Process.OutputDataReceived Event


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Occurs each time an application writes a line to its redirected StandardOutput stream.

Namespace:   System.Diagnostics
Assembly:  System (in System.dll)

Public Event OutputDataReceived As DataReceivedEventHandler

The OutputDataReceived event indicates that the associated Process has written a line, terminating with a newline character, to its redirected StandardOutput stream.

The event is enabled during asynchronous read operations on StandardOutput. To start asynchronous read operations, you must redirect the StandardOutput stream of a Process, add your event handler to the OutputDataReceived event, and call BeginOutputReadLine. Thereafter, the OutputDataReceived event signals each time the process writes a line to the redirected StandardOutput stream, until the process exits or calls CancelOutputRead.


The application that is processing the asynchronous output should call the WaitForExit method to ensure that the output buffer has been flushed.

The following example illustrates how to perform asynchronous read operations on the redirected StandardOutput stream of the ipconfig command.

The example creates an event delegate for the OutputHandler event handler and associates it with the OutputDataReceived event. The event handler receives text lines from the redirected StandardOutput stream, formats the text, and saves it in an output string that’s later shown in the example’s console window.

Imports System
Imports System.IO
Imports System.Diagnostics
Imports System.Text

Module Module1
    Dim lineCount As Integer = 0
    Dim output As StringBuilder = New StringBuilder()

    Sub Main()
        Dim process As New Process()
        process.StartInfo.FileName = "ipconfig.exe"
        process.StartInfo.UseShellExecute = False
        process.StartInfo.RedirectStandardOutput = True
        AddHandler process.OutputDataReceived, AddressOf OutputHandler

        ' Asynchronously read the standard output of the spawned process. 
        ' This raises OutputDataReceived events for each line of output.



        Console.WriteLine(Environment.NewLine + Environment.NewLine + "Press any key to exit.")
    End Sub

    Sub OutputHandler(sender As Object, e As DataReceivedEventArgs)
        If Not String.IsNullOrEmpty(e.Data) Then
            lineCount += 1

            ' Add the text to the collected output.
            output.Append(Environment.NewLine + "[" + lineCount.ToString() + "]: " + e.Data)
        End If
    End Sub
End Module


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.NET Framework
Available since 2.0
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