The C# Language in Visual C#.NET

Draw QR-Code in Visual C#.NET The C# Language

if(b==null) ConsoleWriteLine("The cast in b = (B) a is NOT allowed"); else ConsoleWriteLine("The cast in b = (B) a is allowed"); } }

Here is the output, which is the same as before:

In this version, the as statement checks the validity of the cast and then, if valid, performs the cast, all in one statement

Although useful in their own ways, the as and is operators simply test the compatibility of two types Often, you will need to obtain information about a type To do this, C# supplies the typeof operator It retrieves a SystemType object for a given type Using this object, you can determine the type s characteristics The typeof operator has this general form: typeof(type) Here, type is the type being obtained The Type object returned encapsulates the information associated with type Once you have obtained a Type object for a given type, you can obtain information about it through the use of various properties, fields, and methods defined by Type Type is a large class with many members, and a discussion is deferred until the next section, where reflection is examined However, to briefly demonstrate Type, the following program uses three of its properties: FullName, IsClass, and IsAbstract To obtain the full name of the type, use FullName IsClass returns true if the type is a class IsAbstract returns true if a class is abstract

// Demonstrate typeof using System; using SystemIO; class UseTypeof { static void Main() { Type t = typeof(StreamReader); ConsoleWriteLine(tFullName); if(tIsClass) ConsoleWriteLine("Is a class"); if(tIsAbstract) ConsoleWriteLine("Is abstract"); else ConsoleWriteLine("Is concrete"); } }

This program outputs the following:

17:

This program obtains a Type object that describes StreamReader It then displays the full name, and determines if it is a class and whether it is abstract

Reflection is the feature that enables you to obtain information about a type The term reflection comes from the way the process works: A Type object mirrors the underlying type that it represents To obtain information, you ask the Type object questions, and it returns (reflects) the information associated with the type back to you Reflection is a powerful mechanism because it allows you to learn and use the capabilities of types that are known only at runtime Many of the classes that support reflection are part of the NET Reflection API, which is in the SystemReflection namespace Thus, you will normally include the following in programs that use reflection:

using SystemReflection;

SystemType is at the core of the reflection subsystem because it encapsulates a type It contains many properties and methods that you will use to obtain information about a type at runtime Type is derived from an abstract class called SystemReflectionMemberInfo MemberInfo defines the following read-only properties:

Property Type DeclaringType MemberTypes MemberType Description Obtains the type of the class or interface in which the member is declared Obtains the kind of the member This value indicates if the member is a field, method, property, event, or constructor, among others Obtains a value associated with a specific metadata Obtains a Module object that represents the module (an executable file) in which the reflected type resides The name of the member The type of the object being reflected

Notice that the return type of MemberType is MemberTypes MemberTypes is an enumeration that defines values that indicate the various member types Among others, these include MemberTypesConstructor MemberTypesMethod MemberTypesField MemberTypesEvent MemberTypesProperty Thus, the type of a member can be determined by checking MemberType For example, if MemberType equals MemberTypesMethod, then that member is a method

Part I:

MemberInfo includes two abstract methods: GetCustomAttributes( ) and IsDefined( ) These both relate to attributes The first obtains a list of the custom attributes associated with the invoking object The second determines if an attribute is defined for the invoking object The NET Framework Version 40 adds a method called GetCustomAttributesData( ), which returns information about custom attributes (Attributes are described later in this chapter) To the methods and properties defined by MemberInfo, Type adds a great many of its own For example, here are several commonly used methods defined by Type:

Method ConstructorInfo[ ] GetConstructors( ) EventInfo[ ] GetEvents( ) FieldInfo[ ] GetFields( ) Type[ ] GetGenericArguments( ) Purpose Obtains a list of the constructors for the specified type Obtains a list of events for the specified type Obtains a list of the fields for the specified type Obtains a list of the type arguments bound to a closed constructed generic type or the type parameters if the specified type is a generic type definition For an open constructed type, the list may contain both type arguments and type parameters (See 18 for a discussion of generics) Obtains a list of the members for the specified type Obtains a list of methods for the specified type Obtains a list of properties for the specified type

MemberInfo[ ] GetMembers( ) MethodInfo[ ] GetMethods( ) PropertyInfo[ ] GetProperties( )

Here are several commonly used, read-only properties defined by Type:

Property Assembly Assembly TypeAttributes Attributes Type BaseType string FullName bool IsAbstract bool isArray bool IsClass bool IsEnum bool IsGenericParameter bool IsGenericType string Namespace Purpose Obtains the assembly for the specified type Obtains the attributes for the specified type Obtains the immediate base type for the specified type Obtains the complete name of the specified type Is true if the specified type is abstract Is true if the specified type is an array Is true if the specified type is a class Is true if the specified type is an enumeration Is true if the specified type is a generic type parameter (See 18 for a discussion of generics) Is true if the specified type is a generic type (See 18 for a discussion of generics) Obtains the namespace of the specified type

17: