Task.WaitAll Method (Task())

Imports System.Threading
Imports System.Threading.Tasks

Module WaitAllDemo
    ' Demonstrated features: 
    '   Task.Factory 
    '   Task.Result 
    '   Task group manipulation 
    '   Exception handling 
    ' Expected results: 
    '   10 tasks are started, each passed an index as a state object. 
    '   The tasks that receive an argument between 2 and 5 throw exceptions. 
    '   Task.WaitAll() wraps all exceptions in an AggregateException and propagates it to the main thread. 
    ' Documentation: 
    '   http://msdn.microsoft.com/en-us/library/system.threading.tasks.taskfactory_members(VS.100).aspx 
    Sub Main()
        ' Define a delegate that prints and returns the system tick count 
        Dim action As Func(Of Object, Integer) = Function(obj As Object)
                                                     Dim i As Integer = CInt(obj)

                                                     ' Make each thread sleep a different time in order to return a different tick count
                                                     Thread.Sleep(i * 100)

                                                     ' The tasks that receive an argument between 2 and 5 throw exceptions 
                                                     If 2 <= i AndAlso i <= 5 Then 
                                                         Throw New InvalidOperationException("SIMULATED EXCEPTION")
                                                     End If 

                                                     Dim tickCount As Integer = Environment.TickCount
                                                     Console.WriteLine("Task={0}, i={1}, TickCount={2}, Thread={3}", Task.CurrentId, i, tickCount, Thread.CurrentThread.ManagedThreadId)

                                                     Return tickCount
                                                 End Function 

        Const n As Integer = 10

        ' Construct started tasks 
        Dim tasks As Task(Of Integer)() = New Task(Of Integer)(n - 1) {}
        For i As Integer = 0 To n - 1
            tasks(i) = Task(Of Integer).Factory.StartNew(action, i)
        Next 

        ' Exceptions thrown by tasks will be propagated to the main thread 
        ' while it waits for the tasks. The actual exceptions will be wrapped in AggregateException. 
        Try 
            ' Wait for all the tasks to finish.
            Task.WaitAll(tasks)

            ' We should never get to this point
            Console.WriteLine("WaitAll() has not thrown exceptions. THIS WAS NOT EXPECTED.")
        Catch e As AggregateException
            Console.WriteLine(vbLf & "The following exceptions have been thrown by WaitAll(): (THIS WAS EXPECTED)")
            For j As Integer = 0 To e.InnerExceptions.Count - 1
                Console.WriteLine(vbLf & "-------------------------------------------------" & vbLf & "{0}", e.InnerExceptions(j).ToString())
            Next 
        End Try 
    End Sub 

End Module