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Easy to use F#-like ~discriminated~ unions for C# with exhaustive compile time matching


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OneOf NuGet GitHub

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

Getting Started

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.

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. 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);
    return user;

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");

As an 'Option' Type

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)
  • 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 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.

Advantages over switch or if or exception based control flow:

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.


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 = ...;
    str => AddEntry(DateTime.Parse(str), foo),
    int => AddEntry(int, foo)

TryPick𝑥 method

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>
      return StatusCode(500);

    return Ok(thing);

Reusable OneOf Types using OneOfBase

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() =>
            s => (int.TryParse(s, out var n), n),
            i => (true, i)

StringOrNumber x = 5;
// prints 5

x = "5";
// prints 5

x = "abcd";
// prints False

OneOfBase Source Generation

You 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:

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;