GC.KeepAlive(Object) Método

Referência

Definição

Namespace:: System

Assembly:: System.Runtime.dll

Assembly:: mscorlib.dll

Assembly:: netstandard.dll

Importante

Algumas informações se referem a produtos de pré-lançamento que podem ser substancialmente modificados antes do lançamento. A Microsoft não oferece garantias, expressas ou implícitas, das informações aqui fornecidas.

Referencia o objeto especificado, o que o torna não qualificado para a coleta de lixo, do início da rotina atual ao ponto em que esse método é chamado.

public:
 static void KeepAlive(System::Object ^ obj);

public static void KeepAlive (object obj);

public static void KeepAlive (object? obj);

static member KeepAlive : obj -> unit

Public Shared Sub KeepAlive (obj As Object)

Parâmetros

obj: Object

O objeto a ser referenciado.

Exemplos

O exemplo de código a seguir cria um objeto no início de seu Main método e não se refere ao objeto novamente até o final, quando o KeepAlive método é chamado. O objeto persiste durante a duração de 30 segundos do Main método, apesar das chamadas e Collect WaitForPendingFinalizers métodos.

using namespace System;
using namespace System::Threading;
using namespace System::Runtime::InteropServices;

// A simple class that exposes two static Win32 functions.
// One is a delegate type and the other is an enumerated type.
public ref class MyWin32
{
public:

   // An enumerated type for the control messages sent to the handler routine.
   enum class CtrlTypes
   {
      CTRL_C_EVENT = 0,
      CTRL_BREAK_EVENT, CTRL_CLOSE_EVENT, CTRL_LOGOFF_EVENT = 5,
      CTRL_SHUTDOWN_EVENT
   };

   delegate Boolean HandlerRoutine(    // A delegate type to be used as the Handler Routine for SetConsoleCtrlHandler.
   CtrlTypes CtrlType );

   // Declare the SetConsoleCtrlHandler function as external and receiving a delegate.

   [DllImport("Kernel32")]
   static Boolean SetConsoleCtrlHandler( HandlerRoutine^ Handler, Boolean Add );
};

public ref class MyApp
{
private:

   // A private static handler function in the MyApp class.
   static Boolean Handler( MyWin32::CtrlTypes CtrlType )
   {
      String^ message = "This message should never be seen!";
      
      // A switch to handle the event type.
      switch ( CtrlType )
      {
         case MyWin32::CtrlTypes::CTRL_C_EVENT:
            message = "A CTRL_C_EVENT was raised by the user.";
            break;

         case MyWin32::CtrlTypes::CTRL_BREAK_EVENT:
            message = "A CTRL_BREAK_EVENT was raised by the user.";
            break;

         case MyWin32::CtrlTypes::CTRL_CLOSE_EVENT:
            message = "A CTRL_CLOSE_EVENT was raised by the user.";
            break;

         case MyWin32::CtrlTypes::CTRL_LOGOFF_EVENT:
            message = "A CTRL_LOGOFF_EVENT was raised by the user.";
            break;

         case MyWin32::CtrlTypes::CTRL_SHUTDOWN_EVENT:
            message = "A CTRL_SHUTDOWN_EVENT was raised by the user.";
            break;
      }
      
      // Use interop to display a message for the type of event.
      Console::WriteLine( message );
      return true;
   }


public:
   static void Test()
   {
      
      // Use interop to set a console control handler.
      MyWin32::HandlerRoutine^ hr = gcnew MyWin32::HandlerRoutine( Handler );
      MyWin32::SetConsoleCtrlHandler( hr, true );
      
      // Give the user some time to raise a few events.
      Console::WriteLine( "Waiting 30 seconds for console ctrl events..." );
      
      // The Object hr is not referred to again.
      // The garbage collector can detect that the object has no
      // more managed references and might clean it up here while
      // the unmanaged SetConsoleCtrlHandler method is still using it.
      // Force a garbage collection to demonstrate how the hr
      // object will be handled.
      GC::Collect();
      GC::WaitForPendingFinalizers();
      GC::Collect();
      Thread::Sleep( 30000 );
      
      // Display a message to the console when the unmanaged method
      // has finished its work.
      Console::WriteLine( "Finished!" );
      
      // Call GC::KeepAlive(hr) at this point to maintain a reference to hr.
      // This will prevent the garbage collector from collecting the
      // object during the execution of the SetConsoleCtrlHandler method.
      GC::KeepAlive( hr );
   }

};

int main()
{
   MyApp::Test();
}

using System;
using System.Threading;
using System.Runtime.InteropServices;

// A simple class that exposes two static Win32 functions.
// One is a delegate type and the other is an enumerated type.
public class MyWin32
{
    // Declare the SetConsoleCtrlHandler function
    // as external and receiving a delegate.
    [DllImport("Kernel32")]
    public static extern Boolean SetConsoleCtrlHandler(HandlerRoutine Handler,
        Boolean Add);

