"Ah! It's like a compile time checked switch statement!" - Mike Giorgaras

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! :)

The most frequent use case is as a return value, when you need to return different results from a method. Here's how you might use it in an MVC controller action:

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;
}

[HttpPost]
public IActionResult Register(string username)
{
    OneOf<User, InvalidName, NameTaken> createUserResult = CreateUser(username);
    return createUserResult.Match(
        user => new RedirectResult("/dashboard"),
        invalidName => { 
            ModelState.AddModelError(nameof(username), $"Sorry, that is not a valid username.");
            return View("Register");
        },
        nameTaken => {
            ModelState.AddModelError(nameof(username), "Sorry, that name is already in use.");
            return View("Register");
        }
    );            
}

• 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

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 number 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.

You use the TOut Match(Func<T0, TOut> f0, ... Func<Tn,TOut> fn) method to get a value out. Note how the number of handlers matches the number of generic arguments.

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

• requires every parameter to be handled
• No fallback - if you add another generic parameter, 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)
   );
   

You can declare a OneOf as a Type, by inheriting from OneOfBase.

    public abstract class PaymentResult : OneOfBase<PaymentResult.Success, PaymentResult.Declined, PaymentStatus.Failed>
    {
        public class Success : PaymentResult { }  
        public class Declined : PaymentResult { }
        public class Failed  : PaymentResult { public string Reason { get; set; } }
    }
    
    

The PaymentResult class will inherit the .Match and .Switch methods.

As an alternative to .Switch or .Match you can use the .TryPick𝑥 methods.

    //TryPick𝑥 methods for OneOf<T0, T1, T2>
    public bool TryPickT0(out T0 value, out OneOf<T1, T2> remainder) { ... } 
    public bool TryPickT1(out T1 value, out OneOf<T0, T2> remainder) { ... } 
    public bool TryPickT2(out T2 value, out OneOf<T0, T1> remainder) { ... } 

The return value indicates if the OneOf contains a T𝑥 or not. If so, then value will be set to the inner value from the OneOf. If not, then the remainder will be a OneOf of the remaining generic types. You can use them like this:

//Suppose you have a method call returning a OneOf...
OneOf<Thing, NotFound, Error> GetThingFromDb(string id){ ... }

IActionResult Get(string id)
{
	OneOf<Thing, NotFound, Error> thingOrNotFoundOrError = GetThingFromDb(string id);

	if (thingOrNotFoundOrError.TryPickT1(out NotFound notFound, out var thingOrError))
		return StatusCode(404);

	if (thingOrError.TryPickT1(out var error, out var thing))
	{
		_logger.LogError(error.Message);
		return StatusCode(500);
	}

	return Ok(thing);
}