Each implementation of Equals
must fulfil the following requirements:
Reflexive: An object must equal itself.x.Equals(x)
returns true
.
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 y.Equals(x)
.
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.
if (x.Equals(y) && y.Equals(z))
returns true
, then x.Equals(z)
returns true
.
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 null
.x.Equals(null)
returns false
.
Implementations of GetHashCode
:
Compatible with 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.