Question

I’ve seen posts here on how to make a dictionary that has multiple values per key, like one of the solutions presented in this link:

Multi Value Dictionary

it seems that I have to use a List<> as the value for the keys, so that a key can store multiple values.

the solution in the link is fine if you want to add values. But my problem now is how to remove specific values from a single key.

I have this code for adding values to a dictionary:

private Dictionary<TKey, List<TValue>> mEventDict;
    // this is for initializing the dictionary


public void Subscribe(eVtEvtId inEvent, VtEvtDelegate inCallbackMethod)
    {
        if (mEventDict.ContainsKey(inEvent))
        {
            mEventDict[inEvent].Add(inCallbackMethod);
        }
        else
        {
            mEventDict.Add(inEvent, new List<TValue>() { v });
        }
    }
// this is for adding values to the dictionary.
// if the "key" (inEvent) is not yet present in the dictionary,
// the key will be added first before the value

my problem now is removing a specific value from a key. I have this code:

public void Unsubscribe(eVtEvtId inEvent, VtEvtDelegate inCallbackMethod)
    {
        try
        {
            mEventDict[inEvent].Remove(inCallbackMethod);
        }

        catch (ArgumentNullException)
        {
            MessageBox.Show("The event is not yet present in the dictionary");
        }
    }

basically, what I did is just replace the Add() with Remove() . Will this work?

Also, if you have any problems or questions with the code (initialization, etc.), feel free to ask.

Thanks for the advice.

Answer 1

TylerOhlsen’s answer is a step in the right direction, but it has 6 key lookups (calls to Remove, ContainsKey, and the indexer). This can be reduced to three by using TryGetValue:

private Dictionary<TKey, List<TValue>> mEventDict;

public void Subscribe(TKey inEvent, TValue inCallbackMethod)
{
    List<TValue> list;

    if (mEventDict.TryGetValue(inEvent, out list))
        list.Add(inCallbackMethod);
    else
        mEventDict.Add(inEvent, new List<TValue> { inCallbackMethod });
}

public bool Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
    List<TValue> list;

    if (!mEventDict.TryGetValue(inEvent, out list))
        return false;

    bool removed = list.Remove(inCallbackMethod);

    if (list.Count == 0)
        mEventDict.Remove(inEvent);

    return removed;
}

If you don’t care about removing empty lists:

public bool Unsubscribe(TKey inEvent, TValue inCallbackMethod)
{
    List<TValue> list;

    if (!mEventDict.TryGetValue(inEvent, out list))
        return false;

    return list.Remove(inCallbackMethod);
}

If you don’t need to report whether the item was in the list (and therefore removed from it), change the return type to void, and (in the first version) get rid of the removed variable.