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Attribute Constructor and Field/Property Data Types
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When defining your own custom attribute type, you can define a constructor that takes parameters that a developer applying an instance of your attribute type must specify. In addition, you can define nonstatic, public fields and properties in your type that identify settings a developer applying an instance of your attribute type can optionally specify. When defining an attribute type s instance constructor, fields, and properties, you must restrict yourself to a small subset of data types. Specifically, the legal set of data types is limited to any of the following: Boolean, Char, Byte, SByte, Int16, UInt16, Int32, UInt32, Int64, UInt64, Single, Double, String, Type, Ob ject, or an enumerated type. In addition, you can use a single dimension, zero based array of any of these types. When applying an attribute, you must pass a compile time constant expression that matches the type defined by the attribute class. Wherever the attribute class defines a Type parameter, Type field, or Type property, you must use C# s typeof operator as shown in the following code. Wherever the attribute class defines an Object parameter, Object field, or Object property, you can pass an Int32, a String, or any other constant expression. If the constant expression represents a 278
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value type, the value type will be boxed at run time when an instance of the attribute is constructed. Here s an example of an attribute and an application of it:
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[AttributeUsage(AttributeTargets.All)] class SomeAttribute : Attribute { public SomeAttribute(String name, Object o, Type[] types) { // name refers to a String. // o refers to one of the legal types (boxing if necessary). // types refers to a 1 dimension, 0 based array of Types. } } [Some("Jeff", Color.Red, new Type[] { typeof(Math), typeof(Console) })] public class SomeType { ... }
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Logically, when a compiler detects a custom attribute applied to a target, the compiler constructs an instance of the attribute type by calling its constructor, passing it any specified parameters. Then the compiler initializes any public fields and properties that have also been specified. Now that the custom attribute object is initialized, the compiler serializes the object out to the target s metadata table entry. Important I ve found that this is the best way for developers to think of custom attributes: instances of types that have been serialized to a byte stream that resides in metadata. Later, at run time, an instance of the type can be constructed by deserializing the bytes contained in the metadata. Note Each 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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Defining an attribute type is useless by itself. Sure, you could define attribute types all you want and apply them all you want; but this would just cause additional metadata to be written out to the managed module the behavior of your application code wouldn t change. In 13, you saw that applying the Flags attribute to an enumerated type altered the behavior of System.Enum s ToString, Format, and Parse methods. The reason these methods behave differently is that they check at run time whether the enumerated type they re operating on has the Flags attribute metadata associated with it. Code can look for the presence of attributes using a technology called reflection. I ll give some brief demonstrations of reflection here, but I ll discuss it fully in 20. 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
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// 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 type is associated with it. If IsDefined returns true, then 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 Is Defined returns false, then Format treats the value as a normal enumerated type. So if you define your own attribute types, you must also implement some code that checks for the existence of an instance of your attribute type (on some target) and then executes 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 type. You ll recall that all CLS compliant attributes are derived from System.Attribute and this type 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 16 1 briefly describes what each method does. If you just want to see whether an attribute has been applied to a target, you should call IsDefined because it s much faster than the other two methods. However, you know that when an attribute is applied to a target, you can specify parameters to the attribute s constructor and you can also optionally set fields and properties. Using IsDefined won t construct an attribute object, call its constructor, or set its fields and properties. If you want to construct an attribute object, you must call either GetCustomAttributes or GetCustomAttribute. Every time one of these methods is called, it constructs instances of the specified attribute type and sets each instance s fields and properties based on the values specified in the source code. These methods return references to fully constructed instances of the applied attribute types. When you call any of these methods, internally, they must scan the managed module s metadata, performing string comparisons to locate the specified custom attribute class. Obviously, these operations take time. If you re performance conscious, you should consider caching the result of calling these methods rather than calling them repeatedly asking for the same information. Table 16 1: System.Attribute s Methods That Reflect over Metadata Looking for Instances of CLS Compliant Custom Attributes
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