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10.12 Destructors

Visual Studio .NET 2003

A destructor is a member that implements the actions required to destruct an instance of a class. A destructor is declared using a destructor-declaration:

destructor-declaration:
attributesopt   externopt   ~   identifier   (   )    destructor-body
destructor-body:
block
;

A destructor-declaration may include a set of attributes (Section 17).

The identifier of a destructor-declarator must name the class in which the destructor is declared. If any other name is specified, a compile-time error occurs.

When a destructor declaration includes an extern modifier, the destructor is said to be an external destructor. Because an external destructor declaration provides no actual implementation, its destructor-body consists of a semicolon. For all other destructors, the destructor-body consists of a block that specifies the statements to execute in order to destruct an instance of the class. A destructor-body corresponds exactly to the method-body of an instance method with a void return type (Section 10.5.8).

Destructors are not inherited. Thus, a class has no destructors other than the one that may be declared in that class.

Since a destructor is required to have no parameters, it cannot be overloaded, so a class can have, at most, one destructor.

Destructors are invoked automatically, and cannot be invoked explicitly. An instance becomes eligible for destruction when it is no longer possible for any code to use that instance. Execution of the destructor for the instance may occur at any time after the instance becomes eligible for destruction. When an instance is destructed, the destructors in that instance's inheritance chain are called, in order, from most derived to least derived. A destructor may be executed on any thread. For further discussion of the rules that govern when and how a destructor is executed, see Section 3.9.

The output of the example

using System;
class A
{
   ~A() {
      Console.WriteLine("A's destructor");
   }
}
class B: A
{
   ~B() {
      Console.WriteLine("B's destructor");
   }
}
class Test
{
    static void Main() {
      B b = new B();
      b = null;
      GC.Collect();
      GC.WaitForPendingFinalizers();
   }
}

is

B's destructor
A's destructor

since destructors in an inheritance chain are called in order, from most derived to least derived.

Destructors are implemented by overriding the virtual method Finalize on System.Object. C# programs are not permitted to override this method or call it (or overrides of it) directly. For instance, the program

class A 
{
   override protected void Finalize() {}   // error
   public void F() {
      this.Finalize();                     // error
   }
}

contains two errors.

The compiler behaves as if this method, and overrides of it, do not exist at all. Thus, this program:

class A 
{
   void Finalize() {}                     // permitted
}

is valid, and the method shown hides System.Object's Finalize method.

For a discussion of the behavior when an exception is thrown from a destructor, see Section 16.3.

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