How to run Process one by one in a loop?

Question

I am trying to pass a file path to the Process in a for loop and let ffmpeg.exe read only one video file's format each time. However, it stops after the textbox's text contains the first video's content. I have tried process.close(), process.hasExited, etc, but those couldn't stop the program from starting more than one Process and caused the delegate massively updated the textbox (the outputs weren't listed in the right order and I couldn't tell which lines belong to which video, and a few outputs were missing or showing twice in the text). How to execute only one process each time before the next in a for loop?

            foreach (String file in inputList)
            {
                if (flag == true)
                {
                    flag = false;
                    //textBox1.Text += file + Environment.NewLine;
                    checkvideoFormat(file);
                }

            }
            
            

        }
        catch (Exception err)
        {
            MessageBox.Show(err.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
    }
    private void checkvideoFormat(String filePath)
    {
        Process process1 = new Process();
        process1.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
        process1.StartInfo.CreateNoWindow = true;
        process1.StartInfo.UseShellExecute = false;
        process1.StartInfo.FileName = ".\\ffmpeg.exe";
        process1.StartInfo.WorkingDirectory = ".\\";
        process1.EnableRaisingEvents = true;
        process1.StartInfo.RedirectStandardOutput = true; //if this is true, UseShellExecute must be false. true if output should be written to StandardOutput
        process1.StartInfo.RedirectStandardError = true;
        //indicates that the associated Process has written a line that's terminated with a newline
        process1.ErrorDataReceived += new DataReceivedEventHandler(inputHandler);
        
        process1.Exited += (ending, p) =>
        {
            flag = true;
            Console.WriteLine(flag);
            //textBox1.Text += "Hey " + file + Environment.NewLine;
            //process1.CancelOutputRead();
            //process1.CancelErrorRead();//
        };
        
        process1.StartInfo.Arguments = "-i " + " \"" + filePath + "\"";
        Console.WriteLine(process1.StartInfo.Arguments);
        process1.Start();
        process1.BeginOutputReadLine();
        process1.BeginErrorReadLine();
        //process1.Close();
        //process1.WaitForExit();
        /*
        if(process1.HasExited == true)
        {
            //process1.Refresh();
            flag = true;
            //process1.Close();
            //process1.Kill();
            Thread.Sleep(1000);
        }
        */
    }



    int test_123 = 0;
    private void inputHandler(object sender, DataReceivedEventArgs l)
    {
        cba.Append(test_123 + l.Data + "\n");
        videoInput = test_123 + l.Data + Environment.NewLine;
        //Console.WriteLine(cba);
        //Process p = sender as Process;
        Console.WriteLine(videoInput);
        
        this.Invoke(new MethodInvoker(() =>
        {

            if (!String.IsNullOrEmpty(videoInput))
            {
                textBox1.Text += videoInput;
            }

        }));

        test_123++;

    }

Answer 1

I would suggest rewriting your program to use an asynchronous pattern. In .Net 5+ you can use WaitForExitAsync

private async Task CheckvideoFormat(string filePath)
{
   using var process1 = new Process();

   ...

    return await process1.WaitForExitAsync() ; // make sure process is not disposed before the task has completed.
}

and make sure to await this call in your loop. This pattern should avoid blocking the UI thread.

For older versions you could probably simulate the behavior using the exited event and a TaskCompletionSource

Answer 2

I've tried writing the code using the suggestion from @JonasH using asynchronous pattern. Furthermore I've tried letting all the actual work run in parallel and then only writing to the textbox when it's all done - thereby making sure it's in the right order. I'm unsure if there are any unfourtunate side effects by doing this. Also I'm unsure what you're trying to write for each file, but that should be fairly easy to correct.

Hope it is of use

    private async void Button_Click(object sender, RoutedEventArgs e)
    {
            string[] inputList = { "fileName1", "fileName2" };

        try
        {
            int fileNumber = 0;

            var output = await Task.WhenAll(inputList.Select(x => checkvideoFormat(x, fileNumber++)));

            foreach (var outputLine in output.Where(x => x != null))
            {
                textBox1.Text += outputLine + Environment.NewLine;
            }

        }
        catch (Exception err)
        {
            MessageBox.Show(err.Message, "Error", MessageBoxButton.OK);
        }
    }

    private async Task<string?> checkvideoFormat(string filePath, int fileNumber)
    {
        string? backText = null;
        Process process1 = new Process();
        process1.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
        process1.StartInfo.CreateNoWindow = true;
        process1.StartInfo.UseShellExecute = false;
        process1.StartInfo.FileName = ".\\ffmpeg.exe";
        process1.StartInfo.WorkingDirectory = ".\\";
        process1.EnableRaisingEvents = true;
        process1.StartInfo.RedirectStandardOutput = true; //if this is true, UseShellExecute must be false. true if output should be written to StandardOutput
        process1.StartInfo.RedirectStandardError = true;
        //indicates that the associated Process has written a line that's terminated with a newline
        process1.ErrorDataReceived += (sender, args) =>
        {
            backText = $"{fileNumber} {args.Data}";
        };

        process1.Exited += (ending, p) =>
        {
            // Don't know if you want to add something to backText here?
        };

        process1.StartInfo.Arguments = "-i " + " \"" + filePath + "\"";
        Console.WriteLine(process1.StartInfo.Arguments);
        process1.Start();
        await process1.WaitForExitAsync();

        return backText;
    }