ATTRIBUTES in C#.NET

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CHAPTER 21 ATTRIBUTES
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Applying an Attribute
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The purpose of an attribute is to tell the compiler to emit a certain set of metadata about a program construct, to the assembly. You do this by applying the attribute to the construct. You apply an attribute by placing an attribute section immediately before the construct. An attribute section consists of square brackets enclosing an attribute name and sometimes a parameter list. For example, the following code shows the headings of two classes. The first few lines of code show an attribute named Serializable applied to a class called MyClass. Notice that Serializable has no parameter list. The second class declaration has an attribute called MyAttribute, which has a parameter list with two string parameters. [ Serializable ] public class MyClass { ... [ MyAttribute("Simple class", "Version 3.57") ] public class MyOtherClass { ... // Attribute
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// Attribute with parameters
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Some important things to know about attributes are the following: Most attributes apply only to the construct immediately following the attribute section or sections. A construct with an attribute applied to it is sometimes said to be decorated, or adorned, with the attribute. Both terms are common.
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CHAPTER 21 ATTRIBUTES
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You can apply multiple attributes to a single construct. Multiple attributes can be listed in either of the following formats: Separate attribute sections stacked on top of each other A single attribute section, with the attributes separated by commas You can list the attributes in any order. For example, the following two sections of code show the two ways of applying multiple attributes. The sections of code are equivalent. [ Serializable ] [ MyAttribute("Simple class", "Version 3.57") ] [ MyAttribute("Simple class", "Version 3.57"), Serializable ] Attribute Attribute // Stacked
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// Commas
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Besides classes, you can also apply attributes to other program constructs such as fields and properties. The following declaration shows an attribute on a field, and multiple attributes on a method. [MyAttribute("Holds a value", "Version 3.2")] public int MyField; [Obsolete] [MyAttribute("Prints out a message.", "Version 3.6")] public void PrintOut() { ... // On a field
// On a method
You can also explicitly label attributes to apply to a particular construct. To use an explicit target, place the target type, followed by a colon, at the beginning of the attribute section. For example, the following code adorns the method with an attribute, and also applies an attribute to the return value.
CHAPTER 21 ATTRIBUTES
Explicit target [method: MyAttribute("Prints out a message.", "Version 3.6")] [return: MyAttribute("This value represents ...", "Version 2.3")] public long ReturnSetting() { ... The C# language defines 10 standardized attribute targets, which are listed in Table 21-1. Most of the target names are self-explanatory, but type covers classes, structs, delegates, enums, and interfaces. The typevar target name specifies type parameters to constructs that use generics. Table 21-1. Attribute Targets
event method property type assembly field param return typevar module
Global Attributes
You can also use an explicit target to set attributes at the assembly and module level, by using the assembly and module target names. Some important points about assembly-level attributes are the following: Assembly-level attributes must be placed outside any namespace scope and are usually placed in the AssemblyInfo.cs file. The AssembyInfo.cs file usually contains metadata about the company, product, and copyright information. The following are lines from an AssemblyInfo.cs file. [assembly: [assembly: [assembly: [assembly: [assembly: [assembly: [assembly: [assembly: AssemblyTitle("SuperWidget")] AssemblyDescription("Implements the SuperWidget product.")] AssemblyConfiguration("")] AssemblyCompany("McArthur Widgets, Inc.")] AssemblyProduct("Super Widget Deluxe")] AssemblyCopyright("Copyright McArthur Widgets 2006")] AssemblyTrademark("")] AssemblyCulture("")]
CHAPTER 21 ATTRIBUTES
Predefined Attributes
The .NET Framework predefines a number of attributes that are understood and interpreted by the compiler and the CLR. Table 21-2 lists some of these attributes. Table 21-2. Important Attributes Defined in .NET
CLSCompliant Declares that the publicly exposed members should be checked by the compiler for compliance with the CLS. Compliant assemblies can be used by any .NET-compliant language. Declares that the construct can be serialized. Declares that the construct cannot be serialized. Declares that the construct should not be used. The compiler also produces a compile time warning or error message, if the construct is used. Declares that the implementation is unmanaged code. Declares that the method should be exposed as part of an XML web service. Declares what types of program constructs the attribute can be applied to. This attribute is applied to attribute declarations.
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