"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! :)
PS If you like OneOf, you might want to check out ValueOf, for one-line Value Object Type definitions.
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");
}
);
}It's simple to use OneOf as an Option type - just declare a OneOf<Something, None>. OneOf comes with a variety of useful Types in the OneOf.Types namespace, including Yes, No, Maybe, Unknown, True, False, All, Some, and None.
- 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)
- No need to return a custom result base type e.g
- 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)
);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:
IActionResult Get(string id)
{
OneOf<Thing, NotFound, Error> thingOrNotFoundOrError = GetThingFromDb(string id);
if (thingOrNotFoundOrError.TryPickT1(out NotFound notFound, out var thingOrError)) //thingOrError is a OneOf<Thing, Error>
return StatusCode(404);
if (thingOrError.TryPickT1(out var error, out var thing)) //note that thing is a Thing rather than a OneOf<Thing>
{
_logger.LogError(error.Message);
return StatusCode(500);
}
return Ok(thing);
}You can declare a OneOf as a type, either for reuse of the type, or to provide additional members, by inheriting from OneOfBase. The derived class will inherit the .Match, .Switch, and .TryPick𝑥 methods.
public class StringOrNumber : OneOfBase<string, int>
{
StringOrNumber(OneOf<string, int> _) : base(_) { }
// optionally, define implicit conversions
// you could also make the constructor public
public static implicit operator StringOrNumber(string _) => new StringOrNumber(_);
public static implicit operator StringOrNumber(int _) => new StringOrNumber(_);
public (bool isNumber, int number) TryGetNumber() =>
Match(
s => (int.TryParse(s, out var n), n),
i => (true, i)
);
}
StringOrNumber x = 5;
Console.WriteLine(x.TryGetNumber().number);
// prints 5
x = "5";
Console.WriteLine(x.TryGetNumber().number);
// prints 5
x = "abcd";
Console.WriteLine(x.TryGetNumber().isNumber);
// prints FalseYou can automatically generate OneOfBase hierarchies using GenerateOneOfAttribute and partial class that extends OneOfBase using
a Source Generator (thanks to @romfir for the contribution :D). Install it via
Install-Package OneOf.SourceGenerator
and then define a stub like so:
[GenerateOneOf]
public partial class StringOrNumber : OneOfBase<string, int> { }During compilation the source generator will produce a class implementing the OneOfBase boiler plate code for you. e.g.
public partial class StringOrNumber
{
public StringOrNumber(OneOf.OneOf<System.String, System.Int32> _) : base(_) { }
public static implicit operator StringOrNumber(System.String _) => new StringOrNumber(_);
public static explicit operator System.String(StringOrNumber _) => _.AsT0;
public static implicit operator StringOrNumber(System.Int32 _) => new StringOrNumber(_);
public static explicit operator System.Int32(StringOrNumber _) => _.AsT1;
}
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