C#•3mo ago

Design Patterns / Factory Method

I'm having some serious issues understanding what the Factory method does. We were given an exercise to learn it but i'm completely lost on how / what it is that i have to do.

71 Replies

Merineth 🇸🇪OP•3mo ago

Like specifically the picture they provide doesn't really make sense to me. So far the only thing i know is that we use the "Factory Method" such that we don't instantiate a constructor directly into main. But we let the "factory" (w/e that is) do it for us. So what exactly are we meant to do in main? Call an interface/abstract class which in turn calls something?

Pobiega•3mo ago

in main, you call your factory method. This gives you the object the factory created. There was a pretty good example at the end of your last thread

Merineth 🇸🇪OP•3mo ago

I sadly don't have my own thread open Or can't find it rather

Pobiega•3mo ago

ok, here is an example

var factory = new ProductFactory();

List<Product> products = new()
{
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300),
};

foreach (var product in products)
{
    Console.WriteLine($"Product {product.Id}: {product.Name} costs {product.Price} dollars");
}

public class ProductFactory
{
    private int _id = 1;

    public Product Create(string name, int price)
    {
        return new Product(_id++, name, price);
    }
}

public record Product(int Id, string Name, int Price);

var factory = new ProductFactory();

List<Product> products = new()
{
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300),
};

foreach (var product in products)
{
    Console.WriteLine($"Product {product.Id}: {product.Name} costs {product.Price} dollars");
}

public class ProductFactory
{
    private int _id = 1;

    public Product Create(string name, int price)
    {
        return new Product(_id++, name, price);
    }
}

public record Product(int Id, string Name, int Price);

we have a Product class that has 3 properties: Id, Name and Price

Merineth 🇸🇪OP•3mo ago

And this is all inside main?

Pobiega•3mo ago

no, just the stuff before public class ProductFactory we want Ids to be assigned automatically, so we make a factory that keeps track of IDs we then create instances of products via the factory, and they automatically get their IDs assigned and incremented

Merineth 🇸🇪OP•3mo ago

Ok. So we create an object from ProductFactoru().

List<Product> products = new()
{
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300),
};

List<Product> products = new()
{
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300),
};

I'm not sure what this does tho

Pobiega•3mo ago

it creates a list it makes 3 products via the factory and adds them all to the list

Merineth 🇸🇪OP•3mo ago

I see. Is the list an array?

Pobiega•3mo ago

its a List.

Merineth 🇸🇪OP•3mo ago

Never seen List<Product>Syntax before

Pobiega•3mo ago

Product[] products =
[
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300)
];

Product[] products =
[
    factory.Create("Water Cup", 100),
    factory.Create("Cheese", 200),
    factory.Create("Cheesecake", 300)
];

Merineth 🇸🇪OP•3mo ago

I know of all the basic ones like array, linked list, stack, heap etc..

Pobiega•3mo ago

now its an array. happy? 😛

Merineth 🇸🇪OP•3mo ago

Yeah that makes sense Just wasn't aware c# had it's own type of list

Pobiega•3mo ago

a List<T> is a smart wrapper around an array, and is used for when you dont know the size of the list ahead of time it uses an array internally, but has a lot of helper methods to automatically resize the array when needed

Merineth 🇸🇪OP•3mo ago

I see. Couldn't we just define the array like you did Product[]. This makes the Array scale with input? If the purpose of List<T> is used for when we don't know the size of the array. Shouldn't Product[] do the same thing? Also where is the abstract class / interface in this example?

Pobiega•3mo ago

there is none.

Merineth 🇸🇪OP•3mo ago

So the factory method doesn't always have an interface/abstract class?

Pobiega•3mo ago

no. you are probably mixing the two patterns up again

Merineth 🇸🇪OP•3mo ago

:harold:

Pobiega•3mo ago

there is a pattenr called ABSTRACT FACTORY and one called FACTORY :p

Merineth 🇸🇪OP•3mo ago

Oh. I had no idea that was the case But it seems this exercise is asking to use the Factory Method

Merineth 🇸🇪OP•3mo ago

So what are the differences between these classes?

