How many hash buckets does a .net Dictionary use?

Question

I know that this is an implementation detail, but I'm curious: Is there a bound on the number of hash buckets used in a .NET Dictionary? I assume that it will be somewhere around 2 * numberOfElements, but does anyone know for sure (or is it documented anywhere)?

Answer 1

In short: it uses a size equal to the first prime number greater than [numberOfElements]. However it does not consider every prime number: it uses a table for sizes upto a limit, and for bigger sizes it computes a prime number the hard way.

If you look in the source, you can spot the following table in a class called HashHelpers: (I guess this means you need 7.2 million items before it starts computing prime numbers)

public static readonly int[] primes = {
        3, 7, 11, 17, 23, 29, 37, 47, 59, 71, 89, 107, 131, 163, 197, 239, 293, 353, 431, 521, 631, 761, 919,
        1103, 1327, 1597, 1931, 2333, 2801, 3371, 4049, 4861, 5839, 7013, 8419, 10103, 12143, 14591,
        17519, 21023, 25229, 30293, 36353, 43627, 52361, 62851, 75431, 90523, 108631, 130363, 156437,
        187751, 225307, 270371, 324449, 389357, 467237, 560689, 672827, 807403, 968897, 1162687, 1395263,
        1674319, 2009191, 2411033, 2893249, 3471899, 4166287, 4999559, 5999471, 7199369};