How to spawn a process and capture its STDOUT in NET

Question

I need to spawn a child process that is a console application  and capture its output   I wrote up the following code for a method   string retMessage   String Empty  ProcessStartInfo startInfo   new ProcessStartInfo    Process p   new Process     startInfo CreateNoWindow   true  startInfo RedirectStandardOutput   true  startInfo RedirectStandardInput   true   startInfo UseShellExecute   false  startInfo Arguments   command  startInfo FileName   exec   p StartInfo   startInfo  p Start     p OutputDataReceived    new DataReceivedEventHandler       delegate object sender  DataReceivedEventArgs e                using  StreamReader output   p StandardOutput                        retMessage   output ReadToEnd                        p WaitForExit     return retMessage    However  this does not return anything  I don t believe the OutputDataReceived event is being called back  or the WaitForExit   command may be blocking the thread so it will never callback   Any advice   EDIT  Looks like I was trying too hard with the callback  Doing   return p StandardOutput ReadToEnd       Appears to work fine

User · Answer

It looks like two of your lines are out of order. You start the process before setting up an event handler to capture the output. It's possible the process is just finishing before the event handler is added.

Switch the lines like so.

p.OutputDataReceived += ...
p.Start();

User · Answer

Here s code that I ve verified to work  I use it for spawning MSBuild and listening to its output   process StartInfo UseShellExecute   false  process StartInfo RedirectStandardOutput   true  process OutputDataReceived     sender  args    gt  Console WriteLine  received output   0    args Data   process Start    process BeginOutputReadLine

User · Answer

Here s code that I ve verified to work  I use it for spawning MSBuild and listening to its output   process StartInfo UseShellExecute   false  process StartInfo RedirectStandardOutput   true  process OutputDataReceived     sender  args    gt  Console WriteLine  received output   0    args Data   process Start    process BeginOutputReadLine

User · Answer

Redirecting the stream is asynchronous and will potentially continue after the process has terminated  It is mentioned by Umar to cancel after process termination process CancelOutputRead    However that has data loss potential   This is working reliably for me   process WaitForExit           while  process StandardOutput EndOfStream    false        Thread Sleep 100       I didn t try this approach but I like the suggestion from Sly   if  process WaitForExit timeout         process WaitForExit

User · Answer

The answer from Judah did not work for me  or is not complete  as the application was exiting after the first BeginOutputReadLine     This works for me as a complete snippet  reading the constant output of a ping           var process   new Process            process StartInfo FileName    ping           process StartInfo Arguments    google com -t           process StartInfo RedirectStandardOutput   true          process StartInfo UseShellExecute   false          process OutputDataReceived     sender  a    gt  Console WriteLine a Data           process Start            process BeginOutputReadLine            process WaitForExit

User · Answer

It looks like two of your lines are out of order. You start the process before setting up an event handler to capture the output. It's possible the process is just finishing before the event handler is added.

Switch the lines like so.

p.OutputDataReceived += ...
p.Start();

User · Answer

Here s some full and simple code to do this  This worked fine when I used it   var processStartInfo   new ProcessStartInfo       FileName     C  SomeProgram       Arguments    Arguments       RedirectStandardOutput   true      UseShellExecute   false    var process   Process Start processStartInfo   var output   process StandardOutput ReadToEnd    process WaitForExit      Note that this only captures standard output  it doesn t capture standard error  If you want both  use this technique for each stream

User · Answer

You need to call p Start   to actually run the process after you set the StartInfo   As it is  your function is probably hanging on the WaitForExit   call because the process was never actually started

User · Answer

Here s a method that I use to run a process and gets its output and errors    public static string ShellExecute this string path  string command  TextWriter writer  params string   arguments                using  var process   Process Start new ProcessStartInfo   WorkingDirectory   path  FileName   command  Arguments   string Join      arguments   UseShellExecute   false  RedirectStandardOutput   true  RedirectStandardError   true                           using  process StandardOutput                                writer WriteLine process StandardOutput ReadToEnd                               using  process StandardError                                writer WriteLine process StandardError ReadToEnd                                      return path          For example      E  Temp MyWorkingDirectory  ShellExecute   C  Program Files Microsoft SDKs Windows v6 0A Bin svcutil exe   Console Out

User · Answer

Here s some full and simple code to do this  This worked fine when I used it   var processStartInfo   new ProcessStartInfo       FileName     C  SomeProgram       Arguments    Arguments       RedirectStandardOutput   true      UseShellExecute   false    var process   Process Start processStartInfo   var output   process StandardOutput ReadToEnd    process WaitForExit      Note that this only captures standard output  it doesn t capture standard error  If you want both  use this technique for each stream

User · Answer

It looks like two of your lines are out of order. You start the process before setting up an event handler to capture the output. It's possible the process is just finishing before the event handler is added.