Pobiega•3mo ago

that picture is for the abstract factory

Merineth 🇸🇪OP•3mo ago

Because in my exercise we have two type of brands of cars. Hyundai and Mazda. And these two brands have 2 types each. HyundaiCoupe, Hyundail30 and Mazda6, MazdaMX5. So would i need two types of factories one for Hyundai and one for Mazda?

Pobiega•3mo ago

yeah welllll it depends on several things

Merineth 🇸🇪OP•3mo ago

Really? The exercise is described as "Factory Method" and it says FactoryBaseShould be an interface/abstract method

Pobiega•3mo ago

you could in theory just have a single factory with 4 methods, but thats not very scaleable FactoryBase is literally an abstract class, so yes.

Merineth 🇸🇪OP•3mo ago

Ok. And this abstract class should contain the two Concrete Factories? Which is Mazda and Hyundai

Pobiega•3mo ago

no it doesnt contain them they are subclasses of it

Merineth 🇸🇪OP•3mo ago

Yeah sorry meant subclass And these subclasses are the one that makes the cars?

Pobiega•3mo ago

yes

Merineth 🇸🇪OP•3mo ago

And that is essentially it?

Pobiega•3mo ago

they are the ConcreteFactories

Merineth 🇸🇪OP•3mo ago

Aah i see! And they make the ConcreteProduct ?

Pobiega•3mo ago

yes

Merineth 🇸🇪OP•3mo ago

What is the ProductBaseFor?

Pobiega•3mo ago

public abstract class FactoryBase
{
    public abstract Car Create();
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create()
    {
        return new Hyundai("Hyundai");
    }
}

public abstract class FactoryBase
{
    public abstract Car Create();
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create()
    {
        return new Hyundai("Hyundai");
    }
}

for the abstract factory to know what to make

Pobiega•3mo ago

you can see how the two structures mirror eachother

Merineth 🇸🇪OP•3mo ago

Ah yeah So the right one is An abstract Factory and left one is a normal Factory?

Pobiega•3mo ago

>_> the left ones are the products the cars the right ones are the factories the top one is the factoryBase, the abstract factory the bottom ones are the concrete factories

Merineth 🇸🇪OP•3mo ago

This is so abstract, it's extremely tough to understand

Pobiega•3mo ago

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create(string name)
    {
        return name switch
        {
            "Coupe" => new HyundaiCoupe(),
            "IL30" => new HyundaiIL30(),
            _ => throw new ArgumentException($"Invalid hyundai model name {name}")
        };
    }
}

public class MazdaFactory : FactoryBase
{
    public override Mazda Create(string name)
    {
        return name switch
        {
            "3" => new Mazda3(),
            "MX5" => new MazdaMX5(),
            _ => throw new ArgumentException($"Invalid mazda model name {name}")
        };
    }
}

public abstract record Car;
public record Mazda(string Name) : Car;
public record Mazda3() : Mazda("3");
public record MazdaMX5() : Mazda("MX5");
public record Hyundai(string Name) : Car;
public record HyundaiCoupe() : Hyundai("Coupe");
public record HyundaiIL30() : Hyundai("IL30");

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create(string name)
    {
        return name switch
        {
            "Coupe" => new HyundaiCoupe(),
            "IL30" => new HyundaiIL30(),
            _ => throw new ArgumentException($"Invalid hyundai model name {name}")
        };
    }
}

public class MazdaFactory : FactoryBase
{
    public override Mazda Create(string name)
    {
        return name switch
        {
            "3" => new Mazda3(),
            "MX5" => new MazdaMX5(),
            _ => throw new ArgumentException($"Invalid mazda model name {name}")
        };
    }
}

public abstract record Car;
public record Mazda(string Name) : Car;
public record Mazda3() : Mazda("3");
public record MazdaMX5() : Mazda("MX5");
public record Hyundai(string Name) : Car;
public record HyundaiCoupe() : Hyundai("Coupe");
public record HyundaiIL30() : Hyundai("IL30");

does this help? we have 2 concrete factories, that make specific cars we have the abstract factory that they both inherit from

Merineth 🇸🇪OP•3mo ago

Pobiega•3mo ago

yeah thats exactly what we have here

Merineth 🇸🇪OP•3mo ago

And the stringname is the parameter which decides which car to make? Or rather which concrete Factory to enter?

