_recognizer = new SpeechRecognitionEngine();
_recognizer.RequestRecognizerUpdate();
_recognizer.LoadGrammar(new Grammar(new GrammarBuilder("hello computer")));
_recognizer.SpeechRecognized += _recognizer_SpeechRecognized;
_recognizer.SetInputToDefaultAudioDevice();
_recognizer.RecognizeAsync(RecognizeMode.Multiple);

void _recognizer_SpeechRecognized(object sender, SpeechRecognizedEventArgs e)
{
     if (e.Result.Text == "hello computer")
     {
           saidhello = true;
     }
}