Relationship Have-one in C#?

Question

I tried to do a have-one relationship in C# and I’m not getting it and I don’t even know if it’s recommended to do.

Follow my error code:

System.Nullreferenceexception Undefined object reference for an instance of an object.

class Program
{
    static void Main(string[] args)
    {
        Cliente cliente = new Cliente();
        cliente.endereco.rua = "guaranesia";

        Console.WriteLine(cliente.endereco.rua);
        Console.ReadKey();
    }
}

class Cliente
{
    public string name{get; set;}
    public string idade { get; set; }
    public Endereco endereco;
}

class Endereco
{
    public string rua { get; set; }
    public string bairro { get; set; }
}

Answer 1

I would do so:

using static System.Console;

public class Program {
    public static void Main(string[] args) {
        Cliente cliente = new Cliente();
        cliente.endereco.rua = "guaranesia";
        WriteLine(cliente.endereco.rua);
    }
}

class Cliente {
    public string name{get; set;}
    public string idade { get; set; }
    public Endereco endereco = new Endereco();
}

class Endereco {
    public string rua { get; set; }
    public string bairro { get; set; }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

Make the class work for you. Encapsulate the implementation. Make the consumer cleaner and don’t force him to be aware of how he should use the parts of the class. You used the tag encapsulation which shows that you understand that you should use this form, now you need to implement it like this. You should also consider making the field your own endereco as a property.

In fact in a real class very likely you should have builders. Something like that:

using static System.Console;

public class Program {
    public static void Main(string[] args) {
         var cliente = new Cliente("João", "20", "Rua Torta", "Centro");
         WriteLine(cliente.Endereco.Rua);

    }
}

class Cliente {
    public string Nome {get; set;}
    public string Idade {get; set;}
    public Endereco Endereco {get; set;}

    public Cliente(string nome, string idade, string rua, string bairro) {
        Nome = nome;
        Idade = idade;
        Endereco = new Endereco(rua, bairro);
    }
}

class Endereco {
    public string Rua {get; set;}
    public string Bairro {get; set;}
    //Note que não é preciso criar um construtor Endereco() já que ele não fará nada extra
    //O compilador criará um para você
    public Endereco(string rua, string bairro) {
        Rua = rua;
        Bairro = bairro;
    }
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

Answer 2

You have to initialize the property endereco before you can use it.

The best place to do it is in the constructor.

class Cliente
{
    public string Name { get; set; }
    public string Idade { get; set; }
    public Endereco Endereco { get; set; }

    public Cliente()
    {
        Endereco = new Endereco();
    }
}

Two notes:

• In C#, the convention is 'to use Pascalcase for property names
• Almost never should be exposed Fields publicos! In the above code, replace the field with a property (note the {get; set;}).

To read: Jon Skeet - Why Properties Matter

Answer 3

you have to instance address first:

static void Main(string[] args)
{
    var cliente = new Cliente();
    cliente.endereco = new Endereco();
    cliente.endereco.rua = "guaranesia";

    Console.WriteLine(cliente.endereco.rua);
    Console.ReadKey();
}