C# Language Overloading equality operators


Overloading just equality operators is not enough. Under different circumstances, all of the following can be called:

  1. object.Equals and object.GetHashCode
  2. IEquatable<T>.Equals (optional, allows avoiding boxing)
  3. operator == and operator != (optional, allows using operators)

When overriding Equals, GetHashCode must also be overriden. When implementing Equals, there are many special cases: comparing to objects of a different type, comparing to self etc.

When NOT overridden Equals method and == operator behave differently for classes and structs. For classes just references are compared, and for structs values of properties are compared via reflection what can negatively affect performance. == can not be used for comparing structs unless it is overridden.

Generally equality operation must obey the following rules:

  • Must not throw exceptions.
  • Reflexivity: A always equals A (may not be true for NULL values in some systems).
  • Transitvity: if A equals B, and B equals C, then A equals C.
  • If A equals B, then A and B have equal hash codes.
  • Inheritance tree independence: if B and C are instances of Class2 inherited from Class1: Class1.Equals(A,B) must always return the same value as the call to Class2.Equals(A,B).
class Student : IEquatable<Student>
    public string Name { get; set; } = "";

    public bool Equals(Student other)
        if (ReferenceEquals(other, null)) return false;
        if (ReferenceEquals(other, this)) return true;
        return string.Equals(Name, other.Name);

    public override bool Equals(object obj)
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;

        return Equals(obj as Student);

    public override int GetHashCode()
        return Name?.GetHashCode() ?? 0;

    public static bool operator ==(Student left, Student right)
        return Equals(left, right);

    public static bool operator !=(Student left, Student right)
        return !Equals(left, right);