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Criar threads e passar dados no momento da inicialização

Quando um processo do sistema operacional é criado, o sistema operacional injeta um thread para executar o código nesse processo, incluindo qualquer domínio de aplicativo original. Desse ponto em diante, os domínios de aplicativo podem ser criados e destruídos sem que os threads do sistema operacional sejam necessariamente criados ou destruídos. Se o código que está sendo executado for um código gerenciado, um Thread objeto para o thread em execução no domínio do aplicativo atual poderá ser obtido recuperando a propriedade estática CurrentThread do tipo Thread. Este tópico descreve a criação de threads e discute alternativas para passar dados para o procedimento de thread.

Criando um thread

A criação de um novo Thread objeto cria um novo thread gerenciado. A classe Thread tem construtores que aceitam um delegado ThreadStart ou um delegado ParameterizedThreadStart; o delegado envolve o método que é invocado pela nova thread quando o método Start é chamado. Chamar Start mais de uma vez faz com que um ThreadStateException seja lançado.

O Start método retorna imediatamente, muitas vezes antes que o novo thread seja realmente iniciado. Você pode usar as propriedades ThreadState e IsAlive para determinar o estado da thread a qualquer momento, mas essas propriedades nunca devem ser usadas para sincronizar as atividades das threads.

Observação

Depois que um thread é iniciado, não é necessário manter uma referência ao Thread objeto. O thread continua a ser executado até que o procedimento de thread termine.

O exemplo de código a seguir cria dois novos threads para chamar a instância e métodos estáticos em outro objeto.

using System;
using System.Threading;

public class ServerClass
{
    // The method that will be called when the thread is started.
    public void InstanceMethod()
    {
        Console.WriteLine(
            "ServerClass.InstanceMethod is running on another thread.");

        // Pause for a moment to provide a delay to make
        // threads more apparent.
        Thread.Sleep(3000);
        Console.WriteLine(
            "The instance method called by the worker thread has ended.");
    }

    public static void StaticMethod()
    {
        Console.WriteLine(
            "ServerClass.StaticMethod is running on another thread.");

        // Pause for a moment to provide a delay to make
        // threads more apparent.
        Thread.Sleep(5000);
        Console.WriteLine(
            "The static method called by the worker thread has ended.");
    }
}

public class Simple
{
    public static void Main()
    {
        ServerClass serverObject = new ServerClass();

        // Create the thread object, passing in the
        // serverObject.InstanceMethod method using a
        // ThreadStart delegate.
        Thread InstanceCaller = new(new ThreadStart(serverObject.InstanceMethod));

        // Start the thread.
        InstanceCaller.Start();

        Console.WriteLine("The Main() thread calls this after "
            + "starting the new InstanceCaller thread.");

        // Create the thread object, passing in the
        // serverObject.StaticMethod method using a
        // ThreadStart delegate.
        Thread StaticCaller = new(new ThreadStart(ServerClass.StaticMethod));

        // Start the thread.
        StaticCaller.Start();

        Console.WriteLine("The Main() thread calls this after "
            + "starting the new StaticCaller thread.");
    }
}
// The example displays the output like the following:
//    The Main() thread calls this after starting the new InstanceCaller thread.
//    The Main() thread calls this after starting the new StaticCaller thread.
//    ServerClass.StaticMethod is running on another thread.
//    ServerClass.InstanceMethod is running on another thread.
//    The instance method called by the worker thread has ended.
//    The static method called by the worker thread has ended.

Imports System.Threading

Public class ServerClass
    ' The method that will be called when the thread is started.
    Public Sub InstanceMethod()
        Console.WriteLine(
            "ServerClass.InstanceMethod is running on another thread.")

        ' Pause for a moment to provide a delay to make
        ' threads more apparent.
        Thread.Sleep(3000)
        Console.WriteLine(
            "The instance method called by the worker thread has ended.")
    End Sub

    Public Shared Sub SharedMethod()
        Console.WriteLine(
            "ServerClass.SharedMethod is running on another thread.")

