more effective csharp notes

• Never redefine
• Return true: same reference identity
• Specially, return false when compare two value type variable(due to boxing)

• Never redefine
• Call ReferenceEquals and instance Equals() to comapre two object instances

public virtual bool Equals(object right);

• Equality
  • reflexive: a == a
  • symmetric: a == b <--> b == a
  • transitive: a == b, b == c –> a == c
• Default
  • Reference type: same with Object.ReferenceEquals()
  • Value type: have been override by System.ValueType, but we should create an override of ValueType.Equals() when we create a value type.
• How to override
  • Impliment IEquatable<T>
  • Impliment GetHashCode()
    • Equal objects must produce equal hash codes
    • Hash code must be object invariants
    • Must produce an even distribution to be efficient
  • Standard pattern
    

    public class Foo : IEquatable<Foo> { public override bool (object right) { // This reference is never null in C# methods if (object.ReferenceEquals(right, null)) return false; if (object.ReferenceEquals(this, right)) return true; // To handle inheritance hierarchy problem if (this.GetType() != right.GetType()) return false; // Compare this type's contents here: return this.Equals(right as Foo); } // IEquatable<Foo> Members public bool (Foo other) { // Elided: this.x == other.x && this.y == other.y, etc. return true; } }

public static bool operator ==(MyClass left, MyClass right);

• Redefine for value type, not reference type