I'm writing some library code, and classes that use that code are required to have two attributes specified (one custom attribute and one from .NET).
It's a bit of a hassle to document this requirement and copy-and-paste both of those attributes onto every class that uses my library, so I thought it might be better to have my custom attribute just imply the presence of the other. That other attribute from the .NET framework is sealed, though, so I can't just inherit it...
So is there any way to do this? Can I maybe add the .NET attribute at runtime?
Thanks.
Whilst you can use inheritance in attributes it is recommended you don't.
Since this is only a performance rule you will not violate correctness but it can be a source of some confusion and you must spend a great deal of effort to ensure you do not violate the semantics of the base attribute. You may cause issues if you attempt to expand the possible locations for the attribute with the AttributeUsage flags. The rules for the attribute in question are quite complex.
A method can have either of the two attributes applied, but not both. Any operation that has neither applied uses the attribute applied to the containing class. If the containing class does not have either attribute applied, the DataContractSerializer is used.
Give this making it sealed sounds like a sensible plan on their part. This API is not designed with this sort of extension in mind.
There is no way (short of AOP style 'code injection') to add attributes to existing classes at runtime.
I do not believe there is a way to imply secondary attributes or to add attributes at runtime. However, you could use a custom code template to add the attributes to every class as you add them. And/or for classes that are already developed, I suspect you could write a macro that goes through each file and adds the attributes to the classes, however I have not done this in the past, so I'm not sure what it would entail, I know making a code template to add to attributes to a class declaration is relatively simple. But this only works for class you're adding from the point of template addition on. If you need to do cleanup to old classes as well, I would look into macros if you have a lot of files/classes this needs to be done for. Or do it manually if there are not too many.
Related
I want to facade or proxy a library's implemented attribute.
for example the property mapping attributes.
I want to control which of the libraries attributes to be used in project.
some kind of a DI or Module control.
Is there an easy way to achieve it without using reflection?
something to transfer these two types of attributes into one:
[MapsFromAndToProperty(typeof(fooClass), nameof(fooClass.PropertyName))]
and
[AdaptMember(name)]
into
[MyAttributeToControlWhichOneToUse(typeof(fooClass), nameof(fooClass.PropertyName))]
And Thank you for your attention.
Any help would be appreciated.
As noted in the comments, it is not possible to alter attributes in the general case. This is simply because attributes are effectively part of the assembly metadata and doing what you want means to change that assembly metadata.
There are two ways how this could potentially work, though:
You do change the assembly metadata. Here, you are in the realm of aspect-oriented programming. You could try to create an aspect e.g. based on PostSharp that unrolls your MyAttributeToControlWhichOneToUse attribute into the required attributes at compile-time. This solution is independent of the way how the attributs are read, but requires changes to the build infrastructure.
You alter the way how the attributes are loaded. MEF is an example that does that for its convention-based programming model that effectively emulates attributes on the conventional classes by altering the way how attributes are retrieved. This only works, if foreseen in the lib that reads the properties (and honestly, I have never seen anybody else than MEF to do that).
Syntactically I understand how C# attributes are applied (i.e. denoted in square brackets []). But it is not obvious what effects adding attributes actually has.
For instance:
Are attributes meant to be a type of commenting?
How are these attributes handled by the C# compiler?
Will these C# attributes change the C# program execution in any way?
Attributes are not comments; they are classes that get added to the metadata of "things" in C#. By "things" I mean classes, properties, methods, etc.
An attribute absolutely can change the execution of a program... if something uses Reflection to read and act on the attributes. For example; the [DataMember] attribute will allow a DataContractSerializer to include that member in a serialized object. The [Export] attribute will be picked up by MEF code as a plugin.
There are many other examples and you can also create your own.
For far more information about attributes see: Attributes in C#
Attributes are a powerful construct that can indirectly affect the code execution. You can inspect self or other classes using Reflection and change behaviour based on the presence of certain attributes.
Take a look at the c# programming guide: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/attributes/
I'm trying to find out if there's a way to stop functions/methods from being added (EDIT: by other developers) to a class for the case where the object is a Model or DTO which should not contain methods (to prevent 'abuse' of the Models/DTOs by others, who may try and add 'helper' methods etc).
Is there any way to achieve this?
Use reflection and write a unit test that fails if a model-class has methods.
Mark all you model classes with a custom attribute. Then make a unit test that uses reflection to load a given assembly, iterate all classes in that assembly and check that classes marked with the model attribute does not have methods. This should be fairly straight forward using reflection.
I believe you are trying to solve a procedural issue with code where you should be using communication.
Your colleagues (i assume) are operating on the code files with 'full trust' privileges. If they break that privilege you should open a dialogue. Use the change as an opportunity to educate them on the intended design. Perhaps they are correct and you will be educated!
I suggest simply making the intended design obvious in the class name and with a comment stating the intended nature. Perhaps quote the design document(s) that informed the class.
