C# program that starts process and combines its standart output and standard error in one stream

a.cs

using System;
using System.Diagnostics;
using System.IO;
using System.Threading.Tasks;

class App
{
    public static int ExecuteProcess(
        string fileName,
        string arguments,
        int timeout,
        out string combinedOutput)
    {
        int exitCode;

        using (var process = new Process())
        {
            process.StartInfo.UseShellExecute = false;
            process.StartInfo.CreateNoWindow = true;
            process.StartInfo.RedirectStandardError = true;
            process.StartInfo.RedirectStandardOutput = true;
            process.StartInfo.FileName = fileName;
            process.StartInfo.Arguments = arguments;

            process.Start();

            using (var combinedStream = new StringWriter())
            using (Task<bool> processWaiter = Task.Factory.StartNew(() => process.WaitForExit(timeout)))
            using (Task outputReader = Task.Factory.StartNew((Action<object>)AppendLinesFunc, Tuple.Create(combinedStream, "stdout", process.StandardOutput)))
            using (Task errorReader = Task.Factory.StartNew((Action<object>)AppendLinesFunc, Tuple.Create(combinedStream, "stderr", process.StandardError)))
            {
                bool waitResult = processWaiter.Result;

                if (!waitResult)
                {
                    process.Kill();
                }

                Task.WaitAll(outputReader, errorReader);

                if (!waitResult)
                {
                    throw new TimeoutException("Process wait timeout expired");
                }

                exitCode = process.ExitCode;

                combinedOutput = combinedStream.ToString();
            }
        }

        return exitCode;
    }

    private static void AppendLinesFunc(object packedParams)
    {
        // Unpack params from tuple object
        var paramsTuple = (Tuple<StringWriter, string, StreamReader>)packedParams;
        StringWriter writer = paramsTuple.Item1;
        string marker = paramsTuple.Item2;
        StreamReader reader = paramsTuple.Item3;

        // Collect output from reader
        string line;
        while ((line = reader.ReadLine()) != null)
        {
            // Append to writer is critical section in order to prevent concurrent
            // writes from stdout and stderr to interfere with each other.
            lock (writer)
            {
                writer.WriteLine("{0} {1}: {2}", marker, DateTime.Now, line);
            }
        }
    }

    private static void Main(string[] args)
    {
        string output;
        int timeout = 10 * 1000; // ten seconds
        int exitCode = ExecuteProcess(args[0], args.Length > 1 ? args[1] : null, timeout, out output);
        Console.WriteLine("Exit code: {0}", exitCode);
        Console.WriteLine("Combined output:\n{0}", output);
    }
}

b.cs

using System;

class App
{
	static void Main()
	{
        var random = new Random();
        int n = 10;

        for (int i = 0; i < n; ++i)
        {
            int v = random.Next();
            if (v % 2 == 0)
            {
                Console.Out.WriteLine("even number - {0}", v);
            }
            else
            {
                Console.Error.WriteLine("odd number - {0}", v);
            }
        }
	}
}

how to test.txt

Compile a.cs and b.cs with commands:
  csc.exe a.cs
  csc.exe b.cs

Run with:
  a.exe b.exe

Sample output:
D:\tmp>a.exe b.exe
Exit code: 0
Combined output:
stderr 27.04.2014 21:09:57: odd number - 1519580101
stdout 27.04.2014 21:09:57: even number - 900702068
stdout 27.04.2014 21:09:57: even number - 1576951340
stdout 27.04.2014 21:09:57: even number - 1966632972
stderr 27.04.2014 21:09:57: odd number - 854441047
stderr 27.04.2014 21:09:57: odd number - 1356825953
stderr 27.04.2014 21:09:57: odd number - 64614431
stderr 27.04.2014 21:09:57: odd number - 713766487
stderr 27.04.2014 21:09:57: odd number - 1509132475
stderr 27.04.2014 21:09:57: odd number - 1756029823