Can anyone explain why Dictionary<> in C# doesn't work like map<T,U> in STL?

Answer 1

Which value would you want it to return if it didn't exist in the map already? It could return default(TValue) - but then it would be somewhat easy to accidentally use a value assuming it was correct. Note that if you want that behaviour, you can use:

get {
  string value;
  if (!_mydic.TryGetValue(key, out value))
    value = default(string);
  return value;
}

I wouldn't personally recommend it in general, but if you're happy not to be able to distinguish (via this indexer) between a key with a null value and a missing key, it will do the trick.

Note that your setter can me a lot simpler - just

_mydic[key] = value;

will overwrite if necessary.

Answer 2

I agree handling the exception is kind of annoying, especially for reference types when getting a null usually makes sense. In either case though you can use Dictionary<>.TryGetValue() to get the value in one call.

Answer 3

I'm confused. It does, here. The docs even say so:

When you set the property value, if the key is in the Dictionary(TKey, TValue), the value associated with that key is replaced by the assigned value. If the key is not in the Dictionary(TKey, TValue), the key and value are added to the dictionary.

and

KeyNotFoundException: The property is retrieved and key does not exist in the collection.

(no mention of it being thrown during assignment).