在哪里/何时填充查找值？

Question

I've tried googling for two days but can't seem to find an answer.

I'd like the Category class to provide the description based on the id entered and return an error if the id is not valid. Is this the best approach?

public class Category
{
    private int _id;
    private string _desc;

    public Category(int id)
    {
        ID = id;
    }

    public int ID 
    {
        get 
        {
            return _id;
        }

        set 
        {
            _id = value;

            //_desc = value from data access layer or throw error if the ID is invalid               
        }
    }

    public string Description 
    {
        get 
        {
            return _desc;
        }       
    }
}

public class Person
{
    public int ID {get; set;}

    public Category Category {get; set;}
}

public class MyApp
{
    static void Main()
    {
        Person p = new Person();

        Category c = new Category(2);

        p.Category = c;
    }
}

Answer 1

Since there will be potentially several instances of the class Category, it would be a waste memory-wise to include the look up values in the class itself. Instead they should be accessed elsewhere. For instance a static function in another class.

public class CategoryHelper
{
    public static string GetCategoryDesc(int CatgeoryId)
    {
        ...access database to get description
    }
}

Which we could use in the Description getter in the Category class:

public string Description 
{
    get 
    {
        return CategoryHelper.GetCategoryDesc(this.ID);
    }       
}

Now, since we have the GetCategoryDesc in a separate class we can now optimize it for performance. For instance, if you are fairly certain that the values for the lookup won't change for the duration of the run you can cache the descriptions in memory to avoid DB trips. In the following code we only call the DB the first time the call is made and the the results are cached. This is called "memoization".

public class CategoryHelper
{
    Dictionary<int,string> cachedDesc; //Dictionary used to store the descriptions
    public static string GetCategoryDesc(int CatgeoryId)
    {
        if (cachedDesc==null) cachedDesc = new Dictionary<int,string>(); // Instatiate the dictionary the first time only
        if(cachedDesc.ContainsKey(CatgeoryId)) //We check to see if we have cached this value before
        {
            return cachedDesc[CatgeoryId];
        }
        else
        {
            var description = .... get value from DB
            cachedDesc.add(CatgeoryId, description); //Store the value for later use
            return description;
        }
    }
}

You can make this simpler and even more complex, and since it is isolated in its own function you will have to do little to no changes elsewhere.