Process.CancelErrorRead Method ()
Cancels the asynchronous read operation on the redirected StandardError stream of an application.
Assembly: System (in System.dll)
| Exception | Condition |
|---|---|
| InvalidOperationException | The StandardError stream is not enabled for asynchronous read operations. |
BeginErrorReadLine starts an asynchronous read operation on the StandardError stream. CancelErrorRead ends the asynchronous read operation.
After canceling, you can resume the asynchronous read operation by calling BeginErrorReadLine again.
When you call CancelErrorRead, all in-progress read operations for StandardError are completed and then the event handler is disabled. All further redirected output to StandardError is saved in a buffer. If you re-enable the event handler with a call to BeginErrorReadLine, the saved output is sent to the event handler and asynchronous read operations resume. If you want to change the event handler before resuming asynchronous read operations, you must remove the existing event handler before adding the new event handler:
// At this point the DataReceivedEventHandler(ErrorHandler1)
// has executed a CancelErrorRead.
// Remove the prior event handler.
process.ErrorDataReceived -=
new DataReceivedEventHandler(ErrorHandler1);
// Register a new event handler.
process.ErrorDataReceived +=
new DataReceivedEventHandler(ErrorHandler2);
// Call the corresponding BeginErrorReadLine.
process.BeginErrorReadLine();
Note |
|---|
You cannot mix asynchronous and synchronous read operations on the redirected StandardError 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. If you cancel an asynchronous read operation on StandardError and then need to read from the stream again, you must use BeginErrorReadLine to resume asynchronous read operations. Do not follow CancelErrorRead with a call to the synchronous read methods of StandardError such as Read, ReadLine, or ReadToEnd. |
The following example starts the nmake command with user supplied arguments. The error and output streams are read asynchronously; the collected text lines are displayed to the console as well as written to a log file. If the command output exceeds a specified number of lines, the asynchronous read operations are canceled.
' Define the namespaces used by this sample. Imports System Imports System.Text Imports System.IO Imports System.Diagnostics Imports System.Threading Imports System.ComponentModel Imports Microsoft.VisualBasic Class ProcessNMakeStreamRedirection ' Define static variables shared by class methods. Private Shared buildLogStream As StreamWriter = Nothing Private Shared logMutex As Mutex = New Mutex() Private Shared maxLogLines As Integer = 25 Private Shared currentLogLines As Integer = 0 Public Shared Sub RedirectNMakeCommandStreams() Dim nmakeArguments As String = Nothing Dim nmakeProcess As Process ' Get the input nmake command-line arguments. Console.WriteLine("Enter the NMake command line arguments" + _ " (@commandfile or /f makefile, etc):") Dim inputText As String = Console.ReadLine() If Not String.IsNullOrEmpty(inputText) Then nmakeArguments = inputText End If Console.WriteLine("Enter max line limit for log file (default is 25):") inputText = Console.ReadLine() If Not String.IsNullOrEmpty(inputText) Then If Not Int32.TryParse(inputText, maxLogLines) Then maxLogLines = 25 End If End If Console.WriteLine("Output beyond {0} lines will be ignored.", _ maxLogLines) ' Initialize the process and its StartInfo properties. nmakeProcess = New Process() nmakeProcess.StartInfo.FileName = "NMake.exe" ' Build the nmake command argument list. If Not String.IsNullOrEmpty(nmakeArguments) Then nmakeProcess.StartInfo.Arguments = nmakeArguments End If ' Set UseShellExecute to false for redirection. nmakeProcess.StartInfo.UseShellExecute = False ' Redirect the standard output of the nmake command. ' Read the stream asynchronously using an event handler. nmakeProcess.StartInfo.RedirectStandardOutput = True AddHandler nmakeProcess.OutputDataReceived, _ AddressOf NMakeOutputDataHandler ' Redirect the error output of the nmake command. nmakeProcess.StartInfo.RedirectStandardError = True AddHandler nmakeProcess.ErrorDataReceived, _ AddressOf NMakeErrorDataHandler logMutex.WaitOne() currentLogLines = 0 ' Write a header to the log file. Const buildLogFile As String = "NmakeCmd.Txt" Try buildLogStream = new StreamWriter(buildLogFile, true) Catch e As Exception Console.WriteLine("Could not open output file {0}", buildLogFile) Console.WriteLine("Exception = {0}", e.ToString()) Console.WriteLine(e.Message) buildLogStream = Nothing End Try If Not buildLogStream Is Nothing Then Console.WriteLine("Nmake output logged to {0}", _ buildLogFile) buildLogStream.WriteLine() buildLogStream.WriteLine(DateTime.Now.ToString()) If Not String.IsNullOrEmpty(nmakeArguments) Then buildLogStream.Write("Command line = NMake {0}", _ nmakeArguments) Else buildLogStream.Write("Command line = Nmake") End If buildLogStream.WriteLine() buildLogStream.Flush() logMutex.ReleaseMutex() ' Start the process. Console.WriteLine() Console.WriteLine("\nStarting Nmake command...") Console.WriteLine() nmakeProcess.Start() ' Start the asynchronous read of the error stream. nmakeProcess.BeginErrorReadLine() ' Start the asynchronous read of the output stream. nmakeProcess.BeginOutputReadLine() ' Let the nmake command run, collecting the output. nmakeProcess.WaitForExit() nmakeProcess.Close() buildLogStream.Close() logMutex.Dispose() End If End Sub Private Shared Sub NMakeOutputDataHandler(sendingProcess As Object, _ outLine As DataReceivedEventArgs) ' Collect the output, displaying it to the screen and ' logging it to the output file. Cancel the read ' operation when the maximum line limit is reached. If Not String.IsNullOrEmpty(outLine.Data) Then logMutex.WaitOne() currentLogLines = currentLogLines + 1 If currentLogLines > maxLogLines Then ' Display the line to the console. ' Skip writing the line to the log file. Console.WriteLine("StdOut: {0}", outLine.Data) Else If currentLogLines = maxLogLines Then LogToFile("StdOut", "<Max build log limit reached!>", _ true) ' Stop reading the output streams. Dim p As Process = sendingProcess If Not (p Is Nothing) Then p.CancelOutputRead() p.CancelErrorRead() End If Else ' Write the line to the log file. LogToFile("StdOut", outLine.Data, true) End If logMutex.ReleaseMutex() End If End Sub Private Shared Sub NMakeErrorDataHandler(sendingProcess As Object, _ errLine As DataReceivedEventArgs) ' Collect the error output, displaying it to the screen and ' logging it to the output file. Cancel the error output ' read operation when the maximum line limit is reached. If Not String.IsNullOrEmpty(errLine.Data) Then logMutex.WaitOne() currentLogLines = currentLogLines + 1 If currentLogLines > maxLogLines Then ' Display the line to the console. ' Skip writing the line to the log file. Console.WriteLine("StdErr: {0}", errLine.Data) Else If currentLogLines = maxLogLines Then LogToFile("StdErr", "<Max build log limit reached!>", _ true) ' Stop reading the output streams. Dim p As Process = sendingProcess If Not (p Is Nothing) Then p.CancelOutputRead() p.CancelErrorRead() End If Else ' Write the line to the log file. LogToFile("StdErr", errLine.Data, true) End If logMutex.ReleaseMutex() End If End Sub Private Shared Sub LogToFile(logPrefix As String, _ logText As String, _ echoToConsole As String) ' Write the specified line to the log file stream. Dim logString As StringBuilder = New StringBuilder() If Not String.IsNullOrEmpty(logPrefix) Then logString.AppendFormat("{0}> ", logPrefix) End If If Not String.IsNullOrEmpty(logText) Then logString.Append(logText) End If If Not buildLogStream Is Nothing Then buildLogStream.WriteLine("[{0}] {1}", _ DateTime.Now.ToString(), logString.ToString()) buildLogStream.Flush() End If If echoToConsole Then Console.WriteLine(logString.ToString()) End If End Sub End Class
for full trust for the immediate caller. This member cannot be used by partially trusted code.
Available since 2.0
