Each implementation of
Equals must fulfil the following requirements:
Reflexive: An object must equal itself.
Symmetric: There is no difference if I compare x to y or y to x - the result is the same.
x.Equals(y) returns the same value as
Transitive: If one object is equal to another object and this one is equal to a third one, the first has to be equal to the third.
(x.Equals(y) && y.Equals(z)) returns
Consistent: If you compare an object to another multiple times, the result is always the same.
Successive invocations of
x.Equals(y) return the same value as long as the objects referenced by x and y are not modified.
Comparison to null: No object is equal to
Equals: If two objects are equal (meaning that
Equals returns true), then
GetHashCode must return the same value for each of them.
Large range: If two objects are not equal (
Equals says false), there should be a high probability their hash codes are distinct. Perfect hashing is often not possible as there is a limited number of values to choose from.
Cheap: It should be inexpensive to calculate the hash code in all cases.