Pobiega•3mo ago

yes to the first one, no to the second one

Merineth 🇸🇪OP•3mo ago

Just to go through it, making sure i get it right.

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

So we call the Create method with the argument string name? How does it know which one concrete factory to enter?

Pobiega•3mo ago

it doesnt you cant have an instance of FactoryBase its impossible

Merineth 🇸🇪OP•3mo ago

Right, i'm onboard with that. I'm just curious to what happens when we call the method Create. Since Create is defined in both factories.

Pobiega•3mo ago

well, Create is an instance method its called on the instance of the factory and since we cant make instances of FactoryBase, its only called on a concrete factory so we already know if its a mazda or a hyundai factory

Merineth 🇸🇪OP•3mo ago

I'm not sure i understand that part. I was taught that when we call on a method such as car Create() it will call that specific method and apply everything inside the {}. However Create() is defined inside both factories. So how does C# know which factory to enter? Does it enter both of them? Or does it go to the appropriate one based on the argument inside create() ?

Pobiega•3mo ago

Show me the code you are thinking about alternatively, do you have a headset at hand and can jump into a VC? I could show you via screenshare exactly how this works.

Merineth 🇸🇪OP•3mo ago

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create(string name)
    {
        return name switch
        {
            "Coupe" => new HyundaiCoupe(),
            "IL30" => new HyundaiIL30(),
            _ => throw new ArgumentException($"Invalid hyundai model name {name}")
        };
    }
}

public class MazdaFactory : FactoryBase
{
    public override Mazda Create(string name)
    {
        return name switch
        {
            "3" => new Mazda3(),
            "MX5" => new MazdaMX5(),
            _ => throw new ArgumentException($"Invalid mazda model name {name}")
        };
    }
}

public abstract class FactoryBase
{
    public abstract Car Create(string name);
}

public class HyundaiFactory : FactoryBase
{
    public override Hyundai Create(string name)
    {
        return name switch
        {
            "Coupe" => new HyundaiCoupe(),
            "IL30" => new HyundaiIL30(),
            _ => throw new ArgumentException($"Invalid hyundai model name {name}")
        };
    }
}

public class MazdaFactory : FactoryBase
{
    public override Mazda Create(string name)
    {
        return name switch
        {
            "3" => new Mazda3(),
            "MX5" => new MazdaMX5(),
            _ => throw new ArgumentException($"Invalid mazda model name {name}")
        };
    }
}

This is what i'm basing it of. When we call the method create from public abstract Car Create(string name);. Both MazdaFactoryand HyundaiFactory has the method Create. So which one will it enter?

Pobiega•3mo ago

neither it cant you cant do what you are saying

Merineth 🇸🇪OP•3mo ago

methods work differently inside abstract classes? Essentially what i'm asking is: What happens when public abstract Car Create(string name); is called

Pobiega•3mo ago

no, but in that code you just pasted, there is no method call can you write out the method call show me HOW you would invoke that method

Merineth 🇸🇪OP•3mo ago

public abstract Car Create(string name); public abstract Car Create(string name);this is a method?

Pobiega•3mo ago

yes but its an abstract method that particular method will never be called.

Merineth 🇸🇪OP•3mo ago

Oh so we aren't actually invoking it inside the abstract class?

Pobiega•3mo ago

there are no invocations anywhere here

Merineth 🇸🇪OP•3mo ago

Shit