Wednesday, September 01, 2010

.NET ValueType Equals()/GetHashCode() Performance

As most people are aware of, .NET Framework's builtin implementations of Equals() and GetHashCode() for ValueTypes are based on Value Equality (as opposed to many ReferenceTypes, where those methods are based on Reference Equality). The problem is, those ValueType.Equals() and GetHashCode() implementations are quite slow for user-defined structs - which is not surprising, as they must apply reflection for obtaining field and property values.

That's why it's better to code your own Equals() and GetHashCode() methods in these cases. The task is pretty much straightforward - just compare those fields and properties, that define equality, in Equals(), and XOR their shifted values (or numeric representations of their values) in GetHashCode():

public override bool Equals(object obj) {
    Nullable<MyType> other = obj as Nullable<MyType>;
    return other != null && 
           this.X == other.Value.X && 
           this.Y == other.Value.Y;

public override int GetHashCode() {
    return (this.X * 31) ^ this.Y;

I recently was asked to tune some code, and replaced several nested loops with dictionary lookups - only to find out that this was not as fast as I had expected, due to structs being applied as dictionary keys. Provided my own Equals() and GetHashCode() implementations, and voila - ten-fold speedup!