Specifying a Return Attribute in Visual C#

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Listing 1410 Specifying a Return Attribute
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[return Description( "Returns true if the object is in a valid state")] public bool IsValid() { // return true; }
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In addition to assembly and return, C# allows for explicit target identifications of module, class, and method, corresponding to attributes that decorate the module, class, and method class and method, however, are optional, as demonstrated earlier One of the conveniences of using attributes is that the language takes into consideration the attribute naming convention, which is to place Attribute at the end of the name However, in all the attribute uses in the preceding listings, no such suffix appears, despite the fact that each attribute used follows the naming convention This is because although the full name (DescriptionAttribute, AssemblyVersionAttribute, and so on) is allowed when applying an attribute, C# makes the suffix optional and generally, no such suffix appears when applying an attribute; it appears only when defining one or using the attribute inline (such as typeof(DescriptionAttribute) )
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Defining a custom attribute is relatively trivial Attributes are objects; therefore, to define an attribute, you need to define a class The characteristic that turns a general class into an attribute is that it derives from SystemAttribute Therefore, you can create a CommandLineSwitchRequiredAttribute class, as shown in Listing 1411
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Listing 1411 Defining a Custom Attribute
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public class CommandLineSwitchRequiredAttribute Attribute { }
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With that simple definition, you now can use the attribute as demonstrated in Listing 147 So far, no code responds to the attribute; therefore, the Out property that includes the attribute will have no effect on command-line parsing
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In addition to providing properties for reflecting on a type's members, Type includes methods to retrieve the Attributes decorating that type Similarly, all the reflection types (PropertyInfo and MethodInfo, for example) include members for retrieving a list of attributes that decorate a type Listing 1412 defines a method to return a list of required switches that are missing from the command line
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Listing 1412 Retrieving a Custom Attribute
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using System; using SystemCollectionsSpecialized; using SystemReflection; public class CommandLineSwitchRequiredAttribute Attribute { public static string[] GetMissingRequiredOptions( object commandLine) { StringCollection missingOptions = new StringCollection(); PropertyInfo[] properties = commandLineGetType()GetProperties(); foreach (PropertyInfo property in properties) { Attribute[] attributes = (Attribute[])propertyGetCustomAttributes( typeof(CommandLineSwitchRequiredAttribute), false); if ((attributesLength > 0) && (propertyGetValue(commandLine, null)== null)) { if (propertyGetValue(commandLine, null) == null) { missingOptionsAdd(propertyName); } } } string[] results = new string[missingOptionsCount]; missingOptionsCopyTo(results, 0); return results; } }
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The code that checks for an attribute is relatively simple Given a PropertyInfo object (obtained via reflection), you call GetCustomAttributes() and specify the attribute sought, followed by whether to check any overloaded methods (Alternatively, you can call the GetCustomAttributes() method without the attribute type to return all of the attributes) Although it is possible to place code for finding the CommandLineSwitchRequiredAttribute attribute within the CommandLineHandler 's code directly, it makes for better object encapsulation to place the code within the CommandLineSwitchRequiredAttribute class itself This is frequently the pattern for custom attributes What better location to place code for finding an attribute than in a static method on the attribute class
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Initializing an Attribute through a Constructor
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The call to GetCustomAttributes() returns an array of objects that will successfully cast to an Attribute array However, since the attribute in this example didn't have any instance members, the only metadata information that it provided in the returned attribute was whether it appeared Attributes can also encapsulate data, however Listing 1413 defines a CommandLineAliasAttribute attribute This is another custom attribute, and it provides alias command-line options For example, you can provide command-line support for /Help or / as an abbreviation Similarly, /S could provide
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an alias to /Subfolders that indicates that the command should traverse all the subdirectories To support this, you need to provide a constructor on the attribute Specifically, for the alias you need a constructor that takes a string argument (Similarly, if you want to allow multiple aliases, you need to define an attribute that has a params string array for a parameter)
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