Providing an Attribute Constructor in Visual C#

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Listing 1413 Providing an Attribute Constructor
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public class CommandLineSwitchAliasAttribute : Attribute { public CommandLineSwitchAliasAttribute(string alias) { Alias = alias; } public string Alias { get { return _Alias; } set { _Alias = value; } } private string _Alias; } class CommandLineInfo { [CommandLineSwitchAliasAttribute(" ")] public bool Help { get { return _Help; } set { _Help = value; } } private bool _Help; // }
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The only restriction on the constructor is that when applying an attribute to a construct, only literal values and types (like typeof(int)) are allowed as arguments This is to enable their serialization into the resulting CIL It is not possible, therefore, to call a static method when applying an attribute; in addition, providing a constructor that takes arguments of type SystemDateTime would be of little value, since there is no SystemDateTime literal Given the constructor call, the objects returned from PropertyInfoGetCustomAttributes() will be initialized with the specified constructor arguments, as demonstrated in Listing 1414
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Listing 1414 Retrieving a Specific Attribute and Checking Its Initialization
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PropertyInfo property = typeof(CommandLineInfo)GetProperty("Help"); CommandLineSwitchAliasAttribute attribute = (CommandLineSwitchAliasAttribute) propertyGetCustomAttributes( typeof(CommandLineSwitchAliasAttribute), false)[0]; if(attributeAlias == " ") { ConsoleWriteLine("Help( )"); };
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Furthermore, as Listing 1415 and Listing 1416 demonstrate, you can use similar code in a GetSwitches() method on CommandLineAliasAttribute that returns a dictionary collection of all the switches, including those from the property names, and associate each name with the corresponding attribute on the command-line object
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Listing 1415 Retrieving Custom Attribute Instances
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using System; using SystemReflection; using SystemCollectionsGeneric; public class CommandLineSwitchAliasAttribute : Attribute { public CommandLineSwitchAliasAttribute(string alias) { Alias = alias; } public string Alias { get { return _Alias; } set { _Alias = value; } } private string _Alias; public static Dictionary<string, PropertyInfo> GetSwitches( object commandLine) { PropertyInfo[] properties = null; Dictionary<string, PropertyInfo> options = new Dictionary<string, PropertyInfo>(); properties = commandLineGetType()GetProperties( BindingFlagsPublic | BindingFlagsNonPublic | BindingFlagsInstance); foreach (PropertyInfo property in properties) { optionsAdd(propertyNameToLower(), property); foreach (CommandLineSwitchAliasAttribute attribute in propertyGetCustomAttributes( typeof(CommandLineSwitchAliasAttribute),false)) {
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optionsAdd(attributeAliasToLower(), property); } } return options; } }
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Listing 1416 Updating CommandLineHandlerTryParse() to Handle Aliases
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using System; using SystemReflection; using SystemCollectionsGeneric; public class CommandLineHandler { // public static bool TryParse( string[] args, object commandLine, out string errorMessage) { bool success = false; errorMessage = null; Dictionary<string, PropertyInfo> options = CommandLineSwitchAliasAttributeGetSwitches( commandLine); foreach (string arg in args) { PropertyInfo property; string option; if (arg[0] == '/' || arg[0] =='-') { string[] optionParts = argSplit( new char[] { ':' }, 2); option = optionParts[0]Remove(0, 1)ToLower(); if (optionsTryGetValue(option,out property)) { success = SetOption( commandLine, property, optionParts, ref errorMessage); } else { success = false; errorMessage = stringFormat( "Option '{0}' is not supported", option); } } } return success; }
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private static bool SetOption( object commandLine, PropertyInfo property, string[] optionParts, ref string errorMessage) { bool success; if (propertyPropertyType == typeof(bool)) { // Last parameters for handling indexers propertySetValue( commandLine, true, null); success = true; } else { if ((optionPartsLength < 2) || optionParts[1] == "" || optionParts[1] == ":") { // No setting was provided for the switch success = false; errorMessage = stringFormat( "You must specify the value for the {0} option", propertyName); } else if ( propertyPropertyType == typeof(string)) { propertySetValue( commandLine, optionParts[1], null); success = true; } else if (propertyPropertyTypeIsEnum) { success = TryParseEnumSwitch( commandLine, optionParts, property, ref errorMessage); } else { success = false; errorMessage = stringFormat( "Data type '{0}' on {1} is not supported", propertyPropertyTypeToString(), commandLineGetType()ToString()); } } return success; } }
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Dictionary<TKey,TValue>
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Listing 1415 uses the generic collection Dictionary<string, PropertyInfo> Unfortunately, this is not available prior to 20, and instead, you have to use SystemCollectionsHashtable This is virtually a search-and-replace substitution, except in the call to tryGetValue() , which is not available on Hashtable In that case, you can retrieve the value using the index operator and then check for null, as follows:
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if((property = (PropertyInfo)options[option])!=null) { // }
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