System.NullReferenceException - 未将对象引用设置为对象的实例

Question

When I try to run the program, it throws me this error: Object reference not set to an instance of an object. Is there something wrong with the code? Help!

private void InitSpeechRecognition()
        {

            _audioSource = new KinectAudioSource
            {
                FeatureMode = true,
                AutomaticGainControl = false,
                SystemMode = SystemMode.OptibeamArrayOnly
            };
            var ri =
              SpeechRecognitionEngine.InstalledRecognizers().
                Where(r => r.Id == RecognizerId).FirstOrDefault();
            _engine = new SpeechRecognitionEngine(ri.Id);
            var gb = new GrammarBuilder { Culture = new CultureInfo("en-US") };


            gb.Append(CommandMessage.Choices);


            var g = new Grammar(gb);
            _engine.LoadGrammar(g);
            _engine.SpeechRecognized += SreSpeechRecognized;

            _audioStream = _audioSource.Start();
            _engine.SetInputToAudioStream(_audioStream,
                                          new SpeechAudioFormatInfo(
                                          EncodingFormat.Pcm, 16000, 16, 1,
                                          32000, 2, null));
            _engine.RecognizeAsync(RecognizeMode.Multiple);
        }

        private void SreSpeechRecognized(object sender, SpeechRecognizedEventArgs e)
        {

            if (CommandMessage.Commands.ContainsKey(e.Result.Text))
            {
                Application.Current.Dispatcher.Invoke(DispatcherPriority.Normal,
                  new Action(() =>
                    Messenger.Default.Send(
                       new CommandMessage { Command = CommandMessage.Commands[e.Result.Text] })));
            }
        }

Answer 1

var ri = SpeechRecognitionEngine.InstalledRecognizers().
            Where(r => r.Id == RecognizerId).FirstOrDefault();

Here, FirstOrDefault() either returns the first object in the sequence or null if no match found. Then in the next line you are trying to access the Id property of the object, which may be null.

Perform a check for null after the assignment, and only use ri if it is not null. Eg

  if (ri != null)
  {
    ...
  } 

Or use the First() method instead of FirstOrDefault() and surround the code with a try catch block to handle the case when the sequence is empty.

But the reason why you are getting null is probably where the real problem is. ri is null because there are no installed recognizers with the id of RecognizerId. I don't see the code where you are setting it, so look around that part.

Also, take a look at the example on this page, it might contain just what you need: http://msdn.microsoft.com/en-us/library/system.speech.recognition.speechrecognitionengine.installedrecognizers.aspx