Dotnet gotchas - 3

Synchronized collections are not synchronized on what you think they are, if you are coming from the java world. Consider the following piece of code:

Hashtable ht = Hashtable.Synchronized(new Hashtable());

addToTable(object o )
{
   ht.Add(o, o);
}

processAll()
{
  lock(ht)
  {
    // enumerate and do something with them. hold lock so that 
    // someone does not modify while you are enumerating...
  }
}

The above code is incorrect since synchronised hashtables sync on an internal object called SyncRoot and not on the object itself!!