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

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

public class Person
{
  public readonly DateTime birthday;
  public string name;

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