How does rehashing occur in a map?

In C#, when increasing the size of a dictionary (Dictionary<TKey, TValue>), a rehash operation occurs — redistributing elements into a new, larger internal structure.

When the number of elements reaches a certain threshold (usually the array size multiplied by the load factor, for example 0.75), the dictionary enlarges its internal array (usually doubling it) and redistributes all existing key-value pairs to new indices calculated based on the key's hash code and the new array size.

This is necessary to maintain the efficiency of search, insert, and delete operations.

The process roughly looks like this:

1. A new larger array is created.
2. For each element from the old array, a new index is calculated considering the new size.
3. Elements are copied into the new array at the new indices.

This operation is time-consuming, so it is minimized by pre-setting an appropriate initial size of the dictionary if the expected data volume is known.