Switch the lines like so.

p.OutputDataReceived += ...
p.Start();

User · Answer

Here s code that I ve verified to work  I use it for spawning MSBuild and listening to its output   process StartInfo UseShellExecute   false  process StartInfo RedirectStandardOutput   true  process OutputDataReceived     sender  args    gt  Console WriteLine  received output   0    args Data   process Start    process BeginOutputReadLine

User · Answer

It looks like two of your lines are out of order. You start the process before setting up an event handler to capture the output. It's possible the process is just finishing before the event handler is added.

Switch the lines like so.

p.OutputDataReceived += ...
p.Start();

User · Answer

The answer from Judah did not work for me  or is not complete  as the application was exiting after the first BeginOutputReadLine     This works for me as a complete snippet  reading the constant output of a ping           var process   new Process            process StartInfo FileName    ping           process StartInfo Arguments    google com -t           process StartInfo RedirectStandardOutput   true          process StartInfo UseShellExecute   false          process OutputDataReceived     sender  a    gt  Console WriteLine a Data           process Start            process BeginOutputReadLine            process WaitForExit

User · Answer

You need to call p Start   to actually run the process after you set the StartInfo   As it is  your function is probably hanging on the WaitForExit   call because the process was never actually started

User · Answer

Redirecting the stream is asynchronous and will potentially continue after the process has terminated  It is mentioned by Umar to cancel after process termination process CancelOutputRead    However that has data loss potential   This is working reliably for me   process WaitForExit           while  process StandardOutput EndOfStream    false        Thread Sleep 100       I didn t try this approach but I like the suggestion from Sly   if  process WaitForExit timeout         process WaitForExit

User · Answer

I just tried this very thing and the following worked for me   StringBuilder outputBuilder  ProcessStartInfo processStartInfo  Process process   outputBuilder   new StringBuilder     processStartInfo   new ProcessStartInfo    processStartInfo CreateNoWindow   true  processStartInfo RedirectStandardOutput   true  processStartInfo RedirectStandardInput   true  processStartInfo UseShellExecute   false  processStartInfo Arguments     lt insert command line arguments here gt    processStartInfo FileName     lt insert tool path here gt     process   new Process    process StartInfo   processStartInfo     enable raising events because Process does not raise events by default process EnableRaisingEvents   true     attach the event handler for OutputDataReceived before starting the process process OutputDataReceived    new DataReceivedEventHandler       delegate object sender  DataReceivedEventArgs e                   append the new data to the data already read-in         outputBuilder Append e Data               start the process    then begin asynchronously reading the output    then wait for the process to exit    then cancel asynchronously reading the output process Start    process BeginOutputReadLine    process WaitForExit    process CancelOutputRead        use the output string output   outputBuilder ToString

User · Answer

I needed to capture both stdout and stderr and have it timeout if the process didn t exit when expected  I came up with this   Process process   new Process    StringBuilder outputStringBuilder   new StringBuilder     try   process StartInfo FileName   exeFileName  process StartInfo WorkingDirectory   args ExeDirectory  process StartInfo Arguments   args  process StartInfo RedirectStandardError   true  process StartInfo RedirectStandardOutput   true  process StartInfo WindowStyle   ProcessWindowStyle Hidden  process StartInfo CreateNoWindow   true  process StartInfo UseShellExecute   false  process EnableRaisingEvents   false  process OutputDataReceived     sender  eventArgs    gt  outputStringBuilder AppendLine eventArgs Data   process ErrorDataReceived     sender  eventArgs    gt  outputStringBuilder AppendLine eventArgs Data   process Start    process BeginOutputReadLine    process BeginErrorReadLine    var processExited   process WaitForExit PROCESS TIMEOUT    if  processExited    false     we timed out          process Kill        throw new Exception  ERROR  Process took too long to finish      else if  process ExitCode    0        var output   outputStringBuilder ToString        var prefixMessage            throw new Exception  Process exited with non-zero exit code of      process ExitCode   Environment NewLine         Output from process      outputStringBuilder ToString         finally                   process Close        I am piping the stdout and stderr into the same string  but you could keep it separate if needed  It uses events  so it should handle them as they come  I believe   I have run this successfully  and will be volume testing it soon

