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CHAPTER 17 ATTRIBUTES
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If the attribute has not been applied or if it has been applied but the constructor argument is set to false, then the base class implementation of the ToString method is used. We don t have to apply the attribute to the Car class, even though this is the class that knows the meaning of it. We can as easily apply it to a class derived from Car and get the desired result. Here is an example of a derived class that has been modified with the DetailLevel attribute: [DetailLevel(true)] class VolvoCar : Car { public VolvoCar(string model, string color) : base("Volvo", model, color) { // do nothing } } This class implements only a constructor. Note that it cannot override the base implementation of the ToString method. This is because I have modified the base implementation with the sealed keyword, which is described in 6. If the VolvoCar class were able to implement a new version of ToString, then we could end up with a base class that understands the meaning of the DetailLevel attribute and a derived class that ignores it. The following statements create an instance of the VolvoCar object and writes out a string representation of it: using System; class CustomAttributeTest { static void Main(string[] args) { VolvoCar myCar = new VolvoCar("C30", "Black"); Console.WriteLine("Car: {0}", myCar); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } } The output from these statements is as follows: Car: Manufacturer: Volvo, Model: C30, Color: Black Press enter to finish If I remove the attribute from the VolvoCar class and rerun the previous statements, I get the following results: Car: VolvoCar Press enter to finish
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CHAPTER 17 ATTRIBUTES
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You can see the effect of the attribute in the string representations of the VolvoCar objects. When the attribute is present and the constructor argument is set to true, we get a string representation that contains more detail than when the attribute is not present.
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Defining Properties in a Custom Attribute
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When you create custom attributes, you also have to create the code that can detect them and read the values you have provided as constructor parameters. The normal way to do this is to create properties that are set using the constructor parameter values. This is what I did with the DetailLevel attribute in Listing 17-8. It is important that the properties be read-only. If the value of the properties can be changed, then the meaning assigned to the class by the attribute will also be changed, leading to inconsistent and unexpected results. You can either make the set accessor of an automatically implemented property private so that it can be used only within the attribute class or use field-backed properties, which is demonstrated in Listing 17-9. Listing 17-9. Using Field-Backed Properties in a Custom Attribute Class using System; public class DetailLevelAttribute : Attribute { private bool useDetailed; public DetailLevelAttribute(bool useDetailedParam) { useDetailed = useDetailedParam; } public bool UseDetailed { get { return useDetailed; } } } It doesn t matter which approach you use, just as long as the value of the properties cannot be modified.
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As it stands, our custom attribute can be applied anywhere to classes, to fields, to methods, and so on. The meaning of our attribute is limited to classes, and we can restrict where our attribute can be used by applying attributes to it, specifically, the AttributeUsage attribute. It may seem odd to apply attributes to attributes, but it works rather well in practice. Listing 17-10 contains a demonstration of restricting the use of our custom attribute to classes. Listing 17-10. Restricting the Use of a Custom Attribute using System; [AttributeUsage(AttributeTargets.Class)] public class DetailLevelAttribute : Attribute {
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