== Why is this an issue?

``++GetHashCode++`` is used to file an object in a ``++Dictionary++`` or ``++Hashtable++``. If ``++GetHashCode++`` uses non-``++readonly++`` fields and those fields change after the object is stored, the object immediately becomes mis-filed in the ``++Hashtable++``. Any subsequent test to see if the object is in the ``++Hashtable++`` will return a false negative.


=== Noncompliant code example

[source,csharp]
----
public class Person 
{
  public int age;
  public string name;

  public override int GetHashCode() 
  {
    int hash = 12;
    hash += this.age.GetHashCode(); // Noncompliant
    hash += this.name.GetHashCode(); // Noncompliant
    return hash;
  }
----


=== Compliant solution

[source,csharp]
----
public class Person 
{
  public readonly DateTime birthday;
  public string name;

  public override int GetHashCode() 
  {
    int hash = 12;
    hash += this.birthday.GetHashCode();
    return hash;
  }
----


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Refactor 'GetHashCode' to not reference mutable fields.


=== Highlighting

* primary: method name
* secondary: uses of mutable fields
** message: Remove this use of "xxx" or make it "readonly".


endif::env-github,rspecator-view[]