Determining Whether a Class or Method Supports Generics in C#.NET

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Determining Whether a Class or Method Supports Generics
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In the SystemType class for CLI 20, a handful of methods were added to determine whether a given type supports generic parameters and arguments A generic argument is a type parameter supplied when a generic class is instantiated You can determine whether a class or method contains generic parameters that have not yet been set by querying the TypeContainsGenericParameters Boolean property, as demonstrated in Listing 175
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Listing 175: Reflection with Generics
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using System; public class Program { static void Main() { Type type; type = typeof(SystemNullable<>); ConsoleWriteLine(typeContainsGenericParameters); ConsoleWriteLine(typeIsGenericType); type = typeof(SystemNullable<DateTime>);
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17: Reflection and Attributes
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ConsoleWriteLine(!typeContainsGenericParameters); ConsoleWriteLine(typeIsGenericType); } }
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Output 173 shows the results of Listing 175
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OUTPUT 173:
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True True True True
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TypeIsGenericType is a Boolean property that evaluates whether a
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type is generic
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Obtaining Type Parameters for a Generic Class or Method
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You can obtain a list of generic arguments, or type parameters, from a generic class by calling the GetGenericArguments() method The result is an array of SystemType instances that corresponds to the order in which they are declared as type parameters to the generic class Listing 176 reflects into a generic type and obtains each type parameter Output 174 shows the results
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Listing 176: Using Reflection with Generic Types
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using System; using SystemCollectionsGeneric; public partial class Program { public static void Main() { Stack<int> s = new Stack<int>(); Type t = sGetType(); foreach(Type types in tGetGenericArguments()) { SystemConsoleWriteLine( "Type parameter: " + typesFullName); }
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Attributes
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// } }
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OUTPUT 174:
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Type parameter: SystemInt32
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Attributes
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Before delving into details on how to program attributes, you should consider a use case that demonstrates its utility In the CommandLineHandler example in Listing 173, you dynamically set a class s properties based on the command-line option matching the property name This approach is insufficient, however, when the command-line option is an invalid property name / , for example, cannot be supported Furthermore, this mechanism doesn t provide any way of identifying which options are required versus which are optional Instead of relying on an exact match between the option name and the property name, attributes provide a way of identifying additional metadata about the decorated construct in this case, the option that the attribute decorates With attributes, you can decorate a property as Required and provide a / option alias In other words, attributes are a means of associating additional data with a property (and other constructs) Attributes appear within square brackets preceding the construct they decorate For example, you can modify the CommandLineInfo class to include attributes, as shown in Listing 177
Listing 177: Decorating a Property with an Attribute
class CommandLineInfo { [CommandLineSwitchAlias (" ")] public bool Help { get { return _Help; } set { _Help = value; } } private bool _Help; [CommandLineSwitchRequired]
17: Reflection and Attributes
public string Out { get { return _Out; } set { _Out = value; } } private string _Out; public SystemDiagnosticsProcessPriorityClass Priority { get { return _Priority; } set { _Priority = value; } } private SystemDiagnosticsProcessPriorityClass _Priority = SystemDiagnosticsProcessPriorityClassNormal; }
In Listing 177, the Help and Out properties are decorated with attributes The purpose of these attributes is to allow an alias of / for /Help, and to indicate that /Out is a required parameter The idea is that from within the CommandLineHandlerTryParse() method, you enable support for option aliases and, assuming the parsing was successful, you can check that all the required switches were specified There are two ways to combine attributes on the same construct You can either separate the attributes with commas within the same square brackets, or place each attribute within its own square brackets, as shown in Listing 178
Listing 178: Decorating a Property with Multiple Attributes
[CommandLineSwitchRequired] [CommandLineSwitchAlias("FileName")] public string Out { get { return _Out; } set { _Out = value; } }
[CommandLineSwitchRequired, CommandLineSwitchAlias("FileName")] public string Out { get { return _Out; } set { _Out = value; } }
Attributes
In addition to decorating properties, developers can use attributes to decorate classes, interfaces, structs, enums, delegates, events, methods, constructors, fields, parameters, return values, assemblies, type parameters, and modules For the majority of these, applying an attribute involves the same square bracket syntax shown in Listing 178 However, this syntax doesn t work for return values, assemblies, and modules Assembly attributes are used to add additional metadata about the assembly Visual Studio s Project Wizard, for example, generates an AssemblyInfocs file that includes a host of attributes about the assembly Listing 179 is an example of such a file