    // A delegate type to be used as the handler routine
    // for SetConsoleCtrlHandler.
    public delegate Boolean HandlerRoutine(CtrlTypes CtrlType);

    // An enumerated type for the control messages
    // sent to the handler routine.
    public enum CtrlTypes
    {
        CTRL_C_EVENT = 0,
        CTRL_BREAK_EVENT,
        CTRL_CLOSE_EVENT,
        CTRL_LOGOFF_EVENT = 5,
        CTRL_SHUTDOWN_EVENT
    }
}

public class MyApp
{
    // A private static handler function in the MyApp class.
    static Boolean Handler(MyWin32.CtrlTypes CtrlType)
    {
        String message = "This message should never be seen!";

        // A switch to handle the event type.
        switch(CtrlType)
        {
            case MyWin32.CtrlTypes.CTRL_C_EVENT:
                message = "A CTRL_C_EVENT was raised by the user.";
                break;
            case MyWin32.CtrlTypes.CTRL_BREAK_EVENT:
                message = "A CTRL_BREAK_EVENT was raised by the user.";
                break;
            case MyWin32.CtrlTypes.CTRL_CLOSE_EVENT:
                message = "A CTRL_CLOSE_EVENT was raised by the user.";
                break;
            case MyWin32.CtrlTypes.CTRL_LOGOFF_EVENT:
                message = "A CTRL_LOGOFF_EVENT was raised by the user.";
                break;
            case MyWin32.CtrlTypes.CTRL_SHUTDOWN_EVENT:
                message = "A CTRL_SHUTDOWN_EVENT was raised by the user.";
                break;
        }

        // Use interop to display a message for the type of event.
        Console.WriteLine(message);

        return true;
    }

    public static void Main()
    {

        // Use interop to set a console control handler.
        MyWin32.HandlerRoutine hr = new MyWin32.HandlerRoutine(Handler);
        MyWin32.SetConsoleCtrlHandler(hr, true);

        // Give the user some time to raise a few events.
        Console.WriteLine("Waiting 30 seconds for console ctrl events...");

        // The object hr is not referred to again.
        // The garbage collector can detect that the object has no
        // more managed references and might clean it up here while
        // the unmanaged SetConsoleCtrlHandler method is still using it.
        
        // Force a garbage collection to demonstrate how the hr
        // object will be handled.
        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();
        
        Thread.Sleep(30000);

        // Display a message to the console when the unmanaged method
        // has finished its work.
        Console.WriteLine("Finished!");

        // Call GC.KeepAlive(hr) at this point to maintain a reference to hr.
        // This will prevent the garbage collector from collecting the
        // object during the execution of the SetConsoleCtrlHandler method.
        GC.KeepAlive(hr);
        Console.Read();
    }
}

open System
open System.Threading
open System.Runtime.InteropServices

// A simple module that exposes two static Win32 functions.
// One is a delegate type and the other is an enumerated type.
module MyWin32 =
    // An enumerated type for the control messages
    // sent to the handler routine.
    type CtrlTypes =
        | CTRL_C_EVENT = 0
        | CTRL_BREAK_EVENT = 1
        | CTRL_CLOSE_EVENT = 2
        | CTRL_LOGOFF_EVENT = 5
        | CTRL_SHUTDOWN_EVENT = 6

    // A delegate type to be used as the handler routine for SetConsoleCtrlHandler.
    type HandlerRoutine = delegate of CtrlTypes -> bool

    // Declare the SetConsoleCtrlHandler function
    // as external and receiving a delegate.
    [<DllImport "Kernel32">]
    extern bool SetConsoleCtrlHandler(HandlerRoutine Handler, bool Add)

// A private static handler function in the MyApp class.
let handler (ctrlType: MyWin32.CtrlTypes) =
    let message =
        // Pattern match to handle the event type.
        match ctrlType with
        | MyWin32.CtrlTypes.CTRL_C_EVENT ->
            "A CTRL_C_EVENT was raised by the user."
        | MyWin32.CtrlTypes.CTRL_BREAK_EVENT ->
            "A CTRL_BREAK_EVENT was raised by the user."
        | MyWin32.CtrlTypes.CTRL_CLOSE_EVENT ->
            "A CTRL_CLOSE_EVENT was raised by the user."
        | MyWin32.CtrlTypes.CTRL_LOGOFF_EVENT ->
            "A CTRL_LOGOFF_EVENT was raised by the user."
        | MyWin32.CtrlTypes.CTRL_SHUTDOWN_EVENT ->
            "A CTRL_SHUTDOWN_EVENT was raised by the user."
        | _ -> "This message should never be seen!"

    // Use interop to display a message for the type of event.
    printfn $"{message}"
    true

// Use interop to set a console control handler.
let hr = MyWin32.HandlerRoutine handler
MyWin32.SetConsoleCtrlHandler(hr, true) |> ignore

// Give the user some time to raise a few events.
printfn "Waiting 30 seconds for console ctrl events..."

