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NaCl
Asked: 2020-04-15 08:05:32 +0800 CST 2020-04-15 08:05:32 +0800 CST

Why are fields of two variables of the same type affected?

  • 772

Good! I have the following code:

public class Persona
{
    // ...
    public string Apellido;
}

public class Program 
{
    public static void Main()
    {
        Persona A = new Persona(); A.Apellido = "Mendez";
        Persona B = A; // El apellido debe ser Mendez.
        //Pero...
        Console.WriteLine("Apellido de A: " + A.Apellido); // Mendez ahora.
        A.Apellido = "Castillo"; // Si por algún "error" debo cambiarlo.
        Console.WriteLine("Nuevo Apellido de A: " + A.Apellido); // Castillo ahora.
        Console.WriteLine("Apellido de B: " + B.Apellido); // Castillo igual.
    }
}

Why is the field Apellidoin variable B affected by the change in variable A and vice versa?

c#

3 Answers

  1. What happens is that class variables only hold a reference to the instance (similar to pointers) so you have a single instance that you can access through two different variables, it goes something like this:

    enter image description here

    When you assign it to another variable it does not mean that a new instance is created and the values ​​of the fields are copied.

    • 9
  2. Best Answer

    It is affected because objects are assigned by reference. When you assign like this

    Persona B = A;

    the instance of B is a pointer to that of A, any change you make to either variable will affect the other.

    To avoid this problem you must create a different instance or create a clone.

    How to: Write a Copy Constructor (C# Programming Guide)

    analyze in the example how it defines a constructor to create a new copy from an existing one.

    You could also analyze the use of the

    Object.MemberwiseClone (Method)

    you could create a copy in another instance.

    • 5

  3. Because you are using the same instance of A for B;

     Persona A = new Persona(); A.Apellido = "Mendez";
 Persona B = A; // El apellido debe ser Mendez.

    When changing some property in A, the changes are reflected in B.

    To avoid problems you can create another instance of the class Persona()for variable B:

     Persona A = new Persona(); 
 Persona B = new Persona();

    Or create a clone, you can create a method to copy an object, this is an example:

    Objeto copyObjeto (Objeto objeto){
  Objeto f = new Objeto();
  //Para todas las propiedades de 
  f.set(objeto.get());
  return f;
}

    About Copy Constructor (English)

    • 4