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The documentation for the first release of .NET states that all user-defined attributes will implicitly have the Attribute string appended to their class name if not done so manually. However, our experience shows that this is not the case. Custom attributes should always be given a name ending in the word Attribute to maintain consistency, enabling the C# compiler to support the dual-name behavior described earlier.
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A custom attribute must have at least one public constructor. The constructor parameters become the attribute's positional parameters. As with any other class, more than one constructor can be specified; providing overloaded constructors gives the users of the attribute the option of using different sets of positional parameters when specifying the attribute.
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Named Parameters
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Any public read/write fields and properties contained in an attribute are automatically exposed as named parameters. In the foregoing example, the Email property is exposed as a named parameter, but the Name property is read-only, meaning that it cannot be used as a named parameter.
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While the majority of attributes are used for specifying run-time-accessible metadata, a number of them are evaluated at compile time. The AttributeUsage attribute discussed earlier is evaluated at compile time. Two other important compile-time attributes are discussed in the following sections.
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System.Diagnostics.ConditionalAttribute
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The Conditional attribute marks a method whose execution depends on the definition of a preprocessor symbol. (See the "#define and #undef" section within the "Preprocessor Directives" section later in this chapter.) If a method is marked with the Conditional attribute, any attempts to execute the method will be removed by the compiler if the specified preprocessor symbol is not defined at the calling point. The Conditional attribute is a more elegant but less flexible alternative to using #ifdef preprocessor directives. It centralizes the conditional logic rather than having many #ifdef statements throughout the code. To be a valid target of the Conditional attribute, a method must return void; otherwise, it won't be possible to remove references without breaking the calling code. For example, to define a method whose execution is conditional on the declaration of the symbol DEBUG, use the following syntax:
//The following method will execute if the DEBUG symbol is defined [Conditional("DEBUG")] public void SomeMethod() { // implementation here}
Multiple instances of the Conditional attribute can be specified for a method producing a logical OR behavior. For example:
// The following method will execute if WIN95 OR WIN2000 // symbols are defined [Conditional("WIN95"), Conditional("WIN2000")] public void SomeMethod() { // implementation here}
Achieving logical AND behavior is messy, involving the unpleasant use of intermediate Conditional methods. For example:
// The following method will execute if the DEBUG symbol is defined [Conditional("DEBUG")] public void SomeMethod() { SomeOtherMethod() } // The following method will only execute if the WIN95 symbol is defined. // When called from SomeMethod above, this has the effect of only executing // SomeOtherMethod if both the DEBUG AND WIN95 symbols are defined [Conditional("WIN95")] private void SomeOtherMethod() { // implementation here}
System.ObsoleteAttribute
The Obsolete attribute marks a program element as obsolete and forces the compiler to raise a warning or an error when other code attempts to use the element. This is similar to marking an element as deprecated in Java but places more control in the hands of the code's author.
6. Advanced Language Features
The two positional parameters of the Obsolete attribute specify the message to be displayed by the compiler and whether the compiler raises a warning or an error. For example:
// Specifying true as the second argument means that code referencing // the following method will generate a compiler error about the // method being deprecated. [Obsolete("This method has been deprecated.", true)] public string MyMethod() { // implementation here} // Specifying false as the second argument means that code referencing // the following method will generate a compiler warning that the // method is in the process of being phased out. [Obsolete("This method is being phased out, use SomeMethod().", false)] public string MyOtherMethod() { // implementation here}
The Obsolete attribute is not limited to specification on methods. Valid targets are class, struct, enum, constructor, method, property, field, event, interface, and delegate.
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