Question

What are the access times for:

1. Looking up a value in Dictionary
2. Check whether a HashSet has a value

Is it O(1) like unordered_map of C++0x?

Answer 1

Yes, when you use the Contains method or the indexer of the Dictionary.

From the docs:

The Dictionary(Of TKey, TValue) generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary(Of TKey, TValue) class is implemented as a hash table.

http://msdn.microsoft.com/en-us/library/xfhwa508.aspx