Question

I want to implement a dictionary that creates its own elements on the fly only when they are accessed (not in advance). To do that I would like to use a getter method, but I simply don’t find any information how to declare a getter in the context of dictionary elements.

I do understand how to add a getter to the whole dictionary (which must return a dictionary when called), but what I want to do is implement a getter that is called when a single element in the dictionary is accessed so I can create that element on the fly. That getter must receive the key that is used for the request as a parameter and it must return the corresponding value.

I do not find any syntax for that task in the docs.

Answer 1

You just need to reimplement the indexer on Dictionary<,>

    public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    public new TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (!TryGetValue(key, out value))
            {
                value = Activator.CreateInstance<TValue>();
                Add(key, value);
            }
            return value;
        }
        set { base[key] = value; }
    } 
}

If you need a more sophisticated value instantiation, you can use an activator function

 public class MyDictionary<TKey, TValue> : Dictionary<TKey, TValue>
{
    readonly Func<TKey, TValue> _activator;

    public MyDictionary(Func<TKey, TValue> activator)
    {
        _activator = activator;
    }

    public new TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (!TryGetValue(key, out value))
            {
                value = _activator(key);
                Add(key, value);
            }
            return value;
        }
        set { base[key] = value; }
    } 
}

Usage:

static void Main(string[] args)
{
    var dict = new MyDictionary<int, string>(x => string.Format("Automatically created Value for key {0}", x));
    dict[1] = "Value for key 1";
    for (int i = 0; i < 3; i++)
    {
        Console.WriteLine(dict[i]);
    }
    Console.Read();
}