How to implement the default constructor (using constructor chaining) that sets the Category to Miscellaneous

Question

I am very new to programming so I was trying to do some work with and got stuck with my problem. Here is my code:

//Properties
private static readonly List<string> category = new List<string> 
{ 
    "Electric", 
    "Household", 
    "Garden", 
    "Miscellaneous" 
};

Category HAS to be "readonly"

// Constructor
public Product(List<string> category)
{
    // this.category shows error that it cannot be accessed with an instance reference; 
    // qualify it with a type name instead
    this.category = category;
}

Also in the default constructor I cannot pass it

// Default Constructor
public Product() : this("Miscellaneous")
{
}

So, how to pass 1 one of strings within the list? Or should I use arrays for this? And how do I print it out later?

class TestProduct
{
    static void Main(string[] args)
    {
        // Assigning correct properties to the product
        Product p1 = new Product(1234567, "Cake", "Miscellaneous", 7.5, 150);
        Console.WriteLine(p1);
        Console.ReadLine();
    }
}

Hope my question is clear.

Answer 1

If you have a fixed set of categories, you can use an enum to store all values.

public enum ProductCategory
{
    Electric,
    Household,
    Garden,
    Miscellaneous
}

You can create a default constructor like this:

public enum ProductCategory
{
    Electric,
    Household,
    Garden,
    Miscellaneous
}

public class Product
{

    public ProductCategory Category { get; }

    public Product(ProductCategory category)
    {
        this.Category = category;
    }

    public Product() : this(ProductCategory.Miscellaneous)
    {
    }

}

static void Main()
{
    Product p1 = new Product();
    Console.WriteLine(p1.Category); // output: Miscellaneous
    Console.ReadKey();
}

If you still want to store your category in a string, you can adapt this example. If you want a fixed list of valid categories, you can check if the category is valid in the constructor.

Answer 2

Category HAS to be "readonly"

The problem is not that it is readonly, the reason why you're getting cannot be accessed with an instance reference, qualify it with a type name instead is because you declared the property as static. A static property cannot be accessed as an instance property.

Also in the default constructor I cannot pass it

Yes you can, if you actually pass in the type you declared.

public Product() : this("Miscellaneous") //You're trying to pass in a string
{
}

Can you see why you cannot pass in that string into your base constructor?

public Product(List<string> category) //base constructor takes a List<string>
{
    //stuff
}

You could create a base constructor which actually takes a string:

public Product(string cat)
{
    //Validate that the category passed in is valid, I.E. in your list
    foreach (var item in category)
    {
        if(object.Equals(item, cat))
            break;
        if(object.Equals(item, category.Last()))
            throw new Exception("D'oh! Invalid category");
    }

    //do stuff
}

Something like that. There are probably better ways to validate. Or just make your categories an Enum and use something like Enum.TryParse to validate instead.

Answer 3

Remove the this qualifier from this.category because you are accessing a static variable which by definition cannot belong to an instance of a class.

Of course this means you'll get a naming collision because the static category cannot be differentiated from the local category - so fall back to using a naming convention like an underscore at the start of the name for the static variable - _category.

You should also rethink why you made the private variable category static - there is no reason for it to be that way.

[...] that sets the Category to Miscellaneous

To solve your main issue - there is no way to do this without writing a new constructor that takes a single string (because you cannot statically define or enforce the length of a List<>, you can only check that programmatically). A good option is to use enums as suggested by @apk, then throw an ArgumentOutOfRangeException from within the chained constructor if the value Miscellaneous isn't passed in. But you should rethink even that strategy - if the value must be Miscellaneous then consider defaulting to that in the chained constructor, don't allow an arbitrary value to be passed in and then test for the one possible correct value.

Answer 4

I assumed you don't actually want it to be a static variable (this question might help), as long as you are receiving it in the constructor. Maybe what you want it is to be immutable, which is something quite different.

You can define that constructor this way.

public class Product
{
    private readonly List<string> _categories;

    public Product() : this(new List<string>() { "Miscellanous"})
    {

    }

    public Product(List<string> category)
    {
        _categories = category;
    }
}

Or this way

public class Product
{
    // other code
    public Product()
    {
         categories = new List<string>();
         categories.Add("Miscellanous");
    }
}

Also readonly only forbids changing the reference to the _categories variable. If you want the content to be immutable, then you should have a look at other kind of data structures or simply do it in the code.