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Part III Essential Types
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Important I ve found this to be the best way for developers to think of custom attributes: instances of classes that have been serialized to a byte stream that resides in metadata . Later, at runtime, an instance of the class can be constructed by deserializing the bytes contained in the metadata . In reality, what actually happens is that the compiler emits the information necessary to create an instance of the attribute class into metadata . Each constructor parameter is written out with a 1-byte type ID followed by the value . After serializing the constructor s parameters, the compiler emits each of the specified field and property values by writing out the field/property name followed by a 1-byte type ID and then the value . For arrays, the count of elements is saved first, followed by each individual element .
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Detecting the use of a Custom Attribute
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Defining an attribute class is useless by itself . Sure, you could define attribute classes all you want and apply instances of them all you want, but this would just cause additional metadata to be written out to the assembly the behavior of your application code wouldn t change . In 15, Enumerated Types and Bit Flags, you saw that applying the Flags attribute to an enumerated type altered the behavior of System.Enum s ToString and Format methods . The reason that these methods behave differently is that they check at runtime if the enumerated type that they re operating on has the Flags attribute metadata associated with it . Code can look for the presence of attributes by using a technology called reflection . I ll give some brief demonstrations of reflection here, but I ll discuss it fully in 23, Assembly Loading and Reflection . If you were the Microsoft employee responsible for implementing Enum s Format method, you would implement it like this:
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public static String Format(Type enumType, Object value, String format) { // Does the enumerated type have an instance of // the FlagsAttribute type applied to it if (enumType.IsDefined(typeof(FlagsAttribute), false)) { // Yes; execute code treating value as a bit flag enumerated type. ... } else { // No; execute code treating value as a normal enumerated type. ... } ... }
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This code calls Type s IsDefined method, effectively asking the system to look up the metadata for the enumerated type and see whether an instance of the FlagsAttribute class is associated with it . If IsDefined returns true, an instance of FlagsAttribute is associated with the enumerated type, and the Format method knows to treat the value as though it contained a set of bit flags . If IsDefined returns false, Format treats the value as a normal enumerated type .
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18 Custom Attributes
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So if you define your own attribute classes, you must also implement some code that checks for the existence of an instance of your attribute class (on some target) and then execute some alternate code path . This is what makes custom attributes so useful! The FCL offers many ways to check for the existence of an attribute . If you re checking for the existence of an attribute via a System.Type object, you can use the IsDefined method as shown earlier . However, sometimes you want to check for an attribute on a target other than a type, such as an assembly, a module, or a method . For this discussion, let s concentrate on the methods defined by the System.Attribute class . You ll recall that all CLS-compliant attributes are derived from System.Attribute . This class defines three static methods for retrieving the attributes associated with a target: IsDefined, GetCustomAttributes, and GetCustomAttribute . Each of these functions has several overloaded versions . For example, each method has a version that works on type members (classes, structs, enums, interfaces, delegates, constructors, methods, properties, fields, events, and return types), parameters, modules, and assemblies . There are also versions that allow you to tell the system to walk up the derivation hierarchy to include inherited attributes in the results . Table 18-1 briefly describes what each method does .
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TABLE 18-1
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