How to return object from list in constructor

Question

I have retrieved a static table from a database in a list. If someone calls the constructor of that class with an ID, the constructor finds the object in the list and copies all values.

public class MyClass
{
    public int Id { get; set; }
    public string Text { get; set; }

    public MyClass(int instanzId)
    {
        MyClass myClass = CachedList().Find(T => T.Id == instanzId);
        Id = myClass.Id;
        Text = myClass.Text;
    }
}

This will always create a new instance of that object. Is it possible to return the object from the list directly? Like this:

public class MyClass
{
    public int Id { get; set; }
    public string Text { get; set; }

    public MyClass(int instanzId)
    {
        this = CachedList().Find(T => T.Id == instanzId);
    }
}

I know it's easy in a static method, but how can that be done in the constructor?

You can't return any values from a constructor, and you can't assign values to 'this'. — Jyrka98
– Jyrka98, Commented Jul 14, 2015 at 12:11
This will always create a new instance of that object. - and you are expecting what, when calling new MyClass()? — Kuba Wyrostek
– Kuba Wyrostek, Commented Jul 14, 2015 at 12:11
@DanielA.White: At least, not in a class constructor. In a struct constructor you can assign to this... — Jon Skeet
– Jon Skeet, Commented Jul 14, 2015 at 12:11
I would write whatever crap I could think of to have @JonSkeet correct me. :) — Kuba Wyrostek
– Kuba Wyrostek, Commented Jul 14, 2015 at 12:13
@MichaelK, you previously said (implied) that all the values were cached. How does using a factory method reveal whether it's cached? — David Arno
– David Arno, Commented Jul 14, 2015 at 12:30

RvdK · Accepted Answer · 2015-07-14 12:13:22Z

5

Well no, not via constructor. You can do it via a method. For example a factory:

public MyClass FindOrCreate(int instanceId)
{
    MyClass obj = CachedList().Find(T => T.Id == instanzId);
    //create obj when it does not exist

    return obj;
}

answered Jul 14, 2015 at 12:13

RvdK

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2 Comments

David Arno Over a year ago

@MichaelK, it's your only option if you don't want to create a new instance.

MichaelK Over a year ago

Unfortunately. I thought there was a better way than copying the properties.

Dmitrii Bychenko · Accepted Answer · 2015-07-14 12:36:15Z

2

Wrap cache into just another lightweight class:

  public class MyWrapClass {
    private MyClass m_Cache;

    public MyWrapClass() {
      m_Cache = CachedList().Find(T => T.Id == instanzId);
      ...
    }
    ...

    // implicit cast to MyClass if you want it 
    public static implicit operator MyClass(MyWrapClass value) {
      if (null == value)
        return null;
      else
        return value.m_Cache;
    }
  }

  ...
  // Or MyWrapClass test = new MyWrapClass();
  MyClass test = new MyWrapClass();

edited Jul 14, 2015 at 12:36

answered Jul 14, 2015 at 12:22

Dmitrii Bychenko

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1 Comment

Kuba Wyrostek Over a year ago

except the clear wtf? when reading MyClass test = new MyWrapClass(); months later I really like the cleverness of this attempt.

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