Task(Of TResult).Result Property

 

Gets the result value of this Task(Of TResult).

Namespace:   System.Threading.Tasks
Assembly:  mscorlib (in mscorlib.dll)

Public ReadOnly Property Result As TResult

Property Value

Type: TResult

The result value of this Task(Of TResult), which is the same type as the task's type parameter.

Exception Condition
AggregateException

The task was canceled. The AggregateException.InnerExceptions collection contains a TaskCanceledException object.

-or-

An exception was thrown during the execution of the task. The AggregateException.InnerExceptions collection contains information about the exception or exceptions.

Accessing the property's get accessor blocks the calling thread until the asynchronous operation is complete; it is equivalent to calling the Wait method.

Once the result of an operation is available, it is stored and is returned immediately on subsequent calls to the Result property. Note that, if an exception occurred during the operation of the task, or if the task has been cancelled, the Result property does not return a value. Instead, attempting to access the property value throws an AggregateException exception.

The following example is a command-line utility that calculates the number of bytes in the files in each directory whose name is passed as a command-line argument. If the directory contains files, it executes a lambda expression that instantiates a FileStream object for each file in the directory and retrieves the value of its FileStream.Length property. If a directory contains no files, it simply calls the FromResult(Of TResult) method to create a task whose Task(Of TResult).Result property is zero (0). When the tasks finish, the total number of bytes in all a directory's files is available from the Result property.

Imports System.Collections.Generic
Imports System.IO
Imports System.Threading
Imports System.Threading.Tasks

Module Example
   Public Sub Main()
      Dim args() As String = Environment.GetCommandLineArgs()
      If args.Length > 1 Then
         Dim tasks As New List(Of Task(Of Long))
         For ctr = 1 To args.Length - 1
            tasks.Add(GetFileLengthsAsync(args(ctr)))
         Next
         Try
            Task.WaitAll(tasks.ToArray())
         ' Ignore exceptions here.
         Catch e As AggregateException
         End Try

         For ctr As Integer = 0 To tasks.Count - 1
            If tasks(ctr).Status = TaskStatus.Faulted Then
               Console.WriteLine("{0} does not exist", args(ctr + 1))
            Else
               Console.WriteLine("{0:N0} bytes in files in '{1}'",
                                 tasks(ctr).Result, args(ctr + 1))
            End If
         Next
      Else
         Console.WriteLine("Syntax error: Include one or more file paths.")
      End If
   End Sub

   Private Function GetFileLengthsAsync(filePath As String) As Task(Of Long)
      If Not Directory.Exists(filePath) Then
         Return Task.FromException(Of Long)(
                     New DirectoryNotFoundException("Invalid directory name."))
      Else
         Dim files As String() = Directory.GetFiles(filePath)
         If files.Length = 0 Then
            Return Task.FromResult(0L)
         Else
            Return Task.Run( Function()
                                Dim total As Long = 0
                                Dim lockObj As New Object
                                Parallel.ForEach(files, Sub(fileName)
                                                           Dim fs As New FileStream(fileName, FileMode.Open,
                                                                     FileAccess.Read, FileShare.ReadWrite,
                                                                     256, True)
                                                           Dim length As Long = fs.Length
                                                           Interlocked.Add(total, length)
                                                           fs.Close()
                                                        End Sub)
                                Return total
                             End Function )
         End If
      End If
   End Function
End Module
' When launched with the following command line arguments:
'      subdir . newsubdir
' the example displays output like the following:
'       0 bytes in files in 'subdir'
'       2,059 bytes in files in '.'
'       newsubdir does not exist

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 5.0
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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