User · Answer

Here s a method that I use to run a process and gets its output and errors    public static string ShellExecute this string path  string command  TextWriter writer  params string   arguments                using  var process   Process Start new ProcessStartInfo   WorkingDirectory   path  FileName   command  Arguments   string Join      arguments   UseShellExecute   false  RedirectStandardOutput   true  RedirectStandardError   true                           using  process StandardOutput                                writer WriteLine process StandardOutput ReadToEnd                               using  process StandardError                                writer WriteLine process StandardError ReadToEnd                                      return path          For example      E  Temp MyWorkingDirectory  ShellExecute   C  Program Files Microsoft SDKs Windows v6 0A Bin svcutil exe   Console Out

User · Answer

You need to call p Start   to actually run the process after you set the StartInfo   As it is  your function is probably hanging on the WaitForExit   call because the process was never actually started

User · Answer

I just tried this very thing and the following worked for me   StringBuilder outputBuilder  ProcessStartInfo processStartInfo  Process process   outputBuilder   new StringBuilder     processStartInfo   new ProcessStartInfo    processStartInfo CreateNoWindow   true  processStartInfo RedirectStandardOutput   true  processStartInfo RedirectStandardInput   true  processStartInfo UseShellExecute   false  processStartInfo Arguments     lt insert command line arguments here gt    processStartInfo FileName     lt insert tool path here gt     process   new Process    process StartInfo   processStartInfo     enable raising events because Process does not raise events by default process EnableRaisingEvents   true     attach the event handler for OutputDataReceived before starting the process process OutputDataReceived    new DataReceivedEventHandler       delegate object sender  DataReceivedEventArgs e                   append the new data to the data already read-in         outputBuilder Append e Data               start the process    then begin asynchronously reading the output    then wait for the process to exit    then cancel asynchronously reading the output process Start    process BeginOutputReadLine    process WaitForExit    process CancelOutputRead        use the output string output   outputBuilder ToString

User · Answer

Here s code that I ve verified to work  I use it for spawning MSBuild and listening to its output   process StartInfo UseShellExecute   false  process StartInfo RedirectStandardOutput   true  process OutputDataReceived     sender  args    gt  Console WriteLine  received output   0    args Data   process Start    process BeginOutputReadLine

User · Answer

I needed to capture both stdout and stderr and have it timeout if the process didn t exit when expected  I came up with this   Process process   new Process    StringBuilder outputStringBuilder   new StringBuilder     try   process StartInfo FileName   exeFileName  process StartInfo WorkingDirectory   args ExeDirectory  process StartInfo Arguments   args  process StartInfo RedirectStandardError   true  process StartInfo RedirectStandardOutput   true  process StartInfo WindowStyle   ProcessWindowStyle Hidden  process StartInfo CreateNoWindow   true  process StartInfo UseShellExecute   false  process EnableRaisingEvents   false  process OutputDataReceived     sender  eventArgs    gt  outputStringBuilder AppendLine eventArgs Data   process ErrorDataReceived     sender  eventArgs    gt  outputStringBuilder AppendLine eventArgs Data   process Start    process BeginOutputReadLine    process BeginErrorReadLine    var processExited   process WaitForExit PROCESS TIMEOUT    if  processExited    false     we timed out          process Kill        throw new Exception  ERROR  Process took too long to finish      else if  process ExitCode    0        var output   outputStringBuilder ToString        var prefixMessage            throw new Exception  Process exited with non-zero exit code of      process ExitCode   Environment NewLine         Output from process      outputStringBuilder ToString         finally                   process Close        I am piping the stdout and stderr into the same string  but you could keep it separate if needed  It uses events  so it should handle them as they come  I believe   I have run this successfully  and will be volume testing it soon

User · Answer

You need to call p Start   to actually run the process after you set the StartInfo   As it is  your function is probably hanging on the WaitForExit   call because the process was never actually started

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