        ' Pause for a moment to provide a delay to make
        ' threads more apparent.
        Thread.Sleep(5000)
        Console.WriteLine(
            "The Shared method called by the worker thread has ended.")
    End Sub
End Class

Public class Simple
    Public Shared Sub Main()
        Dim serverObject As New ServerClass()

        ' Create the thread object, passing in the
        ' serverObject.InstanceMethod method using a
        ' ThreadStart delegate.
        Dim InstanceCaller As New Thread(AddressOf serverObject.InstanceMethod)

        ' Start the thread.
        InstanceCaller.Start()

        Console.WriteLine("The Main() thread calls this after " _
            + "starting the new InstanceCaller thread.")

        ' Create the thread object, passing in the
        ' serverObject.SharedMethod method using a
        ' ThreadStart delegate.
        Dim SharedCaller As New Thread( _
            New ThreadStart(AddressOf ServerClass.SharedMethod))

        ' Start the thread.
        SharedCaller.Start()

        Console.WriteLine("The Main() thread calls this after " _
            + "starting the new SharedCaller thread.")
    End Sub
End Class
' The example displays output like the following:
'    The Main() thread calls this after starting the new InstanceCaller thread.
'    The Main() thread calls this after starting the new StaticCaller thread.
'    ServerClass.StaticMethod is running on another thread.
'    ServerClass.InstanceMethod is running on another thread.
'    The instance method called by the worker thread has ended.
'    The static method called by the worker thread has ended.

Passando dados para threads

O ParameterizedThreadStart delegado fornece uma maneira fácil de passar um objeto que contém dados para um thread quando você chama Thread.Start(Object). Consulte ParameterizedThreadStart para um exemplo de código.

Usar o ParameterizedThreadStart delegado não é uma maneira segura de passar dados, pois o Thread.Start(Object) método aceita qualquer objeto. Uma alternativa é encapsular o método de thread e os dados em uma classe auxiliar e usar a delegação do ThreadStart para executar o método de thread. O exemplo a seguir demonstra essa técnica:

using System;
using System.Threading;

// The ThreadWithState class contains the information needed for
// a task, and the method that executes the task.
//
public class ThreadWithState
{
    // State information used in the task.
    private string _boilerplate;
    private int _numberValue;

    // The constructor obtains the state information.
    public ThreadWithState(string text, int number)
    {
        _boilerplate = text;
        _numberValue = number;
    }

    // The thread procedure performs the task, such as formatting
    // and printing a document.
    public void ThreadProc()
    {
        Console.WriteLine(_boilerplate, _numberValue);
    }
}

// Entry point for the example.
public class Example
{
    public static void Main()
    {
        // Supply the state information required by the task.
        ThreadWithState tws = new("This report displays the number {0}.", 42);

        // Create a thread to execute the task, and then
        // start the thread.
        Thread t = new(new ThreadStart(tws.ThreadProc));
        t.Start();
        Console.WriteLine("Main thread does some work, then waits.");
        t.Join();
        Console.WriteLine(
            "Independent task has completed; main thread ends.");
    }
}

// The example displays the following output:
//       Main thread does some work, then waits.
//       This report displays the number 42.
//       Independent task has completed; main thread ends.

Imports System.Threading

' The ThreadWithState class contains the information needed for
' a task, and the method that executes the task.
Public Class ThreadWithState
    ' State information used in the task.
    Private boilerplate As String
    Private numberValue As Integer

    ' The constructor obtains the state information.
    Public Sub New(text As String, number As Integer)
        boilerplate = text
        numberValue = number
    End Sub

    ' The thread procedure performs the task, such as formatting
    ' and printing a document.
    Public Sub ThreadProc()
        Console.WriteLine(boilerplate, numberValue)
    End Sub
End Class

' Entry point for the example.
'
Public Class Example
    Public Shared Sub Main()
        ' Supply the state information required by the task.
        Dim tws As New ThreadWithState( _
            "This report displays the number {0}.", 42)

        ' Create a thread to execute the task, and then
        ' start the thread.
        Dim t As New Thread(New ThreadStart(AddressOf tws.ThreadProc))
        t.Start()
        Console.WriteLine("Main thread does some work, then waits.")
        t.Join()
        Console.WriteLine( _
            "Independent task has completed main thread ends.")
    End Sub
End Class
' The example displays the following output:
'       Main thread does some work, then waits.
'       This report displays the number 42.
'       Independent task has completed; main thread ends.

