error-handling

Error Handling

Core Principles

1. Use the Result pattern for expected failures — Don't throw exceptions for things like "order not found" or "validation failed". These are expected outcomes, not exceptional conditions. See ADR-002.
2. Reserve exceptions for unexpected failures — Database connection lost, null reference bugs, network timeouts — these are truly exceptional and should propagate to the global handler.
3. Every API error returns ProblemDetails — RFC 9457 is the standard. Every error response has type, title, status, detail, and optionally errors.
4. Validate at the boundary — Validate incoming requests at the API layer, not deep inside business logic.

Patterns

Result Pattern

A simple, generic result type that carries either a value or errors.

public class Result
{
    public bool IsSuccess { get; }
    public bool IsFailure => !IsSuccess;
    public List<string> Errors { get; }

    protected Result(bool isSuccess, List<string>? errors = null)
    {
        IsSuccess = isSuccess;
        Errors = errors ?? [];
    }

    public static Result Success() => new(true);
    public static Result Failure(params string[] errors) => new(false, [..errors]);
    public static Result<T> Success<T>(T value) => new(value);
    public static Result<T> Failure<T>(params string[] errors) => new(errors);
}

public class Result<T> : Result
{
    public T Value { get; }

    internal Result(T value) : base(true) => Value = value;
    internal Result(IEnumerable<string> errors) : base(false, [..errors]) => Value = default!;
}

Result to ProblemDetails Mapping

public static class ResultExtensions
{
    public static IResult ToProblemDetails(this Result result, int statusCode = 400)
    {
        return TypedResults.Problem(
            title: "One or more errors occurred",
            statusCode: statusCode,
            extensions: new Dictionary<string, object?>
            {
                ["errors"] = result.Errors
            });
    }
}

// Usage in endpoint
group.MapPost("/", async (CreateOrder.Command command, ISender sender, CancellationToken ct) =>
{
    var result = await sender.Send(command, ct);
    return result.IsSuccess
        ? TypedResults.Created($"/api/orders/{result.Value.Id}", result.Value)
        : result.ToProblemDetails();
});