Adding and retrieving from a KeyedCollection

Question

I want to use a KeyedCollection to store a class against a string key value. I have the following code:

public class MyClass
{
    public string Key;
    public string Test;
}

public class MyCollection : KeyedCollection<string, MyClass>
{
    public MyCollection() : base()
    {
    }

    protected override String GetKeyForItem(MyClass cls)
    {
        return cls.Key;
    }
}

class Program
{
    static void Main(string[] args)
    {
        MyCollection col = new MyCollection();
        col.Add(new MyClass()); // Here is want to specify the string Key Value
    }
}

Can anyone tell me what I’m doing wrong here? Where do I specify the key value so that I can retrieve by it?

Answer 1

The KeyedCollection is a base class for creating keyed collections, so there is quite a lot that you need to implement yourself.

Perhaps using a Dictionary would be easier and faster to work with.

Answer 2

Your GetKeyForItem override is what specifies the key for an item. From the docs:

Unlike dictionaries, an element of KeyedCollection is not a key/value pair; instead, the entire element is the value and the key is embedded within the value. For example, an element of a collection derived from KeyedCollectionmight be "John Doe Jr." where the value is "John Doe Jr." and the key is "Doe"; or a collection of employee records containing integer keys could be derived fromKeyedCollection. The abstract GetKeyForItem method extracts the key from the element.

So in order for the item to be keyed correctly, you should set its Key property before adding it to the collection:

MyCollection col = new MyCollection();
MyClass myClass = new MyClass();
myClass.Key = "This is the key for this object";
col.Add(myClass); 

Answer 3

I know it's slightly different, but have you considered implementing an indexer.

public string this[string index]
{
    get { 
      // put get code here
    }
    set {
      // put set code here.
    }
}