How the Dictionary is internally maintained?

Question

When i say

Dictionary<int,string>

is it equivalent to two different arrays such as:

int[] keys =new int[] { 1, 2, 3 };
string[] values=new string[]{"val1","val2","val3"};

Answer 1

That's not too far off. Looking at the source code in Reflector, it seems three internal collections are used:

private Entry<TKey, TValue>[] entries;
private KeyCollection<TKey, TValue> keys;
private ValueCollection<TKey, TValue> values;

Note that there is also a int[] buckets variable to keep track of the buckets required in the case of hash-code collisions.

These variables' purposes should all be fairly self-explanatory. This is not particularly surprising, anyway, since the Dictionary class is known and documented to provide (ideally, with one item per bucket) O(1) lookup time.

Answer 2

It's all clearly written on MSDN:

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.