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Process.StandardOutput Property

Gets a stream used to read the textual output of the application.

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

[BrowsableAttribute(false)]
[MonitoringDescriptionAttribute("ProcessStandardOutput")]
public StreamReader StandardOutput { get; }

Property Value

Type: System.IO.StreamReader
A StreamReader that can be used to read the standard output stream of the application.

ExceptionCondition
InvalidOperationException

The StandardOutput stream has not been defined for redirection; ensure ProcessStartInfo.RedirectStandardOutput is set to true and ProcessStartInfo.UseShellExecute is set to false.

- or -

The StandardOutput stream has been opened for asynchronous read operations with BeginOutputReadLine.

When a Process writes text to its standard stream, that text is normally displayed on the console. By redirecting the StandardOutput stream, you can manipulate or suppress the output of a process. For example, you can filter the text, format it differently, or write the output to both the console and a designated log file.

NoteNote

To use StandardOutput, you must set ProcessStartInfo.UseShellExecute to false, and you must set ProcessStartInfo.RedirectStandardOutput to true. Otherwise, reading from the StandardOutput stream throws an exception.

The redirected 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 StandardOutput stream, 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.

Synchronous read operations introduce a dependency between the caller reading from the StandardOutput stream and the child process writing to that stream. These dependencies can result in deadlock conditions. When the caller reads from the redirected stream of a child process, it is dependent on the child. The caller waits on the read operation until the child writes to the stream or closes the stream. When the child process writes enough data to fill its redirected stream, it is dependent on the parent. The child process waits on the next write operation until the parent reads from the full stream or closes the stream. The deadlock condition results when the caller and child process wait on each other to complete an operation, and neither can proceed. You can avoid deadlocks by evaluating dependencies between the caller and child process.

The following C# code, for example, shows how to read from a redirected stream and wait for the child process to exit.

 Process p = new Process();
 p.StartInfo.UseShellExecute = false;
 p.StartInfo.RedirectStandardOutput = true;
 p.StartInfo.FileName = "Write500Lines.exe";
 p.Start();

 // To avoid deadlocks, always read the output stream first and then wait.
 string output = p.StandardOutput.ReadToEnd();
 p.WaitForExit();

The code example avoids a deadlock condition by calling p.StandardOutput.ReadToEnd before p.WaitForExit. A deadlock condition can result if the parent process calls p.WaitForExit before p.StandardOutput.ReadToEnd and the child process writes enough text to fill the redirected stream. The parent process would wait indefinitely for the child process to exit. The child process would wait indefinitely for the parent to read from the full StandardOutput stream.

There is a similar issue when you read all text from both the standard output and standard error streams. The following C# code, for example, performs a read operation on both streams.

 // To avoid deadlocks, use asynchronous read operations on at least one of the streams.
 // Do not perform a synchronous read to the end of both redirected streams.
 p.BeginOutputReadLine();
 string error = p.StandardError.ReadToEnd();
 p.WaitForExit();

The code example avoids the deadlock condition by performing asynchronous read operations on the StandardOutput stream. A deadlock condition results if the parent process calls p.StandardOutput.ReadToEnd followed by p.StandardError.ReadToEnd and the child process writes enough text to fill its error stream. The parent process would wait indefinitely for the child process to close its StandardOutput stream. The child process would wait indefinitely for the parent to read from the full StandardError stream.

You can use asynchronous read operations to avoid these dependencies and their deadlock potential. Alternately, you can avoid the deadlock condition by creating two threads and reading the output of each stream on a separate thread.

NoteNote

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 BeginOutputReadLine with 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.

The following example runs the ipconfig.exe command and redirects its standard output to the example’s console window.

using System;
using System.IO;
using System.Diagnostics;

class StandardOutputExample
{
    public static void Main()
    {        
        Process process = new Process();
        process.StartInfo.FileName = "ipconfig.exe";        
        process.StartInfo.UseShellExecute = false;
        process.StartInfo.RedirectStandardOutput = true;        
        process.Start();

        // Synchronously read the standard output of the spawned process. 
        StreamReader reader = process.StandardOutput;
        string output = reader.ReadToEnd();

        // Write the redirected output to this application's window.
        Console.WriteLine(output);

        process.WaitForExit();
        process.Close();

        Console.WriteLine("\n\nPress any key to exit.");
        Console.ReadLine();
    }
}

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

  • LinkDemand 

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

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