 : Base
{
public override string Metadata()
{
return "Metadata for Derived1";
}
}
class Derived2 : Base // won't compile, since Metadata has not been provided
{
}
The behaviour is slightly different, of course. With this option, you need a reference to an instance of the derived class instead of just the type information itself. On the other hand, it avoids reflection.
As Daniel says you can't force at compile time.
You could add the attribute(s) to the abstract parent and pick them up.
Another option is to add a method to check for the existence of the attrribute in the parent class and throw an exception if not present. Call that from suitable methods.
Came across this old question due to a similar use case. One way to enforce Attribute usage at compile time is by writing an analyzer, similar to what the xunit framework does. here's an example:
https://github.com/xunit/xunit.analyzers/blob/main/src/xunit.analyzers/TheoryMethodMustHaveTestData.cs
Involves a little more effort but does the work.
There is an Attribute called DataSourceAttribute in C#. It needs to be declared on each Method again and again. Is it possible to declare it one time at the class level so that I don't need to repeat myself. If so, how?
In Data Driven UnitTesting, the data source need to be specified with the help of this attribute: [DataSource (...),...]. There are about 10-15 such methods, and I do not want to declare the attribute for each method. I'd like to declare it once and have all the methods inherit it from the class level usage.
Take a look at Afterthough. It is a framework that applies custom chunks of code, including properties, methods, attributes to your solution post-compile.
You might be able to create a custom class attribute that applies the method attribute to all methods in the class.
Here is the MSDN for custom attributes.
http://msdn.microsoft.com/en-us/library/sw480ze8(v=VS.100).aspx
It is valid (ie. it compiles and runs) to put an attribute on the generic parameter for a class or a method:
public class MyClass<[My] T>
{
private void MyMethod<[My] T>()
{}
}
public class MyAttribute : Attribute
{}
I've never seen this used, and am struggling to come up with a reason as to why you would want to.
Is it just a quirk/side-effect of the language specification, or is there a valid/useful reason to put an attribute in this position?
For the same reason attributes are useful on any construct; they supply meta-data that can be used by Reflection or other post-processors to do various things. For instance, you might have an AOP system that uses an attribute on a type argument to apply certain run-time constraints that otherwise could not be expressed. I'm not sure if there are any systems that actually use these attributes to do anything, but there's no reason to disallow them as metadata.
I'm sure some AOP nut will find a valid reason to decorate generic parameters with attributes. I certainly can't think of any. Try this:
typeof(MyClass<>).GetGenericArguments().GetCustomAttributes().OfType<MyAttribute>();
If this Enumerable has any elements, then it is possible to access the attribute you placed on the class's generic parameter. If not, then you can't and thus having data you'd expect to access from any other class in your codebase is pointless. HOWEVER, they can still have code that runs when instantiated, and they're instantiated by the runtime when the generic class comes into scope, allowing you to perform aspect-oriented logic in the attribute itself. Exactly what that would be, and how it would be any different than decorating the generic class or method directly, is left as an exercise to people who worship AOP far more than I do.
I've been reading a text about an extension to C# and at one point it says that "An attribute decoration X may only be applied to fields of type Y."
I haven't been able to find a definition for attribute decoration, and I'm not making much sense out of this by exchanging the two.
It's probably referring to the Attribute class. For example, you can mark a type as serializable via the SerializableAttribute. When you apply an attribute, you can leave off the "Attribute" suffix.
[Serializable]
public class SomeClass {
}
Attributes provide a means to add meta-data about the code.
Attributes are used to add metadata to .NET (C#) code in a structured manner. What a lot of people don't realise, though, is that there are actually two types of attribute.
The simplest is custom attributes, where you define an attribute that specific classes look for to alter the way they work. A common example is the System.Xml.Serialization attributes which are read by the XmlSerializer to alter its output, e.g. a class could be marked up something like the following to specify its namespace and that the field should be an attribute:
[XmlType(Namespace = "http://mycompany.com/")]
public class MyClass
{
[XmlAttribute]
public string MyField;
}
Custom attributes like this have no meaning to the compiler or the runtime, they are just added to the class as part of its metadata, and can be retrieved by a call to Type.GetCustomAttributes.
The other main group of attributes is pseudo-custom attributes, which actually have meaning to either the compiler or the runtime. The example in the post by Haacked with SerializableAttribute is actually an example of a pseudo-custom attribute. It is actually stored as part of the type definition and cannot be retrieved using Type.GetCustomAttributes. You cannot create your own pseudo-custom attributes.
So it's likely what you're dealing with here is a custom attribute which is being looked for by a specific tool.