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6 Type and Member Basics
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component versioning . Table 6-2 contains the C# keywords related to component versioning and indicates how each keyword affects a type or type member definition .
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TABLE 6-2
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C# Keywords and How They Affect Component Versioning
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Type
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Indicates that no instances of the type can be constructed (not allowed) (not allowed) Indicates that the type cannot be used as a base type
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C# Keyword
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abstract
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Method/Property/Event
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Indicates that the derived type must override and implement this member before instances of the derived type can be constructed Indicates that this member can be overridden by a derived type Indicates that the derived type is overriding the base type s member Indicates that the member cannot be overridden by a derived type . This keyword can be applied only to a method that is overriding a virtual method .
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Constant/Field
(not allowed)
virtual override sealed
(not allowed) (not allowed) (not allowed)
When applied to a nested type, method, property, event, constant, or field, indicates that the member has no relationship to a similar member that may exist in the base class
I will demonstrate the value and use of all these keywords in the upcoming section titled Dealing with Virtual Methods When Versioning Types . But before we get to a versioning scenario, let s focus on how the CLR actually calls virtual methods .
How the CLR Calls Virtual Methods, Properties, and Events
In this section, I will be focusing on methods, but this discussion is relevant to virtual properties and virtual events as well . Properties and events are actually implemented as methods; this will be shown in their corresponding chapters . Methods represent code that performs some operation on the type (static methods) or an instance of the type (nonstatic methods) . All methods have a name, a signature, and a return value (that may be void) . The CLR allows a type to define multiple methods with the same name as long as each method has a different set of parameters or a different return value . So it s possible to define two methods with the same name and same parameters as long as the methods have a different return type . However, except for IL assembly language, I m not aware of any language that takes advantage of this feature ; most languages (including C#) require that methods differ by parameters and ignore a method s return type when determining uniqueness . (C# actually relaxes this restriction when defining conversion operator methods; see 8 for details .)
Part II Designing Types
The Employee class shown below defines three different kinds of methods:
internal class Employee { // A nonvirtual instance method public Int32 GetYearsEmployed() { ... } // A virtual method (virtual implies instance) public virtual String GetProgressReport() { ... } // A static method public static Employee Lookup(String name) { ... } }
When the compiler compiles this code, the compiler emits three entries in the resulting assembly s method definition table . Each entry has flags set indicating if the method is instance, virtual, or static . When code is written to call any of these methods, the compiler emitting the calling code examines the method definition s flags to determine how to emit the proper IL code so that the call is made correctly . The CLR offers two IL instructions for calling a method:
The call IL instruction can be used to call static, instance, and virtual methods . When the call instruction is used to call a static method, you must specify the type that defines the method that the CLR should call . When the call instruction is used to call an instance or virtual method, you must specify a variable that refers to an object . The call instruction assumes that this variable is not null. In other words, the type of the variable itself indicates which type defines the method that the CLR should call . If the variable s type doesn t define the method, base types are checked for a matching method . The call instruction is frequently used to call a virtual method nonvirtually . The callvirt IL instruction can be used to call instance and virtual methods, not static methods . When the callvirt instruction is used to call an instance or virtual method, you must specify a variable that refers to an object . When the callvirt IL instruction is used to call a nonvirtual instance method, the type of the variable indicates which type defines the method that the CLR should call . When the callvirt IL instruction is used to call a virtual instance method, the CLR discovers the actual type of the object being used to make the call and then calls the method polymorphically . In order to determine the type, the variable being used to make the call must not be null. In other words, when compiling this call, the JIT compiler generates code that verifies that the variable s value is not null. If it is null, the callvirt instruction causes the CLR to throw a NullReferenceException . This additional check means that the callvirt IL instruction executes slightly more slowly than the call instruction . Note that this null check is performed even when the callvirt instruction is used to call a nonvirtual instance method .
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