OneOf to IResult

Convert OneOf<T1,T2,...> discriminated unions to IResult in a single call — HTTP status codes are inferred from error type names and HttpStatusCode tags.

// In Minimal API endpoints
app.MapGet("/users/{id}", async (int id) =>
    (await _service.GetUserAsync(id)).ToIResult());

app.MapPost("/users", async (CreateUserRequest req) =>
    (await _service.CreateAsync(req)).ToPostResult());   // 201 Created on success

Setup — add only the arities you use:

[assembly: GenerateOneOf2ExtensionsAttribute]  // OneOf<T1,T2>.ToIResult()
[assembly: GenerateOneOf3ExtensionsAttribute]  // OneOf<T1,T2,T3>.ToIResult()
[assembly: GenerateOneOf4ExtensionsAttribute]  // OneOf<T1,T2,T3,T4>.ToIResult()

Without the corresponding attribute that arity's .ToIResult() extension is not generated.

9.4.1. OneOf<T1,T2>.ToIResult()

OneOf<NotFoundError, User> result = await _service.GetAsync(id);
return result.ToIResult();  // 404 or 200

9.4.2. OneOf<T1,T2,T3>.ToIResult()

OneOf<ValidationError, ConflictError, User> result = await _service.CreateAsync(request);
return result.ToIResult();  // 422 or 409 or 200

9.4.3. OneOf<T1,T2,T3,T4>.ToIResult()

OneOf<ValidationError, UnauthorizedError, NotFoundError, Order> result =
    await _service.GetOrderAsync(id);
return result.ToIResult();  // 422 or 401 or 404 or 200

9.4.4. HTTP Method Variants

Use typed variants for non-GET endpoints to get the correct success status:

Method	Success status	Typical use
.ToIResult()	200 OK	GET
.ToPostResult()	201 Created	POST
.ToPutResult()	200 OK	PUT / PATCH
.ToDeleteResult()	204 No Content	DELETE

app.MapPost("/orders",    async (req) => (await _svc.CreateAsync(req)).ToPostResult());
app.MapPut("/orders/{id}", async (id, req) => (await _svc.UpdateAsync(id, req)).ToPutResult());
app.MapDelete("/orders/{id}", async (id) => (await _svc.DeleteAsync(id)).ToDeleteResult());

9.4.5. Error → HTTP Status Mapping

Status codes are resolved in order of precedence:

1. HttpStatusCode tag set on the error object at construction (domain errors set this automatically)
2. Type-name heuristic — NotFoundError → 404, ValidationError → 422, ConflictError → 409, etc.
3. Default → 400 Bad Request

// Domain errors set HttpStatusCode at construction — no configuration needed
public class NotFoundError : Reason<NotFoundError>      // → 404
public class ValidationError : Reason<ValidationError>  // → 422
public class ConflictError : Reason<ConflictError>      // → 409
public class UnauthorizedError : Reason<UnauthorizedError>  // → 401
public class ForbiddenError : Reason<ForbiddenError>    // → 403

// Custom error with explicit tag
public class PaymentRequiredError : Error
{
    public PaymentRequiredError() => this.WithTag(HttpStatusCode.PaymentRequired);
}

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