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Chapter ‎18   Unsafe code

class C: B { public override void F() { Console.WriteLine("C.F"); } }

class A declares a virtual method, class B overrides this method with an abstract method, and class C overrides the abstract method to provide its own implementation.

10.5.7 External methods

When a method declaration includes an extern modifier, that method is said to be an external method. External methods are implemented externally, typically using a language other than C#. Because an external method declaration provides no actual implementation, the method-body of an external method simply consists of a semicolon.

The extern modifier is typically used in conjunction with a DllImport attribute (§‎17.5.1), allowing external methods to be implemented by DLLs (Dynamic Link Libraries). The execution environment may support other mechanisms whereby implementations of external methods can be provided.

When an external method includes a DllImport attribute, the method declaration must also include a static modifier. This example demonstrates the use of the extern modifier and the DllImport attribute:

using System.Text; using System.Security.Permissions; using System.Runtime.InteropServices;

class Path { [DllImport("kernel32", SetLastError=true)] static extern bool CreateDirectory(string name, SecurityAttribute sa);

[DllImport("kernel32", SetLastError=true)] static extern bool RemoveDirectory(string name);

[DllImport("kernel32", SetLastError=true)] static extern int GetCurrentDirectory(int bufSize, StringBuilder buf);

[DllImport("kernel32", SetLastError=true)] static extern bool SetCurrentDirectory(string name); }

10.5.8 Method body

The method-body of a method declaration consists of either a block or a semicolon.

Abstract and external method declarations do not provide a method implementation, so their method bodies simply consist of a semicolon. For any other method, the method body is a block (§‎8.2) that contains the statements to execute when that method is invoked.

When the return type of a method is void, return statements (§‎8.9.4) in that method’s body are not permitted to specify an expression. If execution of the method body of a void method completes normally (that is, control flows off the end of the method body), that method simply returns to its caller.

When the return type of a method is not void, each return statement in that method’s body must specify an expression of a type that is implicitly convertible to the return type. The endpoint of the method body of a value-returning method must not be reachable. In other words, in a value-returning method, control is not permitted to flow off the end of the method body.

In the example

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