C# Language Specification - Willy .Net

Chapter 17 ClassesSystem.Int32 System.String System.DoubleSystem.Object[]System.Object[]System.Int32 System.String System.DoubleIn the first and last invocations of F, the normal form of F is applicable because an implicit conversion existsfrom the argument type to the parameter type (both are of type object[]). Thus, overload resolution selects thenormal form of F, and the argument is passed as a regular value parameter. In the second and third invocations,the normal form of F is not applicable because no implicit conversion exists from the argument type to theparameter type (type object cannot be implicitly converted to type object[]). However, the expanded form ofF is applicable, so it is selected by overload resolution. As a result, a one-element object[] is created by theinvocation, and the single element of the array is initialized with the given argument value (which itself is areference to an object[]). end example]17.5.2 Static and instance methodsWhen a method declaration includes a static modifier, that method is said to be a static method. When nostatic modifier is present, the method is said to be an instance method.A static method does not operate on a specific instance, and it is a compile-time error to refer to this in a staticmethod.An instance method operates on a given instance of a class, and that instance can be accessed as this (§14.5.7).When a method is referenced in a member-access (§14.5.4) of the form E.M, if M is a static method, E mustdenote a type that has a method M, and if M is an instance method, E must denote an instance of a type that has amethod M.The differences between static and instance members are discussed further in §17.2.5.17.5.3 Virtual methodsWhen an instance method declaration includes a virtual modifier, that method is said to be a virtual method.When no virtual modifier is present, the method is said to be a non-virtual method.The implementation of a non-virtual method is invariant: The implementation is the same whether the method isinvoked on an instance of the class in which it is declared or an instance of a derived class. In contrast, theimplementation of a virtual method can be superseded by derived classes. The process of superseding theimplementation of an inherited virtual method is known as overriding that method (§17.5.4).In a virtual method invocation, the run-time type of the instance for which that invocation takes place determinesthe actual method implementation to invoke. In a non-virtual method invocation, the compile-time type of theinstance is the determining factor. In precise terms, when a method named N is invoked with an argument list Aon an instance with a compile-time type C and a run-time type R (where R is either C or a class derived from C),the invocation is processed as follows:• First, overload resolution is applied to C, N, and A, to select a specific method M from the set of methodsdeclared in and inherited by C. This is described in §14.5.5.1.• Then, if M is a non-virtual method, M is invoked.• Otherwise, M is a virtual method, and the most derived implementation of M with respect to R is invoked.For every virtual method declared in or inherited by a class, there exists a most derived implementation of themethod with respect to that class. The most derived implementation of a virtual method M with respect to a class Ris determined as follows:• If R contains the introducing virtual declaration of M, then this is the most derived implementation of M.• Otherwise, if R contains an override of M, then this is the most derived implementation of M.• Otherwise, the most derived implementation of M is the same as that of the direct base class of R.[Example: The following example illustrates the differences between virtual and non-virtual methods:233