How to create a Custom Attribute?

Question

For some time I’ve been trying to understand the concept of custom attribute of C#. I think it kind of looks like the system of decorators from Python. But I’m not getting it very well.

I read in a article Codeproject, but it is not yet clear. I understood that to create a CustomAttribute, it is necessary to create a class that inherits from Attribute. But let’s say I want to create a CustomAttribute to validate the type property int of a class, to allow only even numbers. In this case where would be the validation code?

An example would be nice.

Answer 1

First there is a mistake in thinking that the attributes of C# are equivalent to decorators of Python, the operation of both are quite different.

The attribute is just static information, it’s a metadata. If they are not called by some mechanism they do nothing. This call is made through reflection (example), so it differs from Python.

Of course, it is possible to use this mechanism to facilitate the creation of a Decorator. In fact this is done in a question in the OS. One direct comparison with Python is made here.

You can create a method in the attribute to perform the validation itself, but it will not be done just because the attribute is present in some element of the code. Your invocation will be at some other location, made by the user attribute programmer or by some framework that automates this for the final programmer. That answer shows this well.

Has a example of how it works and one more complete in the Codeproject. The .NET has an example well complete of how it can be a similar attribute (note that the IsValid() is not called alone, just like the other methods of the class).

Your example:

[TypeUsage(int)] //atributo hipotético
class EvenAttribute : Attribute {
    public string Message { get; set; }
    public bool IsValid(int value) {
        return value % 2 == 0;
    }
}

I put in the Github for future reference.

This hypothetical attribute TypeUsage which can be used by some tool to ensure that your custom attribute is only used in a property of type int. This attribute would need to be created by you and the checking tool as well.

Obviously, as the examples show linked above, it is possible to do something more sophisticated, it can be something within a larger mechanism, with specific contracts established, it depends on the need, but generally this is the example.

Practical example of use in another question.