// Loads all available audio devices into the available_*_devices lists.
        static AudioDeviceEnumerator()
        {
            IntPtr        dummy_device  = IntPtr.Zero;
            ContextHandle dummy_context = ContextHandle.Zero;

            try
            {
                Debug.WriteLine("Enumerating audio devices.");
                Debug.Indent();

                // need a dummy context for correct results
                dummy_device  = Alc.OpenDevice(null);
                dummy_context = Alc.CreateContext(dummy_device, (int[])null);
                bool     dummy_success = Alc.MakeContextCurrent(dummy_context);
                AlcError dummy_error   = Alc.GetError(dummy_device);
                if (!dummy_success || dummy_error != AlcError.NoError)
                {
                    throw new AudioContextException("Failed to create dummy Context. Device (" + dummy_device.ToString() +
                                                    ") Context (" + dummy_context.Handle.ToString() +
                                                    ") MakeContextCurrent " + (dummy_success ? "succeeded" : "failed") +
                                                    ", Alc Error (" + dummy_error.ToString() + ") " + Alc.GetString(IntPtr.Zero, (AlcGetString)dummy_error));
                }

                // Get a list of all known playback devices, using best extension available
                if (Alc.IsExtensionPresent(IntPtr.Zero, "ALC_ENUMERATION_EXT"))
                {
                    version = AlcVersion.Alc1_1;
                    if (Alc.IsExtensionPresent(IntPtr.Zero, "ALC_ENUMERATE_ALL_EXT"))
                    {
                        available_playback_devices.AddRange(Alc.GetString(IntPtr.Zero, AlcGetStringList.AllDevicesSpecifier));
                        default_playback_device = Alc.GetString(IntPtr.Zero, AlcGetString.DefaultAllDevicesSpecifier);
                    }
                    else
                    {
                        available_playback_devices.AddRange(Alc.GetString(IntPtr.Zero, AlcGetStringList.DeviceSpecifier));
                        default_playback_device = Alc.GetString(IntPtr.Zero, AlcGetString.DefaultDeviceSpecifier);
                    }
                }
                else
                {
                    version = AlcVersion.Alc1_0;
                    Debug.Print("Device enumeration extension not available. Failed to enumerate playback devices.");
                }
                AlcError playback_err = Alc.GetError(dummy_device);
                if (playback_err != AlcError.NoError)
                {
                    throw new AudioContextException("Alc Error occured when querying available playback devices. " + playback_err.ToString());
                }

                // Get a list of all known recording devices, at least ALC_ENUMERATION_EXT is needed too
                if (version == AlcVersion.Alc1_1 && Alc.IsExtensionPresent(IntPtr.Zero, "ALC_EXT_CAPTURE"))
                {
                    available_recording_devices.AddRange(Alc.GetString(IntPtr.Zero, AlcGetStringList.CaptureDeviceSpecifier));
                    default_recording_device = Alc.GetString(IntPtr.Zero, AlcGetString.CaptureDefaultDeviceSpecifier);
                }
                else
                {
                    Debug.Print("Capture extension not available. Failed to enumerate recording devices.");
                }
                AlcError record_err = Alc.GetError(dummy_device);
                if (record_err != AlcError.NoError)
                {
                    throw new AudioContextException("Alc Error occured when querying available recording devices. " + record_err.ToString());
                }

#if DEBUG
                Debug.WriteLine("Found playback devices:");
                foreach (string s in available_playback_devices)
                {
                    Debug.WriteLine(s);
                }

                Debug.WriteLine("Default playback device: " + default_playback_device);

                Debug.WriteLine("Found recording devices:");
                foreach (string s in available_recording_devices)
                {
                    Debug.WriteLine(s);
                }

                Debug.WriteLine("Default recording device: " + default_recording_device);
#endif
            }
            catch (DllNotFoundException e)
            {
                Trace.WriteLine(e.ToString());
                openal_supported = false;
            }
            catch (AudioContextException ace)
            {
                Trace.WriteLine(ace.ToString());
                openal_supported = false;
            }
            finally
            {
                Debug.Unindent();

