Object reference not set to an instance of an object. OOP C#

I add a functional code; I have edited it in a project changing certain things in your code. I have added as a class CApartamento, which will serve as a model to organize the data. Your example has several bugs that have already been mentioned in other answers, as well as in my comment on your question. One of the main ones, which I think you're asking your question about, is that when you initialize the array inmobiliaria and send it to your function Check, you don't check if the position has already been initialized: If your array contains 5 values, but you're at position 0, the following 1, 2, 3 and 4 have not been initialized yet so it will give you an error. However, and as you mention that you are learning, you could take a look at the use of lists , which in your case, would be the option that I would choose.

namespace CApartamento
{
class Program
{

    static void Main(string[] args)
    {

        Console.Write("Cuantos Registros?: ");

        CApartamento[] inmobiliaria = new CApartamento[Convert.ToInt32(Console.ReadLine())];


        for (int i = 0; i < inmobiliaria.Length; i++)
        {
            inmobiliaria[i] = new CApartamento();
            Console.Write("CLAVE [A-###]: ");
            String temporary_key = Console.ReadLine();

            while (Check(inmobiliaria, temporary_key))
            {
                Console.WriteLine("    .------------------.");
                Console.WriteLine("    + INGRESANDO DATOS +");
                Console.WriteLine("    .------------------.");
                Console.WriteLine("\n");

                Console.Write("CLAVE [A-###]: ");
                temporary_key = Console.ReadLine();
            }

            inmobiliaria[i].clave = temporary_key;
        }
    }


    private static bool Check(CApartamento[] inmobiliaria, string temporary_key)
    {
        for (int i = 0; i < inmobiliaria.Length; i++)
        {
            if (inmobiliaria[i] != null && inmobiliaria[i].clave != null && inmobiliaria[i].clave.CompareTo(temporary_key) == 0)
            {
                return true;
            }
        }
        return false;
    }


}


public class CApartamento
{
    public String clave;
    /* otros objetos*/



}
}

Your code may be generating the error, because you have instantiated the variable inmobiliariawith a capacity, and you have not assigned values ​​for each of its components, when you refer to one of its components and it is null, it throws the error, but also your method Checkcontains another obvious error and that is that you declare and assign a value trueto the variable existeand then within the block ifyou assign a new value to it trueas a result, your method Checkwill always return truewhatever the result of the conditional is if. With the following code, none of those errors will occur.

   private bool Check(CApartamento[] inmobiliaria, string temporary_key)
           {
              //Se declara y se asigna valor false a la variable existe 
              bool existe = false;

              for(int i=0; i<inmobiliaria.Length;)
              {
                  if(inmobiliaria[i] != null && inmobiliaria[i].clave.CompareTo(temporary_key) == 0) 
///aca se verifica que el valor inmobiliaria[i].clave sea diferente de nulo, evitando el error
                  {
                    //Se asigna valor true a la variable existe.
                     existe = true;
                  }
              }

             return existe;
           } 

The fault may be in the first three lines of code:

Console.Write("CLAVE [A-###]: ");
inmobiliaria[i].clave = Console.ReadLine();
temporary_key = inmobiliaria[i].clave;

What you read from the console you already try to save in real estate[i].key and I think it should be like this:

Console.Write("CLAVE [A-###]: ");
temporary_key = = Console.ReadLine();

After having the data entered in temporary_key is when you check if said element already exists in your real estate[] array. Please note Eduardo Reyes's corrections in the Check function.

I hope I have been of help although you have only shown part of the code, we do not know where that i comes from, nor its value nor what function the conditional j==i has.

Okay; understanding a little more what you want I think it's a matter of organization. I would do it with List< T > and I would pose it like this:

class CApartamento : IEquatable<CApartamento>
{
    public string Clave;
    public string Descripcion;
    public ...

    // Constructor
    public CApartamento(string str_Key)
    {
         Clave = str_Key;
    }

    // Permite la comparación de dos objetos CApartamento
    public bool Equals(CApartamento other)
    {
       return (this.Clave == other.Clave);
    }
}

// Esta quedaría obsoleta si usamos List<T>.Contains()
private bool ExistKey(List<CApartamento> inmobiliaria, string temporary_key)
{
    foreach(CApartamento CA in inmobiliaria)
    {
        if (CA.Clave == temporary_key) return true;      
    }
    return false;
}

Then in your program body or method in charge we put:

List<CApartamento> inmobiliaria = new List<CApartamento>();

bool SeguirMetiendoDatos = true;

while( SeguirMetiendoDatos == true )
{
    // Introducimos una clave
    Console.Write("CLAVE [A-###]: ");
    temporary_key = Console.ReadLine();
    // Creamos nuestro objeto provisional con la clave introducida
    CApartamento CA = new CApartamento(temporary_key);

    // *** if (ExistKey(inmobiliaria, temporary_key)  **
    // Este IF es innecesario ya que dotamos a nuestra clase
    // con la capacidad de compararse y por lo tanto podemos usar
    // el método Contains de la lista inmobiliaria

    if (inmobiliaria.Contains(CA))
    {
        Console.WriteLine("El elemento ya existe, introduzca otra clave.");
    }
    else // Si no existe lo añadimos a la lista
    {
         // Rellenamos nuesto objeto CApartamento con
         // el resto de datos antes de añadirlo a la lista.
         CA.Descripcion = ...;

         // Lo incluimos en la lista  
         inmobiliaria.Add(CA);

         Console.Write("Introducir otra clave (S/N): ");
         string respuesta = Console.ReadLine();

         SeguirMetiendoDatos = (respuesta.ToUpper() == "S");
    }
}

The fact of using List allows us to perform operations such as dynamically adding or deleting elements from the list, ordering them... at the same time that it allows us to simplify the code and add readability to it.