It’s written in the documentation for Dictionary…
The Dictionary 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 class is implemented as a hash table.
And for the Add function:
If Count is less than the capacity, this method approaches an O(1) operation. If the capacity must be increased to accommodate the new element, this method becomes an O(n) operation, where n is Count.