                if (openal_supported)
                {
                    try
                    {
                        // clean up the dummy context
                        Alc.MakeContextCurrent(ContextHandle.Zero);
                        if (dummy_context != ContextHandle.Zero && dummy_context.Handle != IntPtr.Zero)
                        {
                            Alc.DestroyContext(dummy_context);
                        }
                        if (dummy_device != IntPtr.Zero)
                        {
                            Alc.CloseDevice(dummy_device);
                        }
                    }
                    catch
                    {
                        openal_supported = false;
                    }
                }
            }
        }
        /// \internal
        /// <summary>Creates the audio context using the specified device.</summary>
        /// <param name="device">The device descriptor obtained through AudioContext.AvailableDevices, or null for the default device.</param>
        /// <param name="freq">Frequency for mixing output buffer, in units of Hz. Pass 0 for driver default.</param>
        /// <param name="refresh">Refresh intervals, in units of Hz. Pass 0 for driver default.</param>
        /// <param name="sync">Flag, indicating a synchronous context.</param>
        /// <param name="enableEfx">Indicates whether the EFX extension should be initialized, if present.</param>
        /// <param name="efxAuxiliarySends">Requires EFX enabled. The number of desired Auxiliary Sends per source.</param>
        /// <exception cref="ArgumentOutOfRangeException">Occurs when a specified parameter is invalid.</exception>
        /// <exception cref="AudioDeviceException">
        /// Occurs when the specified device is not available, or is in use by another program.
        /// </exception>
        /// <exception cref="AudioContextException">
        /// Occurs when an audio context could not be created with the specified parameters.
        /// </exception>
        /// <exception cref="NotSupportedException">
        /// Occurs when an AudioContext already exists.</exception>
        /// <remarks>
        /// <para>For maximum compatibility, you are strongly recommended to use the default constructor.</para>
        /// <para>Multiple AudioContexts are not supported at this point.</para>
        /// <para>
        /// The number of auxilliary EFX sends depends on the audio hardware and drivers. Most Realtek devices, as well
        /// as the Creative SB Live!, support 1 auxilliary send. Creative's Audigy and X-Fi series support 4 sends.
        /// Values higher than supported will be clamped by the driver.
        /// </para>
        /// </remarks>
        void CreateContext(string device, int freq, int refresh, bool sync, bool enableEfx, MaxAuxiliarySends efxAuxiliarySends)
        {
            if (!AudioDeviceEnumerator.IsOpenALSupported)
            {
                throw new DllNotFoundException("openal32.dll");
            }

            if (AudioDeviceEnumerator.Version == AudioDeviceEnumerator.AlcVersion.Alc1_1 && AudioDeviceEnumerator.AvailablePlaybackDevices.Count == 0)    // Alc 1.0 does not support device enumeration.
            {
                throw new NotSupportedException("No audio hardware is available.");
            }
            if (context_exists)
            {
                throw new NotSupportedException("Multiple AudioContexts are not supported.");
            }
            if (freq < 0)
            {
                throw new ArgumentOutOfRangeException("freq", freq, "Should be greater than zero.");
            }
            if (refresh < 0)
            {
                throw new ArgumentOutOfRangeException("refresh", refresh, "Should be greater than zero.");
            }


            if (!String.IsNullOrEmpty(device))
            {
                device_name   = device;
                device_handle = Alc.OpenDevice(device); // try to open device by name
            }
            if (device_handle == IntPtr.Zero)
            {
                device_name   = "IntPtr.Zero (null string)";
                device_handle = Alc.OpenDevice(null); // try to open unnamed default device
            }
            if (device_handle == IntPtr.Zero)
            {
                device_name   = AudioContext.DefaultDevice;
                device_handle = Alc.OpenDevice(AudioContext.DefaultDevice); // try to open named default device
            }
            if (device_handle == IntPtr.Zero)
            {
                device_name = "None";
                throw new AudioDeviceException(String.Format("Audio device '{0}' does not exist or is tied up by another application.",
                                                             String.IsNullOrEmpty(device) ? "default" : device));
            }

            CheckErrors();

            // Build the attribute list
            List <int> attributes = new List <int>();

            if (freq != 0)
            {
                attributes.Add((int)AlcContextAttributes.Frequency);
                attributes.Add(freq);
            }

            if (refresh != 0)
            {
                attributes.Add((int)AlcContextAttributes.Refresh);
                attributes.Add(refresh);
            }

            attributes.Add((int)AlcContextAttributes.Sync);
            attributes.Add(sync ? 1 : 0);

            if (enableEfx && Alc.IsExtensionPresent(device_handle, "ALC_EXT_EFX"))
            {
                int num_slots;
                switch (efxAuxiliarySends)
                {
                case MaxAuxiliarySends.One:
                case MaxAuxiliarySends.Two:
                case MaxAuxiliarySends.Three:
                case MaxAuxiliarySends.Four:
                    num_slots = (int)efxAuxiliarySends;
                    break;

                default:
                case MaxAuxiliarySends.UseDriverDefault:
                    Alc.GetInteger(device_handle, AlcGetInteger.EfxMaxAuxiliarySends, 1, out num_slots);
                    break;
                }

                attributes.Add((int)AlcContextAttributes.EfxMaxAuxiliarySends);
                attributes.Add(num_slots);
            }
            attributes.Add(0);

            context_handle = Alc.CreateContext(device_handle, attributes.ToArray());

            if (context_handle == ContextHandle.Zero)
            {
                Alc.CloseDevice(device_handle);
                throw new AudioContextException("The audio context could not be created with the specified parameters.");
            }

            CheckErrors();

            // HACK: OpenAL SI on Linux/ALSA crashes on MakeCurrent. This hack avoids calling MakeCurrent when
            // an old OpenAL version is detect - it may affect outdated OpenAL versions different than OpenAL SI,
            // but it looks like a good compromise for now.
            if (AudioDeviceEnumerator.AvailablePlaybackDevices.Count > 0)
            {
                MakeCurrent();
            }

            CheckErrors();

            device_name = Alc.GetString(device_handle, AlcGetString.DeviceSpecifier);


            lock (audio_context_lock)
            {
                available_contexts.Add(this.context_handle, this);
                context_exists = true;
            }
        }