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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
{
}

Or you can use generic type.

public partial class OperationDataResult<[GenericUnionType] TResult, [GenericUnionType] TError>
{
}

// extend generic type union with additional Int32 type
[UnionType(typeof(int))]
public partial class ExtendedOperationDataResult<[GenericUnionType] TResult, [GenericUnionType] TError>
{
}

Null values are not allowed by default. This behavior can be overriden by AllowNull = true parameter.

[UnionType(typeof(int?), AllowNull = true)]
[UnionType(typeof(string), AllowNull = true)]
public partial class ResultNullable<[GenericUnionType(AllowNull = true)] T>
{
}

Examples

All examples can be found in examples project

Basic

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();
    }
}

Alias for each variant is generated based on type name. Use alias parameter to override it.

[UnionType(typeof(int))]
[UnionType(typeof(string))]
// default alias is 'ArrayOfTupleOfIntAndString' but it is overriden by alias parameter
[UnionType(typeof(Tuple<int,string>[]), alias: "Items")]
public partial class AliasResult
{
}

Handle all variants

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);
    }
}

JSON serialization (EXPERIMENTAL)

To add JSON support

• add JsonPolymorphicUnion attribute to union type
• add TypeDiscriminator to each type variant

Limitations:

• .NET 7 or newer
• only complex type variants

Example

[UnionType(typeof(JsonTestsFooJ), TypeDiscriminator = "Foo")]
[UnionType(typeof(JsonTestsBarJ), TypeDiscriminator = "Bar")]
[JsonPolymorphicUnion]
public partial class JsonTestsUnion
{
}

Union to union converter

When one union type's variants is subset of another union type's variants use one of the following attributes to convert one type to another: UnionConverterTo, UnionConverterFrom, or UnionConverter.

[UnionConverterFrom(typeof(DataAccessResult))] // use this attribute
public partial class BusinessLogicResult
{
}

[UnionConverterTo(typeof(BusinessLogicResult))] // OR this
public partial class DataAccessResult
{
}

[UnionConverter(typeof(DataAccessResult), typeof(BusinessLogicResult))] // OR this
public static partial class Converters
{
}

public class Repository
{
    public DataAccessResult UpdateItem()
    {
        return new NotFoundError();
    }
}

public class Service
{
    private readonly Repository _repository;

    public BusinessLogicResult Update()
    {
        var isValid = IsValid();
        if (!isValid)
        {
            return new ValidationError("the item is not valid");
        }

        var repositoryResult = _repository.UpdateItem();
        // implicit conversion DataAccessResult to BusinessLogicResult when `UnionConverterTo` or `UnionConverterFrom` attribute is used
        return repositoryResult;
        // OR extension method when UnionConverter attribute is used
        return repositoryResult.Convert();
    }

    private bool IsValid() => throw new NotImplementedException();
}

Configuration

<Project Sdk="Microsoft.NET.Sdk">
    <PropertyGroup>
        <UnionTypesGenerator_ExcludeFromCodeCoverage>false</UnionTypesGenerator_ExcludeFromCodeCoverage>
    </PropertyGroup>
</Project>