OneOf

OneOf is a popular library that enhances C# with discriminated unions, which basically means that you can create types that can represent different types - e.g. an instance of OneOf<int, string, float> will hold a single value that will either be an int, or a string, or a float.

Discriminated unions are a great way to do exhaustive type matching, which means that the compiler can enforce that all possible types are being handled:

• By using OneOf<>.Switch() you need to specify an an Action<T> for each of possible type T
• By using OneOf<>.Match() you need to specify a Func<T, TResult> that will convert each possible type T into a single type TResult

OneOf library intentionally does NOT allow the underlying types to be deconstructed because it kind of defeats the purpose of the library, which is exactly to enforce compile-time exhaustive type matching.

However, in some cases it might be helpful to deconstruct the OneOf object into its underlying types...

OneOf.DeconstructorExtensions

This library extends OneOf<> and OneOfBase<> with deconstructor methods to extract the underlying types, while using the same semantic as discriminated-union: only ONE of the return types will be non-null.

The major problem solved by this library is that deconstruction is not trivial because of non-nullable value types:

• When we have an instance of OneOf<T0, T1, ...> and we deconstruct it into its underlying types (T0, T1, ...) we want to have the same semantic as the union-type: we want only one element to be non-null
• This means that any type that is a non-nullable value type (like a primitive type, or struct or an enum) would have to be converted into an equivalent nullable type. (e.g. int should become Nullable<int> so it can be null).
• The problem is that we can't just have a single deconstructor that converts all types T to Nullable<T>, as this can only be done for non-nullable value types.
• This means that we need a lot of overloads. Let's say we're talking about OneOf<T0,T1> - each one of those types T may or may not be a non-nullable value type, so we will have 4 different combinations, all having same signature but different where constraints for the generic-types. For OneOf<T0,T1,T2> we have 8 combinations, etc - this goes up to OneOf<T0,T1,T2,T3,T4,T5,T6> which has 128 combinations.
• Each one of those combinations will convert only the right types to Nullable<>

All the hundreds of overloads are created by a CodegenCS code generator template (output example here).

As a more concrete example, let's say you have a class SalesOrder and a method with this signature:

public OneOf<SalesOrder, CreateSalesOrderErrorEnum> CreateSalesOrder(etc..)

If you do a deconstruction like var (order, error) = CreateSalesOrder(...) and the method succeeds (returns a sales order), you would expect that the deconstructed order is not-null and error is null.
However, since CreateSalesOrderErrorEnum is enum (non-nullable value type), this deconstruction (where the first type T0 is a reference-type and the second type T1 is a non-nullable value type) should return T0? and Nullable<T1> (in other words SalesOrder? and Nullable<CreateSalesOrderErrorEnum>). So basically our deconstruction overloads will always find the right combination of types to ensure that all return types are nullable, and only one of the results is a non-null value.

OneOf.ToTupleExtensions

This library extends OneOf<> and OneOfBase<> with methods to convert them into Tuple<> or ValueTuple<> (which have methods to deconstruct into the underlying types), and uses the same logic as OneOf.DeconstructorExtensions: non-nullable value types are converted to nullable types so that only one element of the tuple will be non-null.

One specific challenge here (doesn't happen for deconstructors but happens here) is that the compiler does NOT consider constraints as part of the method signature, so the compiler thinks that all overloads are possible matches - and we have to use an overload-resolution hack to let the compiler correctly disambiguate between the multiple overloads (which all have identical signature) and use the right method (where we convert only the right types to Nullable<>).

Subclassing alternative

If you are subclassing from OneOfBase<> (instead of using the standard OneOf<> structs) then instead of using our extension methods you could create your own deconstructor. You can subclass with concrete types:

public class CreateUserResult : OneOfBase<User, CreateUserErrorEnum, List<ValidationError> {...}

...or you can subclass with generic types with explicit constraints

public class Result<TResult, TErrorEnum> : OneOfBase<TResult, TErrorEnum, List<ValidationError>>
    where TResult: class
    where TErrorEnum: struct
{...}

In both cases it's clear to the compiler what types are non-nullable value types and you can just handle that (and convert to nullable types) in your deconstructor.

But why would you manually write your own classes and destructors when you can just use OneOf<> and use our extension-methods? :-)

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Motivation: using Discriminated Unions for the Result Pattern

The Result Pattern is a way of representing the outcome of an operation (whether it's successful or has encountered an error) in a more explicit and structured manner. It's an alternative to exception-based error handling. (Hold your horses - I'm not telling that you should never use exceptions - I'm just telling that there are some other design patterns that might be used in conjunction to exceptions and in some cases might be better than throwing exceptions, especially if you're using exceptions as flow control). This pattern can be used by writing your own wrappers or by using some well-established libraries like FluentResults, error-or, ardalis Result, and many others.

