GetHashCode (key) & int.MaxValue

Question

When looking at the implementation of the generic Dictionary<TKey, TValue> in mscorlib.dll, I noticed that the following was used many times to get the hash key:

 int num = this.comparer.GetHashCode(key) & int.MaxValue; 

GetHashCode () returns an int. Am I mistaken in thinking that bitwise AND between int.MaxValue and any integer x will always return x?

Can someone explain why the operator is used in this way above?

Answer 1

The value of int.MaxValue is 0x7FFFFFFF - the most significant bit is zero. Therefore, when you execute bitwise and with another int, you actually nullify the “sign” bit. Please note that due to the used coding of the two additions, -1 will not become 1, but will be 2 147 483 647.

Apparently, for some reason, only positive integers in the code sample are allowed in the num variable.

Answer 2

This will not affect positive numbers.

• [0, int.MaxValue] → remains unchanged
• [int.MinValue, -1] → will change the sign bit