ErrorOrX

Discriminated unions for .NET with source-generated ASP.NET Core Minimal API integration. Zero boilerplate, full AOT support.

Installation

dotnet add package ErrorOrX.Generators

Quick Start

// Program.cs
var app = WebApplication.CreateSlimBuilder(args).Build();
app.MapErrorOrEndpoints();
app.Run();

// TodoApi.cs
using ErrorOr;

public static class TodoApi
{
    [Get("/todos/{id}")]
    public static ErrorOr<Todo> GetById(int id, ITodoService svc)
        => svc.GetById(id).OrNotFound($"Todo {id} not found");

    [Post("/todos")]
    public static ErrorOr<Todo> Create(CreateTodoRequest req, ITodoService svc)
        => svc.Create(req);  // 201 Created

    [Delete("/todos/{id}")]
    public static ErrorOr<Deleted> Delete(int id, ITodoService svc)
        => svc.Delete(id) ? Result.Deleted : Error.NotFound();
}

Nullable-to-ErrorOr Extensions

Convert nullable values to ErrorOr<T> with fluent extensions:

// Auto-generates error code from type name (e.g., "Todo.NotFound")
return _todos.Find(t => t.Id == id).OrNotFound($"Todo {id} not found");
return user.OrUnauthorized("Invalid credentials");
return record.OrValidation("Record is invalid");

// Custom errors
return value.OrError(Error.Custom(422, "Custom.Code", "Custom message"));
return value.OrError(() => BuildExpensiveError());

Extension	Error Type	HTTP	Description
.OrNotFound()	NotFound	404	Resource not found
.OrValidation()	Validation	400	Input validation failed
.OrUnauthorized()	Unauthorized	401	Authentication required
.OrForbidden()	Forbidden	403	Insufficient permissions
.OrConflict()	Conflict	409	State conflict
.OrFailure()	Failure	500	Operational failure
.OrUnexpected()	Unexpected	500	Unexpected error
.OrError(Error)	Any	Any	Custom error
.OrError(Func)	Any	Any	Lazy custom error

Error Types

Error.Validation("User.InvalidEmail", "Email format is invalid")
Error.NotFound("User.NotFound", "User does not exist")
Error.Conflict("User.Duplicate", "Email already registered")
Error.Unauthorized("Auth.InvalidToken", "Token has expired")
Error.Forbidden("Auth.InsufficientRole", "Admin role required")
Error.Failure("Db.ConnectionFailed", "Database unavailable")
Error.Unexpected("Unknown", "An unexpected error occurred")
Error.Custom(422, "Validation.Complex", "Complex validation failed")

Factory	HTTP	Use Case
Error.Validation()	400	Input/request validation
Error.Unauthorized()	401	Authentication required
Error.Forbidden()	403	Insufficient permissions
Error.NotFound()	404	Resource doesn't exist
Error.Conflict()	409	State conflict (duplicate)
Error.Failure()	500	Known operational failure
Error.Unexpected()	500	Unhandled/unknown error
Error.Custom()	Any	Custom HTTP status code

Fluent API

// Chain operations (railway-oriented programming)
var result = ValidateOrder(request)
    .Then(order => ProcessPayment(order))
    .Then(order => CreateShipment(order))
    .FailIf(order => order.Total <= 0, Error.Validation("Order.InvalidTotal", "Total must be positive"));

// Handle both cases
return result.Match(
    order => Ok(order),
    errors => BadRequest(errors.First().Description));

// Provide fallback on error
var user = GetUser(id).Else(errors => DefaultUser);

// Side effects
GetUser(id).Switch(
    user => Console.WriteLine($"Found: {user.Name}"),
    errors => Logger.LogError(errors.First().Description));

Result Markers

Result.Success   // 200 OK (no body)
Result.Created   // 201 Created (no body)
Result.Updated   // 204 No Content
Result.Deleted   // 204 No Content

Smart Parameter Binding

The generator automatically infers parameter sources:

[Post("/todos")]
public static ErrorOr<Todo> Create(
    CreateTodoRequest req,    // -> Body (POST + complex type)
    ITodoService svc)         // -> Service (interface)
    => svc.Create(req);

[Get("/todos/{id}")]
public static ErrorOr<Todo> GetById(
    int id,                   // -> Route (matches {id})
    ITodoService svc)         // -> Service
    => svc.GetById(id).OrNotFound();

Middleware Attributes

[Post("/admin")]
[Authorize("Admin")]
[EnableRateLimiting("fixed")]
[OutputCache(Duration = 60)]
public static ErrorOr<User> CreateAdmin(CreateUserRequest req) { }
// Generates: .RequireAuthorization("Admin").RequireRateLimiting("fixed").CacheOutput(...)

Native AOT

Fully compatible with PublishAot=true. The generator produces reflection-free code with automatic JSON serialization context generation.

<PropertyGroup>
  <PublishAot>true</PublishAot>
</PropertyGroup>

Documentation

• API Reference
• Parameter Binding
• Diagnostics
• Changelog

License

MIT