You cannot hinder anyone with full write-access to your code-base to do so. The only two things you may do to avoid it are create some CodeAnalysis-rule for FXCop as mentioned by Christian.K in the comments or by writing your DTO-class so that it is undoubtly a DTO that should not have any methods by using a unambigious name for the class and if this is not enough provide some code-comments that notifies the coder to do not so.
However you may need some kind of method if using collections e.g. where you will need some kind of comparision if two instances of your DTO are equal, so you have to provide at least an Equals- and GetHashCode-method.
You don't need to use a struct to prevent additions to a class. You can use the sealed keyword
public sealed class MyDTOObject { ... }
Now, you can not inherent a class and also prevent inheritance (which is essentially what you're asking). The very fact of inheriting MyDTOObject is creating a new class which is based off of not equal to, or restricted, or defined in any way by the implementation of MyDTOObject.
You can use an abstract class, to force derived classes to implement certain methods, but not the other way around.
If you want to prevent others from deriving from your class and implementing helper methods, you must use the sealed keyword, or mark the class internal.
You may prevent the class being extended or inherited by marking it final that way nobody would be able to extend your class and hence not being able to add any behavior. But stop and ask yourself whether you want to do that or not, because then you'd be signing an invisible contract that everything ever required by the class is written in the class and this class needs no further addition.
To be clear, I was talking in Java context.
I'm trying to create a generic layer between the frameworks I am using and my application's code and have been blocked by a framework's need to decorate my classes with attributes.
Is there a way to be able to somehow map attributes to other attributes?
Example:
Class A is decorated with Attribute B
At runtime, map Attribute B to Attribute A
Class A is seen as decorated by Attribute A throughout the application's life.
What you are trying to do sounds like bad design. Attributes are not runtime types, they are in fact just type metadata. Even though some attributes allow you to change their parameters at runtime, that will not change which attribute is applied.
You will find solutions on the internet that suggest using Reflection.Emit. Note that this is a slippery slope and will lead to highly unmaintainable code.
My personal suggestion would be to create your own Class B that is decorated with both Attribute A as well as Attribute B, and then use internal logic to bridge whatever it is that you are wanting to bridge.
Could anyone explain the benefits (or reasons) to use custom attributes in your code. Of course I use (and understand the purpose of) defined attributes in certain scenarios (WCF, Serialization etc.), but I cannot imagine any algorithms where I would need to create and use my own custom attributes. Could someone provide a real-world case where usages of custom defined attributes bring something to a project.
The same reason as for WCF etc, but something that's specific to your project - you want to add some metadata to some members (types, fields, methods, whatever) to specify something about the mechanism involved, and it's not something which is covered by existing attributes.
For example, NUnit wanted to add their own indication that a particular type contained unit tests - there was no such existing attribute, so they created TestFixtureAttribute.
It's a relatively rare event, sure - but it can happen.
If you want to write your own system like WCF, Serialization, etc...
If you write code that iterates over types or members and does things with them, you will frequently want to use your own custom attributes to mark some members as being different or special.
I regularly use custom .Net attributes to support tooling in my infrastructure. One example was from very early in the .Net days (C# 1.0 to be exact). I was working on a research project which had a native C++ front and a brand new C# back end written by yours truly.
The front and back end shared a very similar object model which was evolving very rapidly. Not wanting to have to hand code both a C++ front end model, C++ serialization mechanism and a C# serialization mechanism I chose instead to attribute my C# types with custom attributes. They told me the parts of the model which were shared between the front and back end.
Once those attributes were in place I wrote a quick and dirty tool which
Parsed out the attributes to construct the core shared model
Generated the C# serialization code
Generated the C++ code
Generated the C++ serialization code
This made it dirt simple to keep my model up to date between my 2 projects. Just change the C# code, compile and re-run my tool.
I have used annotations in a custom AOP (Aspect-Oriented Programming) system I developed a while back. Attributes are also very useful for controlling orthogonal concerns like code generation.
Custom validation is a very good use case and can be seen from these links:
http://odetocode.com/blogs/scott/archive/2011/02/21/custom-data-annotation-validator-part-i-server-code.aspx
How to create Custom Data Annotation Validators
They can be used for marking tests, as in MBUnit for example. They can also be useful for code that inspects and loads classes (like a Plugin system) to provide meta-information.
They are really useful in building object mappers / ORM tools as well. If you ever decide to roll your own mapping system they are almost "required" to get all the functionality one would need. It's used more for making methods / classes more generic and using reflection to determine how to handle objects / select objects /etc...
To give you a specific case where I've used them. I once had to interact with a Mainframe screenscraper. I created a custom attribute to annotate which fields I wanted to send from my classes to the Mainframe, names that fell outside of conventions, special rules to deal with formatting and collections. I then had a class which was able to reflect over instances and realise which subset of fields were needed to interact with the mainframe screen scraper appropriately.