The actual implementation of ValueType.GetHashCode() doesn’t quite match the comment. It has two versions of the algorithm, fast and slow. It first checks if the struct contains any members of a reference type and if there is any padding between the fields. Padding is empty space in a structure value, created when the JIT compiler … Read more
.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
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
I’d do it the same way I normally combine hash codes – with an addition and a multiplication: public override int GetHashCode() { unchecked { int hash = 19; foreach (var foo in foos) { hash = hash * 31 + foo.GetHashCode(); } return hash; } } (Note that you shouldn’t add anything to the … 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(); }
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
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
Most types in .NET derive from the type System.Object, simply called object in C#. (E.g. interfaces don’t, however their implementations do.) System.Object declares the methods Equals and GetHashCode as well as other members. (Note: The case matters in C#). The types you create automatically inherit these methods. The task of Equals is to compare an … Read more