Wesner Moise on Enums and Performance

Wesner Moise (.NET Undocumented) writes on enums perf in .NET.

While enums are value types and are often recognized and treated like standard integral values by the runtime (in IL, enums and integers have almost no distinction), there are few performance caveats to using them.

Enumerated types are derived from ValueType and Enum (as well as Object), which are, ironically, reference types. An explicit conversion of an enum value to ValueType, will actually perform boxing and generate an object reference.

Any calls to an inherited method from any of those classes will also actually invoke boxing, prior to calling the base method. This includes the following methods: GetType(), ToString(), GetHashCode() and Equals(). In addition the costs of mplicit boxing is the far larger costs of reflection used to actually complete the said methods.

ToString uses reflection, the first time it is called, to dynamically retrieve enumeration constants from the enumerated type and stores those values into a hash table for future use. However, GetHashCode always uses reflection to retrieve the underlying value. While ValueType.Equals will attempt to do a fast bit check, when a valuetype with no reference methods, such as is the case for enumerated types, it won't be faster than a direct compare.

This is true for any value type, but normally the cost can be eliminated for ToString, GetHashCode, and Equals, by simply overriding those methods and avoiding calls to the base methods. However, those methods CANNOT be overridden for enumerated types.

Another ironic conclusion is that creating your own version of an enumerated type, not derived from Enum, is going to be faster than the CLR versions, because you can ensure that GetHashCode, Equals, ToString, IComparable, and IComparable<T> are not inherited from any of base classes such as ValueType.