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Domain guide

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How to write domain and application code with Result. Everything on this page comes from the Offside package alone — no ASP.NET dependency.

Returning a result

using Offside;

public Result<Order> Get(string id)
{
    var order = _orders.Find(id);
    return order is null
        ? Result<Order>.Failure(Error.NotFound("order", id))
        : Result<Order>.Success(order);
}

For an operation with no return value, use the non-generic Result:

public Result Cancel(string id)
{
    var order = _orders.Find(id);
    if (order is null)
        return Result.Failure(Error.NotFound("order", id));

    if (order.Shipped)
        return Result.Failure(Error.Custom("order.already_shipped", ErrorKind.Conflict, new { orderId = id }));

    order.Cancel();
    return Result.Success();
}

Error factories

Every factory produces a specific ErrorKind and a default catalog code.

Factory	Kind	Code	Arguments
Error.NotFound(resource, id?)	NotFound	not_found	resource, id
Error.Gone(resource, id?)	Gone	gone	resource, id
Error.Conflict(resource, reason?)	Conflict	conflict	resource, reason
Error.Validation(field, code?, attemptedValue?)	Validation	validation, or code	field, attemptedValue
Error.BadRequest(reason?)	BadRequest	bad_request	reason
Error.Unauthorized(reason?)	Unauthorized	unauthorized	reason
Error.Forbidden(reason?)	Forbidden	forbidden	reason
Error.PreconditionFailed(reason?)	PreconditionFailed	precondition_failed	reason
Error.Unprocessable(reason?)	Unprocessable	unprocessable	reason
Error.TooManyRequests(reason?)	TooManyRequests	too_many_requests	reason
Error.Unexpected(detail?)	Unexpected	unexpected	detail
Error.Custom(code, kind, arguments?, field?, errorCode?)	your choice	your choice	your choice

Every factory also takes an optional trailing errorCode. Blank or null uses Error.DefaultErrorCode(Kind) (NOT_FOUND, VALIDATION, TOO_MANY_REQUESTS, …). Surrounding whitespace is trimmed.

Error.Validation is the only factory that sets Field, and the only one that lets you override the code without going through Custom:

Error.Validation("email");                                 // code "validation", field "email"
Error.Validation("email", "email.malformed", input);       // code "email.malformed", field "email"

Custom errors for business rules

When a rule deserves its own message, keep the kind and invent the code:

Error.Custom("order.already_shipped", ErrorKind.Conflict, new { orderId }, errorCode: "ORDER_ALREADY_SHIPPED");
Error.Custom("payment.insufficient_funds", ErrorKind.Unprocessable, new { required, available });
Error.Custom("coupon.expired", ErrorKind.PreconditionFailed, new { coupon = code, expiredOn }, field: "coupon");

The code becomes the catalog key, so add a matching entry to every catalog:

{ "order.already_shipped": "Order {orderId} has already shipped." }

An empty or whitespace code throws ArgumentException. Surrounding whitespace is trimmed.

A dotted, namespaced convention (order.already_shipped) is worth adopting for catalog keys. Clients branch on ErrorCode (ORDER_ALREADY_SHIPPED), not on the catalog key and not on detail. Several codes may share one error code.

Arguments

Arguments is a read-only snapshot taken at construction. Anonymous objects and dictionaries both work:

Error.Custom("quota.exceeded", ErrorKind.TooManyRequests, new { limit = 100, window = "1h" });
Error.Custom("quota.exceeded", ErrorKind.TooManyRequests, new Dictionary<string, object?> { ["limit"] = 100 });

Two rules:

• Arguments feed message templates. A {limit} token in the catalog is filled from the limit entry.
• Never put secrets in them. They shape text that ships to the client. Tokens, password hashes, and internal connection strings do not belong there — put diagnostic material in Error.Unexpected(detail), which is sanitized on the way out.

Reading a result

Value throws InvalidOperationException on a failed result. Pick whichever of these fits:

// Pattern match into a single value
var message = result.Match(
    order => $"Found {order.Id}",
    errors => $"Failed: {errors[0].Code}");

// Try-style
if (result.TryGetValue(out var order))
    Console.WriteLine(order.Id);

// Explicit check
if (result.IsSuccess)
    Use(result.Value);

Composing

Map transforms a value. Bind chains an operation that can itself fail. Both short-circuit — the delegate never runs on a failed result, and the original errors pass through untouched:

Result<OrderDto> dto = _orders.Get(id)
    .Bind(order => _pricing.Apply(order))   // Result<Order>
    .Map(order => OrderDto.From(order));    // plain value

Result.Combine merges independent results, concatenating errors in argument order. This is how you report every validation failure at once instead of one per round-trip:

var combined = Result.Combine(
    ValidateEmail(request.Email),
    ValidateName(request.Name),
    ValidateAge(request.Age));

// All three failed → combined.Errors has three entries, in that order.

There is a Combine<T>(params Result<T>[]) overload for value results; it merges the errors and discards the values, returning a unit Result.

Deliberate omissions

Two things you may reach for and not find:

• No implicit conversion from T to Result<T>. Constructing a result is always explicit, so a value never becomes a success by accident — and a refactor that changes a return type produces a compile error rather than silent behaviour.
• No Apply. Combine covers the accumulate-all-errors case without a second, subtly different way to do it.

The escape hatch

For boundaries that cannot return a Result — a constructor, an interface you do not own:

public Order(string id, int quantity)
{
    if (quantity <= 0)
        throw Error.Validation("quantity", attemptedValue: quantity).ToException();
    ...
}

DomainException.Errors carries the errors through, so a boundary handler can still render them properly. Use it sparingly: a codebase where half the failures throw has lost the guarantee that the signature tells you what can go wrong.

Testing

Errors compare by value, so assertions read plainly:

var result = service.Get("missing");

Assert.True(result.IsFailure);
Assert.Equal(Error.NotFound("order", "missing"), result.Errors[0]);

Assert on Code, ErrorCode, and Kind rather than on resolved text — the text is catalog data and is expected to change without breaking anything.