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DataStax C# Driver for Apache Cassandra (.Net 4.5.2)
I think this is an invalid method
private static readonly Dictionary<TKey, TValue> Empty = new Dictionary<TKey, TValue>();
private volatile Dictionary<TKey, TValue> _map;
public CopyOnWriteDictionary()
{
//Start with an instance without nodes
_map = Empty;
}
SO, _map and Empty is same object
public void Clear()
{
_map = Empty;
}
(_map = Empty ) Can't Clear Dictionary, They are same object...
The Clear() method clears the _map variable in that it resets _map to an empty dictionary. If you look at the rest of the methods in CopyOnWriteDicitonary, you'll see that _map is never added to or removed from directly. All of the modifications are done by cloning the current _map contents, making the modification to the clone, and then setting _map to point to the clone. For example:
public void Add(TKey key, TValue value)
{
if (key == null)
{
throw new ArgumentNullException(nameof(key));
}
lock (_writeLock)
{
CloneMapAndAddUnsafe(key, value);
}
}
Add calls CloneMapAndAddUnsafe, which does this:
private void CloneMapAndAddUnsafe(TKey key, TValue value)
{
var newMap = new Dictionary<TKey, TValue>(_map)
{
{ key, value }
};
_map = newMap;
}
Because _map is never modified directly, the static Dictionary "Empty" is never modified either, so it always remains empty. That is why calling Clear() does effectively clear _map as it simply resets _map to the static Empty Dictionary.
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