Reference types vs value types

All C# types fall into the following categories:

  • Value types
  • Reference types
  • Generic type parameters
  • Pointer types

Value types comprise most built-in types (specifically, all numeric types, the char type, and the bool type) as well as custom struct and enum types.

Reference types comprise all class, array, delegate, and interface types. (This includes the predefined string type.) The fundamental difference between value types and reference types is how they are handled in memory.

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Implementing Value Types – Best Practices

When implementing you own struct you should take into consideration the following:

  • Use structs when you intend to create a great many of them
  • structs should be small. Large structs (i.e. structs with many fields) would lead to poor performance, since structs are copied each time.
  • Use structs when you require high density memory collections.  Structs have have a much simpler memory layout and offer excellent memory density and lack of overhead
  • Override structs Equals(…) method and implement IEquitable<T> interface to  avoid boxing.
  • Overload == and != operators
  • Override GetHashCode method.
  • structs should almost always be immutable. Actually I’ll go further and say always. Mutable structs could lead to confusion if declared as readonly. Any modifications to a mutable readonly struct will be ignored, making it a very difficult bug to spot.

Structs have also got their own limitations. Since they don’t have an object header, you cannot for example use a lock(…) on a struct since this  will result on compile time error. However Monitor.Enter(…) accepts any object, structs included, thus leading to a bug. The value  of the struct will be boxed each time and that would be equivalent to having no lock at all!

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