Should a c# class generate instances of itself?

Question

I have a class that defines a CallRate type. I need to add the ability to create multiple instances of my class by reading the data from a file.

I added a static method to my class CallRate that returns a List<CallRate>. Is it ok for a class to generate new instances of itself by calling one of its own constructors? It works, I just wonder if it's the proper thing to do.

List<CallRates> cr = CallRates.ProcessCallsFile(file);

Answer 1

It is perfectly fine to get object(s) of its own from the static method.

e.g.

One of the dot net libraries does the same thing as you did,

XmlReadrer reader = XmlReader.Create(filepathString);

Answer 2

Sure that's fine, even encouraged in some instances. There are several design patterns that deal with object creation, and a few of them do just what you're describing.

Answer 3

I often use this pattern when I need to check the validity of parameters. It is strongly discouraged to throw an exception from a constructor. It's not so bad from a factory method, or you can choose to return null.

Answer 4

Seems fine to me. In other languages you would probably write a function, but in a language like C#, static methods take up that role.

Answer 5

It is ok. What you just created is something like a simple factory method. You have a static method that creates a valid instance of a type. Actually your method doesn't even have to be static and you still have a valid code. There is a design pattern (Prototype) that creates a new valid object from an existing object. See details at http://www.dofactory.com/Patterns/PatternPrototype.aspx.

Answer 6

Sure, for simple parsing (or similar) scenarios - I actually prefer the factory method be part of the class. Yes - it does break SRP, but it fulfills KISS - so I call it a net win. For larger apps, or more complicated parsing routines - it makes more sense to have it be an external factory class.

For your particular case, I'd probably prefer a method that took in an IEnumerable<string> instead of a filename - that'd still give you the parsing logic, but allow easy unit tests and "reuse". The caller can wrap the file into an IEnumerable easily enough.

Answer 7

Factory methods are often a good design. When I write them in C#, I call them 'New', so that:

new MyClass()

becomes

MyClass.New()

Trivially it's implemented like this:

class MyClass
{
    public static MyClass New()
    {
        return new MyClass();
    }
}

Mostly I do this when there are additional conditions about whether to actually create the class or just return null, or whether to return MyClass or something derived from it.

Answer 8

I sometimes use public static methods as an alternative to constructor overloading.

Especially in situations where it is not nice to rely on parameter types alone to indicate what kind of object construction is intended.

Answer 9

I'm a fan of having static methods return instances, as suggested plenty of times, above.

@Paul: don't forget to tick the comment above, which you find is the best answer.

Answer 10

Just like to point out "generate new instances of itself by calling one of its own constructors"

It is not from the constructor, it is from the static method.

Answer 11

I generally use this when I need instant implementations of a class. For example

    public class Car
    {
        public static Car RedExpensiveCar = new Car("Red", 250000);

        public Car()
        {

        }

        public Car(string color, int price)
        {
            Color = color;
            Price = price;
        }

        public string Color { get; set; }
        public int Price { get; set; }
    }

And with this, I don't need to remember or write constructor parameters in my code.

Car car = Car.RedExpensiveCar;

Answer 12

It's perfectly acceptable to do this. When I do, I typically make the real constructors for the class private so that it's clear that the only way to construct instances is through the static method.

This is very useful in cases where "construction" may not always return a new instance. For example, you may want to return a previously cached object instead.