One downside of using a third-party library for Result pattern is that you will have limited degree of freedom for extensibility. As an example if you use FluentResults your methods will return a Result if they don't return any object in case of a success and will return Result<T> if they return type T in case of success - but in both types the errors are stored in a very generic List<IError> Errors which is very generic: it can both contain one or more errors, they can be of any type that implements IError, they can have different types inside it, etc. So basically the signature of a method Result<SalesOrder> doesn't give us any hint on what kind of errors we may get inside this result object.

By using discriminated unions and more specifically the OneOf library we can solve this problem by making more explicit what are the possible errors that each method can return. As an example, consider the following signature:

public OneOf<Success<User>, Error<OutOfStockError>, Error<string>> CreateSalesOrder(SalesOrder order)

It's clear from the signature that the method will either return a SalesOrder (in case of success) or a OutOfStockError instance (which probably requires some special handling from the immediate-caller) or else some generic error represented by a single string, which probably should just be displayed to the user.

The Success<T> wrapper and Error<T> wrapper are just dummy wrappers that provide a clearer semantic on each type, but they are not required. You could for example have something like this:

public OneOf<SalesOrder, CreateSalesOrderErrorEnum, List<ValidationError>> CreateSalesOrder(SalesOrder order)

It's also clear from the signature that this method will either return a SalesOrder or a single error from a given enum type (which probably the immediate-caller should act upon) or a list of one-or-more validation errors that should probably just displayed in the UI.

To sum, discriminated unions provide a more idiomatic way of using the Result Pattern. Even when we don't want/need exhaustive type matching...

Result-Pattern example (using OneOf and OneOf Tuple Deconstruction)

Let's say your service returns a discriminated-union like this:

/// <summary>
/// CreateUser returns ONLY one of these types (the others will be null): 
/// - a successful result (a User object)
/// - an error (of enum type) 
/// - one-or-more validation errors
/// </summary>
public OneOf<User, CreateUserErrorEnum, List<ValidationError>> CreateUser(CreateUserDTO newUserInfo)
{
    // Validation errors should be the first ones to be tested and early return
    // I model them as "ValidationErrors" because the caller just pass those errors back to the UI, doesn't act upon them
    if (string.IsNullOrEmpty(newUserInfo?.UserName))
        return new List<ValidationError>() { new ValidationError($"{nameof(newUserInfo.UserName)} is required") };

    // Non-Validation errors are errors that might require the caller to act based on the errors
    // (in this bad example there's not much to do other than pass the message to the user, but you get the idea - caller could SWITCH upon the enum and act)
    if (newUserInfo!.UserName == "Rick")
        return CreateUserErrorEnum.USERNAME_NOT_AVAILABLE;
    if (string.IsNullOrEmpty(newUserInfo.Password) || newUserInfo.Password.Length < 8)
        return CreateUserErrorEnum.WEAK_PASSWORD;

    // Early returns reduce the need for nested ifs. If everything went fine we return the resulting object
    var user = new User() { FirstName = newUserInfo.FirstName, LastName = newUserInfo.LastName, UserName = newUserInfo.UserName };
    return user;
}

The standard way of handling this with OneOf would be doing a Switch() with a lambda for each type:

var oneOf = svc.CreateUser(createUserCommand);
oneOf.Switch(
    user => {
        // Successful return. Show created user or something like that
    },
    errorEnum => {
        // handle whatever error we got (instead of an enum we could also get individual types for each possible error)
    },
    validationErrors => {
        // Show the validation errors back to the UI
    });

But sometimes using switch and lambas can make our code complex and harder to maintain. Sometimes all we want is some early-returns.
With ToTuple extensions and deconstruct operators we can use a concise golang-style syntax like this:

// Extensions enables us to use deconstruction (which does not exist in OneOf library)
var (user, error, validationErrors) = service.CreateUser(createUserCommand);

if (validationErrors != null)
{
    // Show the validation errors back to the UI
    return;
}

// If it wasn't for the Nullable conversion then errorEnum
// would never possibly be null
if (errorEnum != null)
{
    // switch and handle
    return;
}

// else, success! (user!=null)
// Usually this is the larger code-block, 
// and that's why having early-returns for the other cases makes this code cleaner and easy to maintain!
// (no need to have a giant method with 3 different lambda-actions inside it)

License

MIT License