// The object hr is not referred to again.
// The garbage collector can detect that the object has no
// more managed references and might clean it up here while
// the unmanaged SetConsoleCtrlHandler method is still using it.

// Force a garbage collection to demonstrate how the hr
// object will be handled.
GC.Collect()
GC.WaitForPendingFinalizers()
GC.Collect()

Thread.Sleep 30000

// Display a message to the console when the unmanaged method
// has finished its work.
printfn "Finished!"

// Call GC.KeepAlive(hr) at this point to maintain a reference to hr.
// This will prevent the garbage collector from collecting the
// object during the execution of the SetConsoleCtrlHandler method.
GC.KeepAlive hr
Console.Read() |> ignore

Imports System.Threading
Imports System.Runtime.InteropServices

' A simple class that exposes two static Win32 functions.
' One is a delegate type and the other is an enumerated type.

Public Class MyWin32

   ' Declare the SetConsoleCtrlHandler function as external 
   ' and receiving a delegate.   
   <DllImport("Kernel32")> _
   Public Shared Function SetConsoleCtrlHandler(ByVal Handler As HandlerRoutine, _
         ByVal Add As Boolean) As Boolean
   End Function


   ' A delegate type to be used as the handler routine 
   ' for SetConsoleCtrlHandler.
   Delegate Function HandlerRoutine(ByVal CtrlType As CtrlTypes) As [Boolean]

   ' An enumerated type for the control messages 
   ' sent to the handler routine.

   Public Enum CtrlTypes
      CTRL_C_EVENT = 0
      CTRL_BREAK_EVENT
      CTRL_CLOSE_EVENT
      CTRL_LOGOFF_EVENT = 5
      CTRL_SHUTDOWN_EVENT
   End Enum
End Class

Public Class MyApp

   ' A private static handler function in the MyApp class.
   Shared Function Handler(ByVal CtrlType As MyWin32.CtrlTypes) As [Boolean]
      Dim message As [String] = "This message should never be seen!"

      ' A select case to handle the event type.
      Select Case CtrlType
         Case MyWin32.CtrlTypes.CTRL_C_EVENT
            message = "A CTRL_C_EVENT was raised by the user."
         Case MyWin32.CtrlTypes.CTRL_BREAK_EVENT
            message = "A CTRL_BREAK_EVENT was raised by the user."
         Case MyWin32.CtrlTypes.CTRL_CLOSE_EVENT
            message = "A CTRL_CLOSE_EVENT was raised by the user."
         Case MyWin32.CtrlTypes.CTRL_LOGOFF_EVENT
            message = "A CTRL_LOGOFF_EVENT was raised by the user."
         Case MyWin32.CtrlTypes.CTRL_SHUTDOWN_EVENT
            message = "A CTRL_SHUTDOWN_EVENT was raised by the user."
      End Select

      ' Use interop to display a message for the type of event.
      Console.WriteLine(message)

      Return True
   End Function


   Public Shared Sub Main()
      ' Use interop to set a console control handler.
      Dim hr As New MyWin32.HandlerRoutine(AddressOf Handler)
      MyWin32.SetConsoleCtrlHandler(hr, True)

      ' Give the user some time to raise a few events.
      Console.WriteLine("Waiting 30 seconds for console ctrl events...")

      ' The object hr is not referred to again.
      ' The garbage collector can detect that the object has no
      ' more managed references and might clean it up here while
      ' the unmanaged SetConsoleCtrlHandler method is still using it.      
      ' Force a garbage collection to demonstrate how the hr
      ' object will be handled.
      GC.Collect()
      GC.WaitForPendingFinalizers()
      GC.Collect()

      Thread.Sleep(30000)

      ' Display a message to the console when the unmanaged method
      ' has finished its work.
      Console.WriteLine("Finished!")

      ' Call GC.KeepAlive(hr) at this point to maintain a reference to hr. 
      ' This will prevent the garbage collector from collecting the 
      ' object during the execution of the SetConsoleCtrlHandler method.
      GC.KeepAlive(hr)
      Console.Read()
   End Sub
End Class

Comentários

A finalidade do KeepAlive método é garantir a existência de uma referência a um objeto que corre o risco de ser recuperado prematuramente pelo coletor de lixo. Um cenário comum em que isso pode acontecer é quando não há referências ao objeto em código ou dados gerenciados, mas o objeto ainda está em uso em código não gerenciado, como APIs do Windows, DLLs não gerenciadas ou métodos usando COM.

Esse método faz referência ao obj parâmetro, tornando esse objeto inelegível para coleta de lixo desde o início da rotina até o ponto, na ordem de execução, em que esse método é chamado. Codifique este método no final, não no início, do intervalo de instruções em obj que deve estar disponível.

O KeepAlive método não executa nenhuma operação e não produz nenhum efeito colateral além de estender o tempo de vida do objeto passado como um parâmetro.

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