speech recognition in C#

Well, you can try what the Microsoft Speech recognition engine could do about that using System.Speech.Recognition.SpeechRecognitionEngine.

Yes, dictating lengthy sentence is quite different from command recognition. The key difference is using the DictationGrammar. Try it with console application. Here is the application entry point:

using System.Speech.Recognition;

//...

[System.MTAThread] //IMPORTANT!
static void Main() {
    SpeechRecognitionEngine recognizer = new SpeechRecognitionEngine();
    Grammar dictationGrammar = new DictationGrammar();
    recognizer.LoadGrammar(dictationGrammar);
    try {
        System.Console.WriteLine("Start talking!");
        recognizer.SetInputToDefaultAudioDevice();
        RecognitionResult result = recognizer.Recognize();
        System.Console.WriteLine(result.Text);
    } catch (InvalidOperationException exception) {
        System.Console.WriteLine(
            String.Format(
                "Could not recognize input; {0}: '{1}'.",
                 exception.GetType(), exception.Message));
    } finally {
        recognizer.UnloadAllGrammars();
    }
    System.Console.WriteLine("Done. Press any key...");
    System.Console.ReadKey(true);
}

While the recognition with command-based grammar works amazingly well, you may find dictation results frustrating…
Not my fault. :-)

—SA