Gets or sets a value that indicates whether the textual output of an application is written to the Process.StandardOutput stream.
Assembly: System (in System.dll)
When a Process writes text to its standard stream, that text is typically displayed on the console. By setting to true to redirect 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.
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 (see OutputDataReceived) 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.
The application that is processing the asynchronous output should call the WaitForExit method to ensure that the output buffer has been flushed.
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 cause deadlock conditions. When the caller reads from the redirected stream of a child process, it is dependent on the child. The caller waits for 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 for 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 for each other to complete an operation, and neither can continue. You can avoid deadlocks by evaluating dependencies between the caller and child process.
For example, the following C# code 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. For example, the following C# code 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.
// Run "csc.exe /r:System.dll /out:sample.exe stdstr.cs". UseShellExecute is false because we're specifying // an executable directly and in this case depending on it being in a PATH folder. By setting // RedirectStandardOutput to true, the output of csc.exe is directed to the Process.StandardOutput stream // which is then displayed in this console window directly. Process compiler = new Process(); compiler.StartInfo.FileName = "csc.exe"; compiler.StartInfo.Arguments = "/r:System.dll /out:sample.exe stdstr.cs"; compiler.StartInfo.UseShellExecute = false; compiler.StartInfo.RedirectStandardOutput = true; compiler.Start(); Console.WriteLine(compiler.StandardOutput.ReadToEnd()); compiler.WaitForExit();