How to notify\raise event when a key value is added to Dictionary in C#

Question

I want to perform some action in an other thread if a new key\value pair is added to static dictionary. One naïve method is to do polling at frequent intervals and check if ContainsKey(givenKey), but I want to do even more quicker instead of delay in polling.

Answer 1

You have to create your own Dictionary. Best practice would be:

public enum Type
{
    AddItem,
    RemoveItem
}

public class DictChangedEventArgs<K, V> : EventArgs
{
    public Type Type
    {
        get;
        set;
    }

    public K Key
    {
        get;
        set;
    }

    public V Value
    {
        get;
        set;
    }
}

public class OwnDict<K, V> : IDictionary<K, V>
{
    public delegate void DictionaryChanged(object sender, DictChangedEventArgs<K, V> e);

    public event DictionaryChanged OnDictionaryChanged;

    private IDictionary<K, V> innerDict;

    public OwnDict()
    {
        innerDict = new Dictionary<K, V>();
    }

    public void Add(K key, V value)
    {
        if (OnDictionaryChanged != null)
        {
            OnDictionaryChanged(this, new DictChangedEventArgs<K, V>() { Type = Type.AddItem, Key = key, Value = value });
        }

        innerDict.Add(key, value);
    }

    // ...
    // ...
}

Now you are free to create event for Before-Add-Item, After-Add-Item. You can also add some Handled parameter to your DictChangedEventArgs-Class to prevent adding a special item. You can also decide to work with the Type-Enum or create an own event for every action.

But be sure, you have to implement all methods from IDictionary, but this should be very easy.