Process.BeginOutputReadLine Method

Process.BeginOutputReadLine Method

.NET Framework 4.5

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Begins asynchronous read operations on the redirected StandardOutput stream of the application.

Namespace: System.Diagnostics

Assembly: System (in System.dll)

Syntax

C#

C++

F#

VB

[ComVisibleAttribute(false)]
public void BeginOutputReadLine()

Exceptions

Exception	Condition
InvalidOperationException	The ProcessStartInfo.RedirectStandardOutput property is false. - or - An asynchronous read operation is already in progress on the StandardOutput stream. - or - The StandardOutput stream has been used by a synchronous read operation.

Remarks

The StandardOutput stream can be read synchronously or asynchronously. Methods such as Read, ReadLine,
and ReadToEnd perform synchronous read operations on the output stream of the process. These synchronous
read operations do not complete until the associated Process writes to its StandardOutputstream,
or closes the stream.

In contrast, BeginOutputReadLine starts asynchronous read operations on the StandardOutput stream.
This method enables a designated event handler for the stream output and immediately returns to the caller, which can perform other work while the stream output is directed to the event handler.

Follow these steps to perform asynchronous read operations on StandardOutput for a Process :

Set UseShellExecute to false.

Set RedirectStandardOutput to true.

Add your event handler to the OutputDataReceived event. The event handler must match
the System.Diagnostics.DataReceivedEventHandler delegate signature.

Start the Process.

Call BeginOutputReadLine for the Process. This call starts asynchronous read
operations on StandardOutput.

When asynchronous read operations start, the event handler is called each time the associated Process writes
a line of text to its StandardOutput stream.

You can cancel an asynchronous read operation by calling CancelOutputRead. The read operation
can be canceled by the caller or by the event handler. After canceling, you can call BeginOutputReadLine again to resume asynchronous read operations.

Note

You cannot mix asynchronous and synchronous read operations on a redirected stream. Once the redirected stream of a Process is opened in either asynchronous or synchronous mode, all further read operations on that stream must be in the same mode. For example, do not follow BeginOutputReadLinewith a call to ReadLine on the StandardOutput stream, or vice versa. However, you can read two different streams in different modes. For example, you can call BeginOutputReadLine and then call ReadLine for the StandardError stream.

Examples

The following example illustrates how to perform asynchronous read operations on the redirected StandardOutput stream
of the sort command. The sortcommand is a console application that reads and sorts text input.

The example creates an event delegate for the SortOutputHandler event handler and associates it with the OutputDataReceived event.
The event handler receives text lines from the redirected StandardOutput stream, formats
the text, and writes the text to the screen.

C#

C++

VB

// Define the namespaces used by this sample.
using System;
using System.Text;
using System.IO;
using System.Diagnostics;
using System.Threading;
using System.ComponentModel;

namespace ProcessAsyncStreamSamples
{
class SortOutputRedirection
{
// Define static variables shared by class methods.
private static StringBuilder sortOutput = null;
private static int numOutputLines = 0;

public static void SortInputListText()
{
// Initialize the process and its StartInfo properties.
// The sort command is a console application that
// reads and sorts text input.

Process sortProcess;
sortProcess = new Process();
sortProcess.StartInfo.FileName = "Sort.exe";

// Set UseShellExecute to false for redirection.
sortProcess.StartInfo.UseShellExecute = false;

// Redirect the standard output of the sort command.
// This stream is read asynchronously using an event handler.
sortProcess.StartInfo.RedirectStandardOutput = true;
sortOutput = new StringBuilder("");

// Set our event handler to asynchronously read the sort output.
sortProcess.OutputDataReceived += new DataReceivedEventHandler(SortOutputHandler);

// Redirect standard input as well.  This stream
// is used synchronously.
sortProcess.StartInfo.RedirectStandardInput = true;

// Start the process.
sortProcess.Start();

// Use a stream writer to synchronously write the sort input.
StreamWriter sortStreamWriter = sortProcess.StandardInput;

// Start the asynchronous read of the sort output stream.
sortProcess.BeginOutputReadLine();

// Prompt the user for input text lines.  Write each
// line to the redirected input stream of the sort command.
Console.WriteLine("Ready to sort up to 50 lines of text");

String inputText;
int numInputLines = 0;
do
{
Console.WriteLine("Enter a text line (or press the Enter key to stop):");

inputText = Console.ReadLine();
if (!String.IsNullOrEmpty(inputText))
{
numInputLines ++;
sortStreamWriter.WriteLine(inputText);
}
}
while (!String.IsNullOrEmpty(inputText) && (numInputLines < 50));
Console.WriteLine("<end of input stream>");
Console.WriteLine();

// End the input stream to the sort command.
sortStreamWriter.Close();

// Wait for the sort process to write the sorted text lines.
sortProcess.WaitForExit();

if (numOutputLines > 0)
{
// Write the formatted and sorted output to the console.
Console.WriteLine(" Sort results = {0} sorted text line(s) ",
numOutputLines);
Console.WriteLine("----------");
Console.WriteLine(sortOutput);
}
else
{
Console.WriteLine(" No input lines were sorted.");
}

sortProcess.Close();
}

private static void SortOutputHandler(object sendingProcess,
DataReceivedEventArgs outLine)
{
// Collect the sort command output.
if (!String.IsNullOrEmpty(outLine.Data))
{
numOutputLines++;

// Add the text to the collected output.
sortOutput.Append(Environment.NewLine +
"[" + numOutputLines.ToString() + "] - " + outLine.Data);
}
}
}
}

namespace ProcessAsyncStreamSamples
{

class ProcessSampleMain
{
/// The main entry point for the application.
static void Main()
{
try
{
SortOutputRedirection.SortInputListText();
}
catch (InvalidOperationException e)
{
Console.WriteLine("Exception:");
Console.WriteLine(e.ToString());
}
}
}
}

Version
Information

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET
Framework Security

LinkDemand

for full trust for the immediate caller. This member cannot be used by partially trusted code.

Platforms

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System
Requirements.

See
Also

Reference

Process Class
System.Diagnostics Namespace
ProcessStartInfo.RedirectStandardOutput
StandardOutput
OutputDataReceived
System.Diagnostics.DataReceivedEventHandler
CancelOutputRead