== 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.

=== Exceptions

This rule does not raise if the type implementing `GetHashCode` is a value type, for example a `struct` or a `record struct`, since when a value type is stored in a `Dictionary` or `Hashtable`, a copy of the value is stored, not a reference to the value.

== How to fix it

=== Code examples

==== Noncompliant code example

[source,csharp,diff-id=1,diff-type=noncompliant]
----
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,diff-id=1,diff-type=compliant]
----
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[]
