CODEWITHSUNDEEP
c#listkey
user1350555
user1350555

Reputation:

How do I refer to an item in a list<object> by Key rather than index?

I have a class that contains a List<> of objects. For this example I will say the objects are a basic class as below:

public class City
{
    private string name;
    private string country;

    public string Name
    {
        get
        {
            return name;
        }
        set
        {
            name = value;
        }
    }
}

Normally I would refer to these objects like so:

List<City> theList = new List<City>();
City theCity = theList[0];

What I would like to do is refer to the list as follows:

List<City> theList = new List<City>();
City theCity = theList["London"];

Where London is the name property of one of the cities.

How do I achieve this? At present I have been constructing "find" type methods that return me the city in question. What I need is to be able to refer by Key.

Upvotes: 0

Views: 306

Answers (4)

phoog
phoog

Reputation: 43056

How about System.Collections.ObjectModel.KeyedCollection?

http://msdn.microsoft.com/en-us/library/ms132438.aspx

Upvotes: 0

Mike Corcoran
Mike Corcoran

Reputation: 14564

You write a wrapper List class CityList and overload the [] operator.

public class City
{
    private string name;
    private string country;

    public City(string cityName)
    {
        Name = cityName;
    }

    public string Name
    {
        get
        {
            return name;
        }
        set
        {
            name = value;
        }
    }
}

public class CityList : CollectionBase
{
    public CityList() : base ()
    {
    }

    public City this[int index]
    {
        get
        {
            return (City)List[index];
        }
        set
        {
            List[index] = value;
        }
    }

    public City this[string name]
    {
        get
        {
            int index = this.IndexOf(name);
            if (index < 0 || index >= this.List.Count) return null; // or assert

            return (City)List[index];
        }
        set
        {
            int index = this.IndexOf(name);
            if (index > 0 || index >= this.List.Count) return; // or assert
            List[index] = value;
        }
    }

    public virtual int IndexOf(City city)
    {
        return List.IndexOf(city);
    }

    public virtual int IndexOf(string name)
    {
        if (name == null) return -1;

        for (int i = 0; i < List.Count; i++)
        {
            if (((City)List[i]).Name.ToLower() == name.ToLower())
                return i;
        }
        return -1;
    }

    public virtual void Insert(int index, City city)
    {
        List.Insert(index, city);
    }

    public virtual int Add(City city)
    {
        return base.List.Add(city);
    }
}

class Program
{
    static void Main()
    {
        City NewYork = new City("New York");
        City Boston = new City("Boston");
        City Tampa = new City("Tampa");

        CityList cities = new CityList();
        cities.Add(NewYork);
        cities.Add(Boston);
        cities.Add(Tampa);

        Console.WriteLine(cities["Boston"].Name); // prints "Boston"

        Console.ReadKey();
    }
}

There might be a slicker way to do it now so you don't need the casting. This is like .NET 2.0 code.

Upvotes: 3

Jon Skeet
Jon Skeet

Reputation: 1502476

Basically, it sounds like you want a Dictionary<string, City> instead of a List<City>. You can easily create this with LINQ:

var dictionary = list.ToDictionary(city => city.Name);

You can use a list and search through it (as per Ethan's answer) if you really want to preserve the order, but if you only need to look up by name, then a dictionary is what you want.

Upvotes: 9

Ethan Brown
Ethan Brown

Reputation: 27282

You can use LINQ:

List<City> theList = new List<City>();
City theCity = theList.FirstOrDefault( x => x.Name == "London" );

Upvotes: 3

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