Como: Percorrer uma árvore binária com tarefas em paralelo

O exemplo a seguir mostra duas maneiras nas quais tarefas em paralelo podem ser usadas para percorrer uma estrutura de dados de árvore. A criação da própria árvore é deixada como um exercício.

Exemplo

Imports System.Threading.Tasks

Public Class TreeWalk

    Shared Sub Main()

        Dim tree As Tree(Of Person) = New Tree(Of Person)()

        ' ...populate tree (left as an exercise)

        ' Define the Action to perform on each node.
        Dim myAction As Action(Of Person) = New Action(Of Person)(Sub(x)
                                                                      Console.WriteLine("{0}  : {1} ", x.Name, x.Number)
                                                                  End Sub)

        ' Traverse the tree with parallel tasks.
        DoTree(tree, myAction)
    End Sub

    Public Class Person

        Public Name As String
        Public Number As Integer
    End Class

    Public Class Tree(Of T)

        Public Left As Tree(Of T)
        Public Right As Tree(Of T)
        Public Data As T
    End Class



    ' By using tasks explicitly.
    Public Shared Sub DoTree(Of T)(ByVal myTree As Tree(Of T), ByVal a As Action(Of T))

        If Not myTree Is Nothing Then
            Return
        End If
        Dim left = Task.Factory.StartNew(Sub() DoTree(myTree.Left, a))
        Dim right = Task.Factory.StartNew(Sub() DoTree(myTree.Right, a))
        a(myTree.Data)

        Try

            Task.WaitAll(left, right)

        Catch ae As AggregateException

            'handle exceptions here
        End Try
    End Sub

    ' By using Parallel.Invoke
    Public Shared Sub DoTree2(Of T)(ByVal myTree As Tree(Of T), ByVal myAct As Action(Of T))

        If Not myTree Is Nothing Then
            Return
        End If
        Parallel.Invoke(
            Sub() DoTree2(myTree.Left, myAct),
            Sub() DoTree2(myTree.Left, myAct),
            Sub() myAct(myTree.Data)
        )
    End Sub

End Class

    public class TreeWalk
    {
        static void Main()
        {
            Tree<MyClass> tree = new Tree<MyClass>();

            // ...populate tree (left as an exercise)

            // Define the Action to perform on each node.
            Action<MyClass> myAction = x => Console.WriteLine("{0} : {1}", x.Name, x.Number);

            // Traverse the tree with parallel tasks.
            DoTree(tree, myAction);
        }

        public class MyClass
        {
            public string Name { get; set; }
            public int Number { get; set; }
        }
        public class Tree<T>
        {
            public Tree<T> Left;
            public Tree<T> Right;
            public T Data;
        }



        // By using tasks explcitly.
        public static void DoTree<T>(Tree<T> tree, Action<T> action)
        {
            if (tree == null) return;
            var left = Task.Factory.StartNew(() => DoTree(tree.Left, action));
            var right = Task.Factory.StartNew(() => DoTree(tree.Right, action));
            action(tree.Data);

            try
            {
                Task.WaitAll(left, right);
            }
            catch (AggregateException )
            {
                //handle exceptions here
            }
        }

        // By using Parallel.Invoke
        public static void DoTree2<T>(Tree<T> tree, Action<T> action)
        {
            if (tree == null) return;
            Parallel.Invoke(
                () => DoTree2(tree.Left, action),
                () => DoTree2(tree.Right, action),
                () => action(tree.Data)
            );
        }

    }

Os dois métodos mostrados são funcionalmente equivalentes. Usando o StartNew() método para criar e executar as tarefas, você obtém um identificador de volta em tarefas que podem ser usadas para aguardar as tarefas e lidar com exceções.

Consulte também

Conceitos

Biblioteca paralela de tarefas