Working with separate classes, global/static class ?

Question

Say I have two separate classes, A and B.

I also have Repository class C which loads some information from a textfile. E.g. It has methods loadLines(), addLine(), deleteLine().

Ignoring databinding, how can I make A and B both work on the same class C. Is it possible?

For example at the moment, in class A and B formload, I have:

var classC = new C();

This causes repeated execution. It would be much better if I could have one copy of class c to work on from either A or B.

edit: so with using the singleton, when does class C's constructor execute? - whichever class first creates it? and it only happens the once I take it?

edit1: does using the singleton pattern imply that you should only have one of them in your project solution? could I have multiple?

Answer 1

You can use the Singleton pattern or if you want to go the route of a Dependency Injection framework (which is about much more than just creating singletons but well worth looking into) most DI containers have the ability to create only a single instance of an object.

class C {

    public static readonly C Instance = new C();

    private C() {
    }

}

Then you would use it like:

private void A_Load(object sender, EventArgs e) {
    var classC = C.Instance;
}

private void B_Load(object sender, EventArgs e) {
    var classC = C.Instance;
}

Answer 2

If you want to ensure you only ever have one instance of class C running around, you can use the singleton pattern.

By making the constructor on class C private, and having a public static method that takes care of giving you the right instance of C, you can ensure that A and B will always get the same instance:

public class C
{
    private static C instance;

    private C()
    {
        // do some stuff
    }

    public static C GetInstance()
    {
        if(instance==null)
           instance = new C();

        return instance;
    }
}

In A and B you can do:

C theSameInstanceOfC = C.GetInstance();