using System;

public class Program

{

    public static void Main()

    {

        bool isBetween;

        DateTime dataInicial = new DateTime(2016, 09, 30);

        DateTime dataFinal = new DateTime(2016, 11, 30);

        DateTime dataAtual = DateTime.Now;

        isBetween = dataAtual.IsBetween(dataInicial, dataFinal);

        Console.WriteLine("Exemplo 1:\ndataAtual.IsBetween(dataInicial, dataFinal) : " + isBetween);

        DateTime? dataNula = null;

        isBetween = dataAtual.IsBetweenNullable(dataNula, dataNula); //Inconsistência

        Console.WriteLine("\nExemplo 2:\ndataAtual.IsBetweenNullable(dataNula, dataNula) : " + isBetween);

        //O correto seria:

        DateTime? dataInicialNullable = new DateTime(2016, 09, 30);

        if(dataInicialNullable.HasValue) //verifica se o valor não é nulo

        {

            isBetween = dataAtual.IsBetween(dataInicialNullable.Value, dataFinal);  //acessa o valor guardado pelo nullable

        }

        Console.WriteLine("\nExemplo 3:\ndataAtual.IsBetween(dataInicialNullable.Value, dataFinal) : " + isBetween);

    }

}

​

public static class ExtensionMethods

{

    public static bool IsBetween(this DateTime dataAtual, DateTime dataInicial, DateTime dataFinal)

    {

        return dataAtual >= dataInicial && dataAtual <= dataFinal;

    }

    public static bool IsBetweenNullable(this DateTime dataAtual, DateTime? dataInicial, DateTime? dataFinal)

    {

        return dataAtual >= dataInicial && dataAtual <= dataFinal;

    }

}