C# Language Idioms - Effective C# - page 47

MyStruct s = new MyStruct ();
s.SetMessage( "Hello" );
return s.GetHashCode() == s.GetMessage().GetHashCode();
The first rule says that two objects that are equal (as defined by opera-
tor==()) must have the same hash code. This rule is followed for value
types under most conditions, but you can break it, just as you could with
for reference types. ValueType.operator==() compares the first field in the
struct , along with every other field. That satisfies rule 1. As long as any
override that you define for operator== uses the first field, it will work.
Any struct whose first field does not participate in the equality of the
type violates this rule, breaking GetHashCode().
The second rule states that the hash code must be an instance invariant.
That rule is followed only when the first field in the struct is an
immutable field. If the value of the first field can change, so can the hash
code. That breaks the rules. Yes, GetHashCode() is broken for any struct
that you create when the first field can be modified during the lifetime of
the object. It's yet another reason why immutable value types are your best
bet (see Item 20).
The third rule depends on the type of the first field and how it is used. If
the first field generates a random distribution across all integers, and the
first field is distributed across all values of the struct , then the struct
generates an even distribution as well. However, if the first field often has
the same value, this rule is violated. Consider a small change to the earlier
struct :
public struct MyStruct
{
private DateTime epoch;
private string msg;
private int id;
}
If the epoch field is set to the current date (not including the time), all
MyStruct objects created in a given date will have the same hash code. That
prevents an even distribution among all hash code values.
Summarizing the default behavior, Object.GetHashCode() works correctly
for reference types, although it does not necessarily generate an efficient
distribution. (If you have overridden Object.operator==(), you can break