Getting the error of a Powershell Script within c# and outputting it with conditional statement?

Question

I have a console application and a method that executes a PowerShell script within the console application. So I'm trying to grab an error text that it outputs in the application and do something with it.

Example/What I'm trying to do:

    If Error.contains("Object") 
{
    // do something here
}

Here is my current method

  public void ExecutePowershellScript()
{
  var file = @"C:\Path\filename.ps1";
           
            var start = new ProcessStartInfo()
            {
                FileName = "powershell.exe",
                Arguments = $"-NoProfile -ExecutionPolicy unrestricted -file \"{file}\"",
                UseShellExecute = false
            };
            Process.Start(start);
}

user16589670 · Accepted Answer · 2021-08-04 03:31:31Z

3

Process.start: how to get the output?

When you create your Process object set StartInfo appropriately:

var proc = new Process 
{
    StartInfo = new ProcessStartInfo
    {
        FileName = "program.exe",
        Arguments = "command line arguments to your executable",
        UseShellExecute = false,
        RedirectStandardOutput = true,
        CreateNoWindow = true
    }
};

then start the process and read from it:

proc.Start();
while (!proc.StandardOutput.EndOfStream)
{
    string line = proc.StandardOutput.ReadLine();
    // do something with line
}

You can use int.Parse() or int.TryParse() to convert the strings to numeric values. You may have to do some string manipulation first if there are invalid numeric characters in the strings you read.

answered Aug 4, 2021 at 3:31

user16589670

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1 Comment

Jason N Over a year ago

Yes, and you can set async error handlers

process.OutputDataReceived += new DataReceivedEventHandler(OutputHandler);     process.ErrorDataReceived += new DataReceivedEventHandler(OutputHandler);

//* Start process and handlers

Daniel · Accepted Answer · 2021-08-04 03:30:15Z

2

~~You can set RedirectStandardError = true and access any errors from process.StandardError~~

public static void ExecutePowershellScript() { var file = @"C:\Path\filename.ps1"; var start = new ProcessStartInfo() { FileName = "powershell.exe", Arguments = $"-NoProfile -ExecutionPolicy unrestricted -file \"{file}\"", UseShellExecute = false, RedirectStandardOutput = true, RedirectStandardError = true }; using Process process = Process.Start(start); string output = process.StandardOutput.ReadToEnd(); string errors = process.StandardError.ReadToEnd(); }

Okay, scratch the above suggestion. After being corrected by mklement0,

This is a perfectly reasonable attempt, but, unfortunately, it can lead to hangs (while waiting for one's stream end, the other, when exceeding the buffer size, may cause process execution to block). If you need to capture both streams, you must collect the output from one of them via events. – mklement0

I changed the solution to use the ErrorDataReceived event

        public static async Task ExecutePowershellScript()
        {
            var file = @"C:\Path\filename.ps1";

            var start = new ProcessStartInfo
            {
                FileName = "powershell.exe",
                Arguments = $"-NoProfile -ExecutionPolicy unrestricted -file \"{file}\"",
                UseShellExecute = false,
                // redirect standard error stream to process.StandardError
                RedirectStandardError = true
            };

            using var process = new Process
            {
                StartInfo = start
            };

            // Subscribe to ErrorDataReceived event
            process.ErrorDataReceived += (sender, e) =>
            {
                //  code to process the error lines in e.Data
            };

            process.Start();

            // Necessary to start redirecting errors to StandardError
            process.BeginErrorReadLine();

            // Wait for process to exit
            await process.WaitForExitAsync();

        }

edited Aug 4, 2021 at 3:30

answered Aug 4, 2021 at 1:50

Daniel

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6 Comments

mklement0 Over a year ago

This is a perfectly reasonable attempt, but, unfortunately, it can lead to hangs (while waiting for one's stream end, the other, when exceeding the buffer size, may cause process execution to block). If you need to capture both streams, you must collect the output from one of them via events.

Daniel Over a year ago

Fair enough. Thanks for the info! @mklement0

Abraham Zinala Over a year ago

@mklement0, ever think of making a book (or article/blog) bridging the gap between C# and PowerShell? 🤔 I'm real persistent on these book ideas lol

mklement0 Over a year ago

Thanks for updating, @Daniel. Just to reiterate: The original approach works robustly when only one stream is captured. When both need to be captured, the event-based approach is the only fully robust one, though, pragmatically speaking, reading from both streams in a loop, line by line, will work too - you would only get in trouble with excessively long individual lines (close to 64KiB and above).

mklement0 Over a year ago

@AbrahamZinala :) I'm starting to get concerned about the amount of homework...

|

user16577818 · Accepted Answer · 2021-08-04 03:09:54Z

0

start.Start();
while (!start.StandardOutput.EndOfStream)
{
    string line = start.StandardOutput.ReadLine();
}

answered Aug 4, 2021 at 3:09

user16577818

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