我的某些班级字段是必需的。 我怎么知道它已初始化？

Question

Ok guys. Im writing code and have one problem. Language is C# but it doesnot matter. For example: I have class Point

Class Point
{
 private _x;
 private _y;

 //getter and setters
}

So, if i want to use object of this class i need to totally know that all fields, like _x and _y was initialized. Default values is not solving. My solutions:

• pass all required paramaters through constructor and initialize
• Always check if my field is not null (i think this is the bad way)

But if i have 7 required fields.

Class Point
{
 private _x; //required it must not be null
 private _y; //required it must not be null
 private _z; //required it must not be null
 private _b; //required it must not be null
 private _a; //required it must not be null
 private _f; //required it must not be null
 private _h; //required it must not be null

 //getter and setters
}

How i can be sure that all of them was initialized? Passing this count of paramaters in constructor is really hard and ugly. If i will use builder pattern, it actually not make me sure. Sorry for bad english.

Answer 1

As people have commented, you can ensure that the fields are initialized by either setting them to always have a default value

Class Point
{
    private int j = 42;
}

or have your constructor always define a default value for each field

Class Point
{
    private int j;
    public Point (int jVal)
    {
       j = -1;
    }
}

This will ensure that whenever a new instance of the class is created, the fields will always be initialized.

If you have references that need to point to objects, you can do it the same way, but just change the value (such as 42 in the previous example) to the language's equivalent of C#'s new Object() .

Answer 2

For knowing if the field was initialised you are force to using a flag.
Make it explicit bool _xIsSet; or if _x is an int then make the type int? and check for the value being null.
The former does not throw any exception but if you are not careful you will work with non-initialised values.
The later way throws errors; possibly better but still at run time.

You could decide for properties instead and wrap the fields with checks for null but the result would be the same; any error would surface run time. (or not at all)

The method I prefer since 20? years is to have a constructor that takes all parameters and even make sure there is no default constructor.

var myPoint = new Point( valueX, valueY, valueK );

With later C# versions one can name the arguments so the code becomes quite easy to read.

var myPoint = new Point( X = valueX, Y = valueY, L = valueK // <-- see here! it is quite easy to spot a possible error );

---

My explanation for using this method, except for the one you state, is that "It is very seldom you want a Customer that is not set fully.". Then... if there are several ways to set a Customer fully I create more constructors.

I have taken this further and avoid null as much as I can, so in my code there is never a Customer that is only half set and only rarely a Customer that is null.

Lately I have started experimenting with using static constructors instead; the call is then var customer = Customer.Create(...) . This has the taste of DDD as Create can express the intention , can express why a class is created. var customer = Customer.CreateFromEntranceScan(...) or var customer = Customer.CreateFromTaxDeduction(...) .