[c#] Properties private set;

I know it only allows the class to set it, but what is the point?

How do I solve the problem of having readonly ids?

Say I have a person class:

public class Person
    {
        public string Name { get;  set; }
        public int Id { get; private set; }
        public int Age { get; set; }
    }

And this is in an Entities.dll, used by a GUI, BL and DAL.

The GUI calls the BL:

   List<Person> p =  BL.PeopleBL.GetPeople();

For the sake of the example calls the DAL:

...
while(dr.read())
{
    returnPersonList.add( new Person{ Age=dr.GetInt32(1), Id=dr.GetInt32(0), Name=dr.GetString(2)})
}
...

of course I cannot do that cause Id is a private set; What is the proper way to do this?

How can I let the BL/Dal set the Id, but not on the GUI?

Or is this not even the proper use of a private set?

I just wanted to add that this is your typical DB app, where the pk is the Id and should not be changed( only by the BL/DAL)

The two common approaches are either that the class should have a constructor for the DAL to use, or the DAL should use reflection to hydrate objects.

This is normally the case then the ID is not a natural part of the entity, but a database artifact that needs be abstracted away.

It is a design decision - to only allow setting the ID during construction or through method invocation, so it is managed internally by the class.

You can write a setter yourself, assuming you have a backing field:

private int Id = 0;
public void SetId (int id)
{
  this.Id = id;
}

Or through a constructor:

private int Id = 0;
public Person (int id)
{
  this.Id = id;
}

while(dr.read())
{
    returnPersonList.add( 
        new Person(dr.GetInt32(1), dr.GetInt32(0), dr.GetString(2)));
}

where:

public class Person
{
    public Person(int age, int id, string name) 
    {
        Age = age;
        Id = id;
        Name = name;
    }
}

Depending on the scope of my application, I like to put the object hydration mechanisms in the object itself. I'll wrap the data reader with a custom object and pass it a delegate that gets executed once the query returns. The delegate gets passed the DataReader. Then, since I'm in my smart business object, I can hydrate away with my private setters.

Edit for Pseudo-Code

The "DataAccessWrapper" wraps all of the connection and object lifecycle management for me. So, when I call "ExecuteDataReader," it creates the connection, with the passed proc (there's an overload for params,) executes it, executes the delegate and then cleans up after itself.

public class User
{
    public static List<User> GetAllUsers()
    {
        DataAccessWrapper daw = new DataAccessWrapper();
        return (List<User>)(daw.ExecuteDataReader("MyProc", new ReaderDelegate(ReadList)));
    }

    protected static object ReadList(SQLDataReader dr)
    {
        List<User> retVal = new List<User>();
        while(dr.Read())
        {
            User temp = new User();
            temp.Prop1 = dr.GetString("Prop1");
            temp.Prop2 = dr.GetInt("Prop2");
            retVal.Add(temp);
        }
        return retVal;
    }
}

Perhaps, you can have them marked as internal, and in this case only classes in your DAL or BL (assuming they are separate dlls) would be able to set it.

You could also supply a constructor that takes the fields and then only exposes them as properties.

Maybe I'm misunderstanding, but if you want truly readonly Ids why not use an actual readonly field?

public class Person
{
   public Person(int id)
   {
      m_id = id;
   }

   readonly int m_id;
   public int Id { get { return m_id; } }
}

Or you can do

public class Person
{
    public Person(int id)
    {
         this.Id=id;
    }

    public string Name { get;  set; }
    public int Id { get; private set; }
    public int Age { get; set; }
}

You can let the user set a read-only property by providing it through the constructor:

public class Person
{
    public Person(int id)
    {
        this.Id = id;
    }

    public string Name { get;  set; }
    public int Id { get; private set; }
    public int Age { get; set; }
}