OneOf

install-package OneOf

This library provides F# style discriminated unions for C#, using a custom type OneOf<T0, ... Tn>. An instance of this type holds a single value, which is one of the types in its generic argument list.

I can't encourage you enough to give it a try! Due to exhaustive matching DUs provide an alternative to polymorphism when you want to have a method with guaranteed behaviour-per-type (i.e. adding an abstract method on a base type, and then implementing that method in each type). It's a really powerful tool, ask any f#/Scala dev! :)

##Use cases

###As a method return value

The most frequent use case is as a return value, when you need to return different results from a method

public OneOf<User, InvalidName, NameTaken> CreateUser(string username)
{
    if (!IsValid(username)) return new InvalidName();
    var user = _repo.FindByUsername(username);
    if(user != null) return new NameTaken();
    var user = new User(username);
    _repo.Save(user);
    return user;
}

Benefits

• True strongly typed method signature
  • No need to return a custom result base type e.g IActionResult, or even worse, a non-descriptive type (e.g. object)
  • The method signature accurately describes all the potential outcomes, making it easier for consumers to understand the code
  • Method consumer HAS to handle all cases (see 'Matching', below)
• You can avoid using "Exceptions for control flow" antipattern by returning custom Typed error objects

###As a method parameter value

You can use also use OneOf as a parameter type, allowing a caller to pass different types without requiring additional overloads. This might not seem that useful for a single parameter, but if you have multiple parameters, the numer of overloads required increases rapidly.

public void SetBackground(OneOf<string, ColorName, Color> backgroundColor) { ... }

//The method above can be called with either a string, a ColorName enum value or a Color instance.

Matching

You use the TOut Match(Func<T0, TOut> f0, ... Func<Tn,TOut> fn) method to get a value out.

Advantages over switch or if (x is T0):

This has a major advantage over a switch statement, as it

• requires every case to be handled
• No fallback - if you extend a methods return types, you HAVE to update all the calling code to handle your changes. In brown-field code-bases this is incredibly useful, as the default handler is often a runtime throw NotImplementedException, or behaviour that wouldn't suit the new result type.

E.g.

   OneOf<string, ColorName, Color> backgroundColor = ...;
   Color c = backgroundColor.Match(
     str => CssHelper.GetColorFromString(str),
     name => new Color(name),
     col => col 
   );
   _window.BackgroundColor = c;
}

There is also a .Switch method, for when you aren't returning a value:

   OneOf<string, DateTime> dateValue = ...;
   dateValue.Switch(
     str => AddEntry(DateTime.Parse(str), foo),
     int => AddEntry(int, foo)
   );
   
}