Discriminated unions for .NET with fluent error handling. ErrorOr<T> provides railway-oriented programming, nullable-to-ErrorOr extensions (OrNotFound, OrValidation, OrError), Match/Then/Else/Switch/FailIf fluent API, and rich typed errors mapped to HTTP status codes.
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();
