CODEWITHSUNDEEP
c#classinstance
linurb
linurb

Reputation: 307

Use variable value to check if particular instance exists, if not - create it

I have a list of images like this:

public List<Image> imageList = new List<Image>();

I also have a picture class in order to collect and manipulate data about the images in the list:

public Class Pic {
    // properties and stuff
}

And then I have a function that takes an integer as an argument. That integer corresponds to an image in the image list. What I want to do in the function is to check if an instance of the Pic class has been created for that particular image. If not, I want to create it, using the value of the variable passed into the function. The following code obviously doesn't work, but it shows what I want:

public void doStuffWithImage(int picNumber) {
    // Check if instance called pic + picNumber exists
    if(pic + picNumber.toString() == null) {
        // Create an instance
        Pic pic + picNumber.toString() = new Pic();
    }
}

Suggestions on how to accomplish this?

Upvotes: 0

Views: 238

Answers (4)

David
David

Reputation: 10708

See Lazy<T> and the Lazy Loading Pattern in general - this is actually a common optimization technique as it defers what can add up to a lot at startup to microdelays during runtime.

Be wary about making sure the microdelays are worth it, and I advise leaving methods about which can force loading.

If you're grabbing from a list, preface with a .Any or .Contains check, and since you're looking up by name like that, consider using a Dictionary instead

Upvotes: 0

Tigran
Tigran

Reputation: 62256

If you are using .net 4.0 or more you can use Lazy type which:

Provides support for lazy initialization.

Which means that the object will be constructed not in the moment of declaration, but when first accessed.

So you can basically declare them like 

List<Lazy<Pic>> ....

Upvotes: 0

Sergey Kalinichenko
Sergey Kalinichenko

Reputation: 726609

You need to create a "registry" of known Pics. DIctionary<int,Pic> would be good collection to hold this registry. You need to store the registry itself somewhere - perhaps in the "factory" object that registers your pictures.

class PicFactory {
    private readonly IDictionary<int,Pic> knownPics = new Dictionary<int,Pic>();
    public Pic GetOrCreate(int id) {
        Pic res;
        if (knownPics.TryGetValue(id, out res)) {
            return res;
        }
        res = new Pic(id.ToString()); // This assumes that Pic(string) exists
        knownPics.Add(id, res);
        return res;
    }
}

This way of implementing a registry may be too primitive for your purposes - for example, if you need your registry to be concurrent, you would need to set up some sort if a locking scheme to protect the knownPics dictionary. The class that accesses pictures would need to create an instance of PicFactory, so that it could access pictures through the GetOrCreate(id) call.

Upvotes: 0

D Stanley
D Stanley

Reputation: 152566

It seems like you're trying to create individual variables pic1, pic2, etc. you'd be better off using a dictionary:

Dictionary<int, Pic> pics = new Dictionary<int, Pic>();

public void doStuffWithImage(int picNumber) {
    // Check if instance called pic + picNumber exists
    if(!pics.ContainsKey(picNumber)) {
        // Create an instance
        pics[picNumber] = new Pic();
    }
}

Upvotes: 1

Related Questions