This gist shows a strange behaviour when using spidermonkey from within an NUnit Testcase.

StreamReaderNUnitSpiderMonkeyFailure.cs

// ----------------------------------------------------------------------------
// This file is subject to the terms and conditions defined in
// file 'LICENSE.txt', which is part of this source code package.
// ----------------------------------------------------------------------------
using System;
using System.Diagnostics;
using System.Threading;
using NUnit.Framework;

namespace NUnitFail
{
        [TestFixture()]
        public class Test
        {
                [Test()]
                public void TestCase ()
                {
                        var psi = new ProcessStartInfo(
                                "js", ""
                                ) {
                                StandardErrorEncoding = System.Text.Encoding.UTF8,

                                RedirectStandardInput = true,
                                RedirectStandardOutput = true,
                                RedirectStandardError = true,
                                CreateNoWindow = true,
                                UseShellExecute = false,
                                WindowStyle = ProcessWindowStyle.Hidden
                        };

                        var stdoutSignal = new ManualResetEventSlim(false);
                        var stderrSignal = new ManualResetEventSlim(false);

                        var _Process = Process.Start(psi);

                        string _StdOut = null, _StdErr = null;
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdOut = _Process.StandardOutput.ReadToEnd();
                                } catch {
                                }
                                stdoutSignal.Set();
                        }));
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdErr = _Process.StandardError.ReadToEnd();
                                } catch {
                                }
                                stderrSignal.Set();
                        }));

                        Action _JoinImpl = () => {
                                stdoutSignal.Wait();
                                stderrSignal.Wait();
                                stderrSignal.Dispose();
                                stderrSignal.Dispose();
                        };
                        //_Process.StandardInput.Encoding = System.Text.Encoding.UTF8;
                        _Process.StandardInput.Write("var t = {}");
                        _Process.StandardInput.Flush();
                        _Process.StandardInput.Close();

                        _JoinImpl();
                        _Process.WaitForExit();
                        var _ExitCode = _Process.ExitCode;

                        if (!string.IsNullOrWhiteSpace(_StdErr)) {
                                Assert.Fail(_StdErr);
                        }
                        Assert.AreEqual(_ExitCode, 0);
                }
                [Test()]
                public void TestCaseRaw ()
                {
                        var psi = new ProcessStartInfo(
                                "js", ""
                                ) {
                                StandardErrorEncoding = System.Text.Encoding.UTF8,

                                RedirectStandardInput = true,
                                RedirectStandardOutput = true,
                                RedirectStandardError = true,
                                CreateNoWindow = true,
                                UseShellExecute = false,
                                WindowStyle = ProcessWindowStyle.Hidden
                        };

                        var stdoutSignal = new ManualResetEventSlim(false);
                        var stderrSignal = new ManualResetEventSlim(false);

                        var _Process = Process.Start(psi);

                        string _StdOut = null, _StdErr = null;
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdOut = _Process.StandardOutput.ReadToEnd();
                                } catch {
                                }
                                stdoutSignal.Set();
                        }));
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdErr = _Process.StandardError.ReadToEnd();
                                } catch {
                                }
                                stderrSignal.Set();
                        }));

                        Action _JoinImpl = () => {
                                stdoutSignal.Wait();
                                stderrSignal.Wait();
                                stderrSignal.Dispose();
                                stderrSignal.Dispose();
                        };
                        //_Process.StandardInput.Encoding = System.Text.Encoding.UTF8;
                        var bytes = System.Text.Encoding.UTF8.GetBytes("var t = {}");
                        _Process.StandardInput.BaseStream.Write(bytes, 0, bytes.Length);
                        _Process.StandardInput.BaseStream.Flush();
                        _Process.StandardInput.BaseStream.Close();

                        _JoinImpl();
                        _Process.WaitForExit();
                        var _ExitCode = _Process.ExitCode;

                        if (!string.IsNullOrWhiteSpace(_StdErr)) {
                                Assert.Fail(_StdErr);
                        }
                        Assert.AreEqual(_ExitCode, 0);
                }


                [Test()]
                public void TestCaseRaw2 ()
                {
                        var psi = new ProcessStartInfo(
                                "js", ""
                                ) {
                                StandardErrorEncoding = System.Text.Encoding.UTF8,

                                RedirectStandardInput = true,
                                RedirectStandardOutput = true,
                                RedirectStandardError = true,
                                CreateNoWindow = true,
                                UseShellExecute = false,
                                WindowStyle = ProcessWindowStyle.Hidden
                        };

                        var stdoutSignal = new ManualResetEventSlim(false);
                        var stderrSignal = new ManualResetEventSlim(false);

                        var _Process = Process.Start(psi);

                        string _StdOut = null, _StdErr = null;
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdOut = _Process.StandardOutput.ReadToEnd();
                                } catch {
                                }
                                stdoutSignal.Set();
                        }));
                        ThreadPool.QueueUserWorkItem(new WaitCallback((_) => {
                                try {
                                        _StdErr = _Process.StandardError.ReadToEnd();
                                } catch {
                                }
                                stderrSignal.Set();
                        }));

                        Action _JoinImpl = () => {
                                stdoutSignal.Wait();
                                stderrSignal.Wait();
                                stderrSignal.Dispose();
                                stderrSignal.Dispose();
                        };
                        //_Process.StandardInput.Encoding = System.Text.Encoding.UTF8;
                        var writer = new System.IO.StreamWriter(_Process.StandardInput.BaseStream, System.Text.Encoding.UTF8);
                        writer.Write("var t = {}");
                        writer.Flush();
                        writer.Close();

                        _JoinImpl();
                        _Process.WaitForExit();
                        var _ExitCode = _Process.ExitCode;

                        if (!string.IsNullOrWhiteSpace(_StdErr)) {
                                Assert.Fail(_StdErr);
                        }
                        Assert.AreEqual(_ExitCode, 0);
                }
        }
}