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Attribute Parameters
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The foregoing examples also demonstrate how parameters are passed to attributes. The Serializable attribute takes no parameters. The CLSCompliant attribute takes one positional parameter, and the WebMethod attribute takes one positional and one named parameter. The way the compiler handles attribute parameters is one of the key behaviors that differentiate attributes from other classes.
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6. Advanced Language Features
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Positional parameters
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Positional parameters are the same as parameters in any normal instance constructor. They are expected in a predefined order, must be of the correct type, and must appear before any named parameters.
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Named parameters are specified by providing a comma-separated list of name/value pairs. Named parameters must be placed after all positional parameters; their order is not important. Providing named parameters that are not supported by an attribute or values that cannot be implicitly converted to the expected type will cause a compile-time error.
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Attribute Target Specification and Global Attributes
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The target of an attribute is normally apparent by its position preceding the target element. However, there are circumstances in which the target is a module or an assembly, or in which the target is ambiguous. Ambiguity occurs most frequently when specifying an attribute on a member that returns a value; both the method and the return value are valid attribute targets. In these situations, the programmer must clarify the target by prefixing the attribute name with the appropriate attribute target specifier from the following list: assembly, field, event, method, module, param, property, return, or type. The most common use of the attribute target specifier is to identify an assembly or module as the target of an attribute; these are called global attributes. For example:
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[assembly:CLSCompliant(true)] [module:DynamicLoad]
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For global attributes, these statements must occur after any top level using statements but before any namespace or type declarations.
It's common to put assembly attributes in a separate file. The wizard-generated projects in Microsoft Visual Studio .NET take this approach, using a file named AssemblyInfo.cs.
Custom Attributes
An attribute is a class that derives from System.Attribute. The following is an example of a simple attribute used to identify the developer who created a class or assembly:
6. Advanced Language Features using System ; // Needed for Attributes [AttributeUsage(AttributeTargets.Class | AttributeTargets.Assembly, AllowMultiple=true, Inherited = false)] public class CreatorAttribute: System.Attribute { private String name; private String email; // creator's name // creator's email
// Declare public constructor public CreatorAttribute (String creator) { name = creator; email = ""; } // Declare a property to get/set the creator's email address public string Email { get { return email; } set { email = value; } } // Declare a property to get the creator's name public string Name { get { return name;} } }
The following examples demonstrate the use of this attribute:
[assembly:Creator("Bob")] [assembly:Creator("Jim" , Email = "Jim@somecompany.com")] [Creator ("Judy", Email = "Judy@someothercompany.com")] public class MyClass { // class implementation }
The System.AttributeUsage Attribute
The AttributeUsage attribute defines how a new attribute can be used. AttributeUsage takes one positional and two named parameters, which are described in Table 6-2. The table also specifies the default value applied to an attribute declaration if the AttributeUsage attribute is omitted or a parameter isn't specified. A compiler error occurs if an attempt is made to use the AttributeUsage attribute on a class that is not derived from System.Attribute.
6. Advanced Language Features
Table 6-2. AttributeUsage Parameter Description
Type Description Default positional Identifies which program elements the attribute AttributeTargets.All is valid against. Valid values are any member of the System.AttributeTargets enumeration. Whether the attribute can be specified more false AllowMultiple named than once for a single element. named Whether the attribute is inherited by derived true Inherited classes or overridden members.
Attribute Declaration
Parameter validOn
The remainder of the attribute declaration is the same as for any other class; however, all attribute classes must be declared public.
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