virzak/N.SourceGenerators.UnionTypes

Discriminated union type source generator

N.SourceGenerators.UnionTypes

Discriminated union type source generator

Motivation

C# doesn't support discriminated unions yet. This source generator helps automate writing union types with set of helper methods.

Getting Started

Add package reference to N.SourceGenerators.UnionTypes

dotnet add package N.SourceGenerators.UnionTypes

Create a partial class or struct that will be used as a union type

public partial class FooResult
{
}

Add types you want to use in a discriminated union

public record Success(int Value);
public record ValidationError(string Message);
public record NotFoundError;

public partial class FooResult
{
}

Add N.SourceGenerators.UnionTypes.UnionTypeAttribute to a union type.

using N.SourceGenerators.UnionTypes;

public record Success(int Value);
public record ValidationError(string Message);
public record NotFoundError;

[UnionType(typeof(Success))]
[UnionType(typeof(ValidationError))]
[UnionType(typeof(NotFoundError))]
public partial class FooResult
{
}

Examples

All examples can be found in examples project

Implicit conversion

public FooResult ImplicitReturn()
{
    // you can return any union type variation without creating FooResult
    return new NotFoundError();
}

Explicit conversion

public ValidationError ExplicitCast(FooResult result)
{
    return (ValidationError)result;
}

Checking value type

public void ValueTypeProperty()
{
    FooResult foo = GetFoo();
    Type valueType = foo.ValueType; // returns typeof(NotFoundError)

    static FooResult GetFoo()
    {
        return new NotFoundError();
    }
}

TryGet method is used to check if union contains a specific type

public void TryGetValue()
{
    FooResult foo = GetFoo();
    if (foo.TryGetNotFoundError(out var notFoundError))
    {
        // make something with notFoundError
    }

    static FooResult GetFoo()
    {
        return new NotFoundError();
    }
}

Match and MatchAsync methods are used to convert union type to another type. These methods force you to handle all possible variations.

public IActionResult MatchMethod(FooResult result)
{
    return result.Match<IActionResult>(
        success => new OkResult(),
        validationError => new BadRequestResult(),
        notFoundError => new NotFoundResult()
    );
}

public async Task<IActionResult> MatchAsyncMethod(FooResult result, CancellationToken cancellationToken)
{
    return await result.MatchAsync<IActionResult>(
        static async (success, ct) =>
        {
            await SomeWork(success, ct);
            return new OkResult();
        }, static async (validationError, ct) =>
        {
            await SomeWork(validationError, ct);
            return new BadRequestResult();
        }, static async (notFoundError, ct) =>
        {
            await SomeWork(notFoundError, ct);
            return new NotFoundResult();
        }, cancellationToken);

    static Task SomeWork<T>(T value, CancellationToken ct)
    {
        return Task.Delay(100, ct);
    }
}

Switch and SwitchAsync methods are used to execute some work based on inner type

 public void SwitchMethod(FooResult result)
{
    result.Switch(
        success => SomeWork(success),
        validationError => SomeWork(validationError),
        notFoundError => SomeWork(notFoundError)
    );

    static void SomeWork<T>(T value)
    {
        throw new NotImplementedException();
    }
}

public async Task SwitchAsyncMethod(FooResult result, CancellationToken cancellationToken)
{
    await result.SwitchAsync(
        static async (success, ct) =>
        {
            await SomeWork(success, ct);
        }, static async (validationError, ct) =>
        {
            await SomeWork(validationError, ct);
        }, static async (notFoundError, ct) =>
        {
            await SomeWork(notFoundError, ct);
        }, cancellationToken);

    static Task SomeWork<T>(T value, CancellationToken ct)
    {
        return Task.Delay(100, ct);
    }
}