Correct way to lock the dictionary object

Question

In my code I have a static dictionary object

private static IDictionary< ConnKey, DbConnection > ConnectionList = new Dictionary< ConnKey, DbConnection >( );

which is throwing this error

System.IndexOutOfRangeException: Index was outside the bounds of the array.
  at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
  at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)

I searched and found that this occurs because multiple threads try to access dictionary, but I do have lock on dictionary

lock( ConnectionList ) {
   ConnectionList.Add( key, res );
}

Then I searched more and found that lock on dictionary doesn't prevent all the operations on it so I should be using lock on SyncRoot object of it like this to achieve what I want

lock( ((IDictionary)ConnectionList).SyncRoot) {

But then I searched that using SyncRoot is not a good practice

On further search I found there is a ConcurrentDictionary for this purpose

1. So can anybody please suggest me which is the best way to lock the dictionary
2. If I use ConcurrentDictionary do I still need to use lock on it or will it handle everything by itself.
3. If I have to use lock on ConcurrentDictionary, I have to use lock on it directly or again I have to lock the SyncRoot object for it

Thanks in advance!

Answer 1

With Dictionary<,> you have to lock both reading and writing. So both

lock( ConnectionList ) {
   ConnectionList.Add( key, res );
}

and

lock( ConnectionList ) {
   res = ConnectionList[ key ];
}

and

lock( ConnectionList ) {
   int cnt = ConnectionList.Count;
}

and

lock( ConnectionList ) {
   ConnectionList.Clear();
}

and

lock( ConnectionList ) {
   foreach ( var kv in ConnectionList ) {
      // Do things
   }
}

and so on :-)

With ConcurrentDictionary<,> you don't need any locking, but note that the syntax is a little different than the one of the Dictionary<,>