Process::CancelErrorRead Method ()

 
System_CAPS_noteNote

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Cancels the asynchronous read operation on the redirected StandardError stream of an application.

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

public:
[ComVisibleAttribute(false)]
void CancelErrorRead()

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();
System_CAPS_noteNote

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.

ref class ProcessNMakeStreamRedirection
{
private:
   // Define static variables shared by class methods.
   static StreamWriter^ buildLogStream = nullptr;
   static Mutex^ logMutex = gcnew Mutex;
   static int maxLogLines = 25;
   static int currentLogLines = 0;

public:
   static void RedirectNMakeCommandStreams()
   {
      String^ nmakeArguments = nullptr;
      Process^ nmakeProcess;

      // Get the input nmake command-line arguments.
      Console::WriteLine( "Enter the NMake command line arguments (@commandfile or /f makefile, etc):" );
      String^ inputText = Console::ReadLine();
      if (  !String::IsNullOrEmpty( inputText ) )
      {
         nmakeArguments = inputText;
      }

      Console::WriteLine( "Enter max line limit for log file (default is 25):" );
      inputText = Console::ReadLine();
      if (  !String::IsNullOrEmpty( inputText ) )
      {
         if (  !Int32::TryParse( inputText, maxLogLines ) )
         {
            maxLogLines = 25;
         }
      }
      Console::WriteLine( "Output beyond {0} lines will be ignored.",
         maxLogLines.ToString() );

      // Initialize the process and its StartInfo properties.
      nmakeProcess = gcnew Process;
      nmakeProcess->StartInfo->FileName = "NMake.exe";

      // Build the nmake command argument list.
      if (  !String::IsNullOrEmpty( nmakeArguments ) )
      {
         nmakeProcess->StartInfo->Arguments = nmakeArguments;
      }

      // 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;
      nmakeProcess->OutputDataReceived += gcnew DataReceivedEventHandler( NMakeOutputDataHandler );

      // Redirect the error output of the nmake command. 
      nmakeProcess->StartInfo->RedirectStandardError = true;
      nmakeProcess->ErrorDataReceived += gcnew DataReceivedEventHandler( NMakeErrorDataHandler );

      logMutex->WaitOne();

      currentLogLines = 0;

      // Write a header to the log file.
      String^ buildLogFile = "NmakeCmd.Txt";
      try
      {
         buildLogStream = gcnew StreamWriter( buildLogFile,true );
      }
      catch ( Exception^ e ) 
      {
         Console::WriteLine( "Could not open output file {0}", buildLogFile );
         Console::WriteLine( "Exception = {0}", e->ToString() );
         Console::WriteLine( e->Message->ToString() );

         buildLogStream = nullptr;
      }

      if ( buildLogStream != nullptr )
      {
         Console::WriteLine( "Nmake output logged to {0}", buildLogFile );

         buildLogStream->WriteLine();
         buildLogStream->WriteLine( DateTime::Now.ToString() );
         if (  !String::IsNullOrEmpty( nmakeArguments ) )
         {
            buildLogStream->Write( "Command line = NMake {0}", nmakeArguments );
         }
         else
         {
            buildLogStream->Write( "Command line = Nmake" );
         }
         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 output stream.
         nmakeProcess->BeginOutputReadLine();

         // Start the asynchronous read of the error stream.
         nmakeProcess->BeginErrorReadLine();

         // Let the nmake command run, collecting the output.
         nmakeProcess->WaitForExit();

         nmakeProcess->Close();
         buildLogStream->Close();
         logMutex->Dispose();
      }
   }

private:
   static void NMakeOutputDataHandler( Object^ sendingProcess,
      DataReceivedEventArgs^ outLine )
   {
      // 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 (  !String::IsNullOrEmpty( outLine->Data ) )
      {
         logMutex->WaitOne();

         currentLogLines++;
         if ( currentLogLines > maxLogLines )
         {
            // Display the line to the console.
            // Skip writing the line to the log file.
            Console::WriteLine( "StdOut: {0}", outLine->Data->ToString() );
         }
         else
         if ( currentLogLines == maxLogLines )
         {
            LogToFile( "StdOut", "<Max build log limit reached!>", true );

            // Stop reading the output streams.
            Process^ p = dynamic_cast<Process^>(sendingProcess);
            if ( p != nullptr )
            {
               p->CancelOutputRead();
               p->CancelErrorRead();
            }
         }
         else
         {
            // Write the line to the log file.
            LogToFile( "StdOut", outLine->Data, true );
         }
         logMutex->ReleaseMutex();
      }
   }

   static void NMakeErrorDataHandler( Object^ sendingProcess,
      DataReceivedEventArgs^ errLine )
   {

      // 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 (  !String::IsNullOrEmpty( errLine->Data ) )
      {
         logMutex->WaitOne();

         currentLogLines++;
         if ( currentLogLines > maxLogLines )
         {

            // Display the line to the console.
            // Skip writing the line to the log file.
            Console::WriteLine( "StdErr: {0}", errLine->Data->ToString() );
         }
         else
         if ( currentLogLines == maxLogLines )
         {
            LogToFile( "StdOut", "<Max build log limit reached!>", true );

            // Stop reading the output streams.
            Process^ p = dynamic_cast<Process^>(sendingProcess);
            if ( p != nullptr )
            {
               p->CancelOutputRead();
               p->CancelErrorRead();
            }
         }
         else
         {
            // Write the line to the log file.
            LogToFile( "StdErr", errLine->Data, true );
         }
         logMutex->ReleaseMutex();
      }
   }

   static void LogToFile( String^ logPrefix, String^ logText,
      bool echoToConsole )
   {
      // Write the specified line to the log file stream.
      StringBuilder^ logString = gcnew StringBuilder;

      if (  !String::IsNullOrEmpty( logPrefix ) )
      {
         logString->AppendFormat( "{0}> ", logPrefix );
      }

      if (  !String::IsNullOrEmpty( logText ) )
      {
         logString->Append( logText );
      }

      if ( buildLogStream != nullptr )
      {
         buildLogStream->WriteLine(  "[{0}] {1}",
            DateTime::Now.ToString(), logString->ToString() );
         buildLogStream->Flush();
      }

      if ( echoToConsole )
      {
         Console::WriteLine( logString->ToString() );
      }
   }
};

LinkDemand

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.NET Framework
Available since 2.0
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