using System.Speech.Recognition;

// ...

SpeechRecognitionEngine recognitionEngine = new SpeechRecognitionEngine();
recognitionEngine.LoadGrammar(new DictationGrammar());
recognitionEngine.SpeechRecognized += delegate(object sender, SpeechRecognizedEventArgs e)
{
    Console.WriteLine("You said: {0}", e.Result.Text);
};
recognitionEngine.SetInputToDefaultAudioDevice();
recognitionEngine.RecognizeAsync(RecognizeMode.Multiple);