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using System; [assembly: SomeAttr] [module: SomeAttr] // Applied to assembly // Applied to module
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[type: SomeAttr] // Applied to type internal sealed class SomeType<[typevar: SomeAttr] T> { [field: SomeAttr] // Applied to field public Int32 SomeField = 0; [return: SomeAttr] // Applied to return value [method: SomeAttr] // Applied to method public Int32 SomeMethod( [param: SomeAttr] // Applied to parameter Int32 SomeParam) { return SomeParam; } [property: SomeAttr] public String SomeProp { [method: SomeAttr] get { return null; } // Applied to property
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// Applied to generic type variable
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// Applied to get accessor method
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Part III Essential Types
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} [event: SomeAttr] // Applied to event [field: SomeAttr] // Applied to compiler-generated field [method: SomeAttr] // Applied to compiler-generated add & remove methods public event EventHandler SomeEvent; }
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Now that you know how to apply a custom attribute, let s find out what an attribute really is . A custom attribute is simply an instance of a type . For Common Language Specification (CLS) compliance, custom attribute classes must be derived, directly or indirectly, from the public abstract System.Attribute class . C# allows only CLS-compliant attributes . By examining the .NET Framework SDK documentation, you ll see that the following classes (from the earlier example) are defined: StructLayoutAttribute, MarshalAsAttribute, DllImportAttribute, InAttribute, and OutAttribute . All of these classes happen to be defined in the System.Runtime.InteropServices namespace, but attribute classes can be defined in any namespace . Upon further examination, you ll notice that all of these classes are derived from System.Attribute, as all CLS-compliant attribute classes must be . Note When applying an attribute to a target in source code, the C# compiler allows you to
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omit the Attribute suffix to reduce programming typing and to improve the readability of the source code . My code examples in this chapter take advantage of this C# convenience . For example, my source code contains [DllImport(...)] instead of [DllImportAttribute(...)] .
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As I mentioned earlier, an attribute is an instance of a class . The class must have a public constructor so that instances of it can be created . So when you apply an attribute to a target, the syntax is similar to that for calling one of the class s instance constructors . In addition, a language might permit some special syntax to allow you to set any public fields or properties associated with the attribute class . Let s look at an example . Recall the application of the DllImport attribute as it was applied to the GetVersionEx method earlier:
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[DllImport("Kernel32", CharSet = CharSet.Auto, SetLastError = true)]
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The syntax of this line should look pretty strange to you because you could never use syntax like this when calling a constructor . If you examine the DllImportAttribute class in the documentation, you ll see that its constructor requires a single String parameter . In this example, "Kernel32" is being passed for this parameter . A constructor s parameters are called positional parameters and are mandatory; the parameter must be specified when the attribute is applied . What are the other two parameters This special syntax allows you to set any public fields or properties of the DllImportAttribute object after the object is constructed . In this example, when the DllImportAttribute object is constructed and "Kernel32" is passed to the constructor, the object s public instance fields, CharSet and SetLastError, are set to CharSet.Auto and true, respectively . The parameters that set fields or properties are
18 Custom Attributes
called named parameters and are optional because the parameters don t have to be specified when you re applying an instance of the attribute . A little later on, I ll explain what causes an instance of the DllImportAttribute class to actually be constructed . Also note that it s possible to apply multiple attributes to a single target . For example, in this chapter s first program listing, the GetVersionEx method s ver parameter has both the In and Out attributes applied to it . When applying multiple attributes to a single target, be aware that the order of attributes has no significance . Also, in C#, each attribute can be enclosed in square brackets, or multiple attributes can be comma-separated within a single set of square brackets . If the attribute class s constructor takes no parameters, the parentheses are optional . Finally, as mentioned earlier, the Attribute suffix is also optional . The following lines behave identically and demonstrate all of the possible ways of applying multiple attributes:
[Serializable][Flags] [Serializable, Flags] [FlagsAttribute, SerializableAttribute] [FlagsAttribute()][Serializable()]
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