Questions[c#]

How can I subtract two dates in C#?

I have two variables DateTimeand I need to get the difference between them in hours.

DateTime fecharegistro = {04/05/2018 8:34:01} //obtenemos este valor de una base de datos
DateTime fechafin = DateTime.Now.Substract(fechaRegistro);

When compiling the above code I get an error saying that an object cannot be implicitly converted TimeStampto System.DateTime, when the variable is DateTime.

On numerous occasions, a , is received NullReferenceException, being one of the exceptions that raises the most questions. What does it mean NullReferenceExceptionand how can it be fixed?

_{For the NullReferenceExceptionin VB.NET, see NullReferenceException: What is it and how can I fix it?}

I have the following code to recognize voice in Spanish, the problem is that there are several words which it does not recognize correctly.

using System;
using System.Speech.Recognition;

namespace SpeechRecognitionApp
{
    class Program
    {
        static void Main(string[] args)
        {

            using (
            SpeechRecognitionEngine recognizer =
              new SpeechRecognitionEngine(
                new System.Globalization.CultureInfo("es-ES")))
            {

                recognizer.LoadGrammar(new DictationGrammar());

                recognizer.SpeechRecognized +=
                  new EventHandler<SpeechRecognizedEventArgs>(recognizer_SpeechRecognized);

                recognizer.SetInputToDefaultAudioDevice();

                recognizer.RecognizeAsync(RecognizeMode.Multiple);

                while (true)
                {
                    Console.ReadLine();
                }
            }
        }

        static void recognizer_SpeechRecognized(object sender, SpeechRecognizedEventArgs e)
        {
            Console.WriteLine("Recognized text: " + e.Result.Text);
        }
    }
}

I have also tried to define the words that it can recognize, in this way it is never wrong, the drawback is that the number of words that it recognizes is limited to the number of words that I define.

Choices colors = new Choices();
                colors.Add(new string[] { "azul", "rojo", "verde" });

                      // Create a GrammarBuilder object and append the Choices object.
                      GrammarBuilder gb = new GrammarBuilder();
                gb.Append(colors);

                      // Create the Grammar instance and load it into the speech recognition engine.
                      Grammar g = new Grammar(gb);


                // Create and load a dictation grammar.
                recognizer.LoadGrammar(g);

Is there a way to recognize multiple words without getting it wrong and without being limited by the defined words?

I had to make a web application and based on certain business rules send SMSthem to clients notifying them of their movements, at that time when investigating I found several pages with apisor services where you make a call sending them certain information and they send the SMS, and charge you for SMS.

So I started to think how I could do that service myself, thinking of paying for a plan with SMSunlimited

I have been researching about delegates in C# but the truth is not that clear to me yet.

Can someone explain to me what it is about?