Interceptors

The generated methods are only used where your code calls them. Interceptors are another way: the compiler replaces the System.Enum calls on the listed enums by calls to the generated code, so existing code gets it without any change. They are disabled by default, because they change what a line of code does without changing how it looks.

Enable interceptors

Set MeziantouFastEnumInterceptors in the project file:

<PropertyGroup>
  <MeziantouFastEnumInterceptors>true</MeziantouFastEnumInterceptors>
</PropertyGroup>

Interceptors require C# 12 or later: with an older version, nothing is intercepted. The package adds the namespace of the generated interceptors to the InterceptorsNamespaces property for you.

The code stays the same, and its results too:

// Plain System.Enum calls: with interceptors enabled, they run the generated code
Assert.Equal("Bravo", InterceptedColor.Bravo.ToString());
Assert.Equal("Alpha", Enum.GetName(InterceptedColor.Alpha));
Assert.True(Enum.IsDefined(InterceptedColor.Charlie));
Assert.Equal(["Alpha", "Bravo", "Charlie"], Enum.GetNames<InterceptedColor>());

Intercepted calls

These calls are intercepted when the enum is listed in a FastEnumAttribute:

The behavior does not change: GetNames and GetValues still return a new array that the caller owns, ordered by value, GetName still returns null for an undeclared value, and HasFlag still throws when the flag is of another enum type.

Limitations

• Only the generic overloads, such as Enum.GetNames<TEnum>(), are intercepted. The overloads that take a Type return object or Array, which an interceptor cannot change.
• Parse and TryParse are not intercepted: call the generated members directly.
• ToString() and HasFlag() are declared on System.Enum, so the value is still boxed at the call site, as it is without interceptors. The gain is to skip the reflection, not the allocation of the box.
• To find the call sites, the generator inspects every call to ToString, HasFlag, IsDefined, GetName, GetNames and GetValues in the project, which costs more in the IDE than the default mode.