Assembly: System (in system.dll)
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The StandardError stream has been opened for asynchronous read operations with BeginErrorReadLine.
When a Process writes text to its standard error stream, that text is normally displayed on the console. By redirecting the StandardError stream, you can manipulate or suppress the error 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.
To use StandardError, you must set ProcessStartInfo.UseShellExecute to false, and you must set ProcessStartInfo.RedirectStandardError to true. Otherwise, reading from the StandardError stream throws an exception.
The redirected StandardError stream can be read synchronously or asynchronously. Methods such as Read, ReadLine, and ReadToEnd perform synchronous read operations on the error output stream of the process. These synchronous read operations do not complete until the associated Process writes to its StandardError stream, or closes the stream.
In contrast, BeginErrorReadLine starts asynchronous read operations on the StandardError 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 StandardError 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.
// Start the child process. Process p = new Process(); // Redirect the error stream of the child process. p.StartInfo.UseShellExecute = false; p.StartInfo.RedirectStandardError = true; p.StartInfo.FileName = "Write500Lines.exe"; p.Start(); // Do not wait for the child process to exit before // reading to the end of its redirected error stream. // p.WaitForExit(); // Read the error stream first and then wait. string error = p.StandardError.ReadToEnd(); p.WaitForExit();
The code example avoids a deadlock condition by calling p.StandardError.ReadToEnd before p.WaitForExit. A deadlock condition can result if the parent process calls p.WaitForExit before p.StandardError.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 StandardError 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.
// Do not perform a synchronous read to the end of both // redirected streams. // string output = p.StandardOutput.ReadToEnd(); // string error = p.StandardError.ReadToEnd(); // p.WaitForExit(); // Use asynchronous read operations on at least one of the 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.
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 BeginErrorReadLine with a call to ReadLine on the StandardError 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 uses the net use command together with a user supplied argument to map a network resource. It then reads the standard error stream of the net command and writes it to console.
Process myProcess = new Process(); ProcessStartInfo myProcessStartInfo = new ProcessStartInfo("net ","use "+ args); myProcessStartInfo.UseShellExecute = false; myProcessStartInfo.RedirectStandardError = true; myProcess.StartInfo = myProcessStartInfo; myProcess.Start(); StreamReader myStreamReader = myProcess.StandardError; // Read the standard error of net.exe and write it on to console. Console.WriteLine( myStreamReader.ReadLine()); myProcess.Close();
Windows 98, Windows 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.