Implementing IEquatable<T> for a Value Type Implementing IEquatable<T> for a value type is a little bit different than for a reference type. Let’s assume we have the Implement-Your-Own-Value-Type archetype, a Complex number struct. public struct Complex { public double RealPart { get; set; } public double ImaginaryPart { get; set; } } Our first step … Read more
It depends what properties you want that hash to have. For example, you could just write something like this: public int HashString(string text) { // TODO: Determine nullity policy. unchecked { int hash = 23; foreach (char c in text) { hash = hash * 31 + c; } return hash; } } So long … Read more
How does that work if the fields that it’s based on are mutable? It doesn’t in the sense that the hash code will change as the object changes. That is a problem for all of the reasons listed in the articles you read. Unfortunately this is the type of problem that typically only show up … Read more
Be sure to obtain the Reference Source source code when you have questions like this. There’s a lot more to it than what you can see from a decompiler. Pick the one that matches your preferred .NET target, the method has changed a great deal between versions. I’ll just reproduce the .NET 4.5 version of … Read more
You can override Equals() and GetHashCode() on your class like this: public override bool Equals(object obj) { var item = obj as RecommendationDTO; if (item == null) { return false; } return this.RecommendationId.Equals(item.RecommendationId); } public override int GetHashCode() { return this.RecommendationId.GetHashCode(); }
.NET 4 and later only Good news, everyone! The perfect tool for this job is built in .NET 4 and it’s called ConditionalWeakTable<TKey, TValue>. This class: can be used to associate arbitrary data with managed object instances much like a dictionary (although it is not a dictionary) does not depend on memory addresses, so is … Read more
For a class, the defaults are essentially reference equality, and that is usually fine. If writing a struct, it is more common to override equality (not least to avoid boxing), but it is very rare you write a struct anyway! When overriding equality, you should always have a matching Equals() and GetHashCode() (i.e. for two … Read more
A bit of background first… Every object in .NET has an Equals method and a GetHashCode method. The Equals method is used to compare one object with another object – to see if the two objects are equivalent. The GetHashCode method generates a 32-bit integer representation of the object. Since there is no limit to … Read more
I usually go with something like the implementation given in Josh Bloch’s fabulous Effective Java. It’s fast and creates a pretty good hash which is unlikely to cause collisions. Pick two different prime numbers, e.g. 17 and 23, and do: public override int GetHashCode() { unchecked // Overflow is fine, just wrap { int hash … Read more