Best way to remove multiple items matching a predicate from a c# Dictionary ?

Question

I need to remove multiple items from a Dictionary. A simple way to do that is as follows :

  List<string> keystoremove= new List<string>();
  foreach (KeyValuePair<string,object> k in MyCollection)
     if (k.Value.Member==foo)
        keystoremove.Add(k.Key);
  foreach (string s in keystoremove)
        MyCollection.Remove(s);

The reason why I can't directly Remove the items in the foreach block is that this would throw an Exception ("Collection was modified...")

I'd like to do the following :

 MyCollection.RemoveAll(x =>x.Member==foo)

But the Dictionary<> class doesn't expose a RemoveAll(Predicate<> Match) method, like the List<> Class does.

What's the best way (both performance wise and elegant wise) to do that?

Answer 1

Can you just change your loop to use an index (i.e. FOR instead of FOREACH)? You'd have to loop backwards, of course, i.e. count-1 down to zero.

Answer 2

Here's an alternate way

foreach ( var s in MyCollection.Where(p => p.Value.Member == foo).ToList() ) {
  MyCollection.Remove(s.Key);
}

Pushing the code into a list directly allows you to avoid the removing while enumerating problem. The .ToList() will force the enumeration before the foreach really starts.

Answer 3

Instead of removing just do the inverse (create a new dictionary from the old one containing only the elements you are interested in) and let the garbage collector take care of the old dictionary:

var newDictionary = oldDictionary.Select(x => x.Value != foo);

Answer 4

you can create extension method:

public static class DictionaryExtensions
{
    public static void RemoveAll<TKey, TValue>(this Dictionary<TKey, TValue> dic, 
        Func<TValue, bool> predicate)
    {
        var keys = dic.Keys.Where(k => predicate(dic[k])).ToList();
        foreach (var key in keys)
        {
            dic.Remove(key);
        }
    }
}

...

dictionary.RemoveAll(x => x.Member == foo);

Answer 5

Instead of removing, just do the inverse. Create a new dictionary from the old one containing only the elements you are interested in.

public Dictionary<T, U> NewDictionaryFiltered<T, U>
(
  Dictionary<T, U> source,
  Func<T, U, bool> filter
)
{
return source
  .Where(x => filter(x.Key, x.Value))
  .ToDictionary(x => x.Key, x => x.Value);
}