Nem o delegado ThreadStart nem o delegado ParameterizedThreadStart tem um valor de retorno, porque não há lugar para retornar os dados de uma chamada assíncrona. Para recuperar os resultados de um método de execução paralela, você pode usar uma função de retorno, conforme mostrado na próxima seção.

Recuperando dados de threads com métodos de retorno de chamada

O exemplo a seguir demonstra um método de retorno de chamada que recupera dados de um thread. O construtor da classe que contém os dados e o método thread também aceita um delegado que representa o método de retorno de chamada; antes que o método thread termine, ele invoca o delegado de retorno de chamada.

using System;
using System.Threading;

// The ThreadWithState2 class contains the information needed for
// a task, the method that executes the task, and a delegate
// to call when the task is complete.
public class ThreadWithState2
{
    // State information used in the task.
    private string _boilerplate;
    private int _numberValue;

    // Delegate used to execute the callback method when the
    // task is complete.
    private ExampleCallback _callback;

    // The constructor obtains the state information and the
    // callback delegate.
    public ThreadWithState2(string text, int number,
        ExampleCallback callbackDelegate)
    {
        _boilerplate = text;
        _numberValue = number;
        _callback = callbackDelegate;
    }

    // The thread procedure performs the task, such as
    // formatting and printing a document, and then invokes
    // the callback delegate with the number of lines printed.
    public void ThreadProc()
    {
        Console.WriteLine(_boilerplate, _numberValue);
        _callback?.Invoke(1);
    }
}

// Delegate that defines the signature for the callback method.
public delegate void ExampleCallback(int lineCount);

// Entry point for the example.
public class Example2
{
    public static void Main()
    {
        // Supply the state information required by the task.
        ThreadWithState2 tws = new(
            "This report displays the number {0}.",
            42,
            new ExampleCallback(ResultCallback)
        );

        Thread t = new(new ThreadStart(tws.ThreadProc));
        t.Start();
        Console.WriteLine("Main thread does some work, then waits.");
        t.Join();
        Console.WriteLine(
            "Independent task has completed; main thread ends.");
    }

    // The callback method must match the signature of the
    // callback delegate.
    public static void ResultCallback(int lineCount)
    {
        Console.WriteLine($"Independent task printed {lineCount} lines.");
    }
}

// The example displays the following output:
//       Main thread does some work, then waits.
//       This report displays the number 42.
//       Independent task printed 1 lines.
//       Independent task has completed; main thread ends.

Imports System.Threading

' The ThreadWithState class contains the information needed for
' a task, the method that executes the task, and a delegate
' to call when the task is complete.
Public Class ThreadWithState
    ' State information used in the task.
    Private boilerplate As String
    Private numberValue As Integer

    ' Delegate used to execute the callback method when the
    ' task is complete.
    Private callback As ExampleCallback

    ' The constructor obtains the state information and the
    ' callback delegate.
    Public Sub New(text As String, number As Integer, _
        callbackDelegate As ExampleCallback)
        boilerplate = text
        numberValue = number
        callback = callbackDelegate
    End Sub

    ' The thread procedure performs the task, such as
    ' formatting and printing a document, and then invokes
    ' the callback delegate with the number of lines printed.
    Public Sub ThreadProc()
        Console.WriteLine(boilerplate, numberValue)
        If Not (callback Is Nothing) Then
            callback(1)
        End If
    End Sub
End Class

' Delegate that defines the signature for the callback method.
'
Public Delegate Sub ExampleCallback(lineCount As Integer)

Public Class Example
    Public Shared Sub Main()
        ' Supply the state information required by the task.
        Dim tws As New ThreadWithState( _
            "This report displays the number {0}.", _
            42, _
            AddressOf ResultCallback)

        Dim t As New Thread(AddressOf tws.ThreadProc)
        t.Start()
        Console.WriteLine("Main thread does some work, then waits.")
        t.Join()
        Console.WriteLine( _
            "Independent task has completed; main thread ends.")
    End Sub

    Public Shared Sub ResultCallback(lineCount As Integer)
        Console.WriteLine( _
            "Independent task printed {0} lines.", lineCount)
    End Sub
End Class
' The example displays the following output:
'       Main thread does some work, then waits.
'       This report displays the number 42.
'       Independent task printed 1 lines.
'       Independent task has completed; main thread ends.

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