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();
});

Global Exception Handler

Catches unexpected exceptions and converts them to ProblemDetails. For the modern IExceptionHandler approach (preferred), see knowledge/common-infrastructure.md. The inline lambda below works for simple cases:

// Program.cs
app.UseExceptionHandler(errorApp =>
{
    errorApp.Run(async context =>
    {
        var exception = context.Features.Get<IExceptionHandlerFeature>()?.Error;
        var logger = context.RequestServices.GetRequiredService<ILogger<Program>>();

        logger.LogError(exception, "Unhandled exception for {Method} {Path}",
            context.Request.Method, context.Request.Path);

        var problem = new ProblemDetails
        {
            Title = "An unexpected error occurred",
            Status = StatusCodes.Status500InternalServerError,
            Type = "https://tools.ietf.org/html/rfc9110#section-15.6.1"
        };

        // Don't leak details in production
        if (context.RequestServices.GetRequiredService<IHostEnvironment>().IsDevelopment())
        {
            problem.Detail = exception?.Message;
        }

        context.Response.StatusCode = problem.Status.Value;
        await context.Response.WriteAsJsonAsync(problem);
    });
});

FluentValidation with Endpoint Filters

// Validator
public class CreateOrderValidator : AbstractValidator<CreateOrderRequest>
{
    public CreateOrderValidator()
    {
        RuleFor(x => x.CustomerId)
            .NotEmpty().WithMessage("Customer ID is required");

        RuleFor(x => x.Items)
            .NotEmpty().WithMessage("At least one item is required");

        RuleForEach(x => x.Items).ChildRules(item =>
        {
            item.RuleFor(x => x.ProductId).NotEmpty();
            item.RuleFor(x => x.Quantity).GreaterThan(0);
        });
    }
}

// Generic validation filter
public class ValidationFilter<TRequest> : IEndpointFilter
{
    public async ValueTask<object?> InvokeAsync(
        EndpointFilterInvocationContext context,
        EndpointFilterDelegate next)
    {
        var validator = context.HttpContext.RequestServices.GetService<IValidator<TRequest>>();
        if (validator is null)
            return await next(context);

        var request = context.Arguments.OfType<TRequest>().FirstOrDefault();
        if (request is null)
            return await next(context);

        var result = await validator.ValidateAsync(request);
        if (!result.IsValid)
        {
            return TypedResults.ValidationProblem(result.ToDictionary());
        }

        return await next(context);
    }
}

// Registration
group.MapPost("/", CreateOrder)
    .AddEndpointFilter<ValidationFilter<CreateOrderRequest>>();

Typed Error Results

For richer error handling, use typed error enums or error objects.

public abstract record Error(string Code, string Message);
public record NotFoundError(string Entity, object Id)
    : Error("not_found", $"{Entity} with ID {Id} was not found");
public record ValidationError(string Field, string Message)
    : Error("validation", Message);
public record ConflictError(string Message)
    : Error("conflict", Message);

// Map to HTTP status codes
public static IResult ToHttpResult(this Error error) => error switch
{
    NotFoundError => TypedResults.Problem(title: error.Message, statusCode: 404),
    ValidationError => TypedResults.Problem(title: error.Message, statusCode: 400),
    ConflictError => TypedResults.Problem(title: error.Message, statusCode: 409),
    _ => TypedResults.Problem(title: error.Message, statusCode: 500)
};

Anti-patterns

Don't Throw Exceptions for Flow Control

// BAD — exceptions for expected outcomes
public Order GetOrder(Guid id)
{
    var order = db.Orders.Find(id)
        ?? throw new NotFoundException($"Order {id} not found");
    return order;
}

// GOOD — Result pattern
public Result<Order> GetOrder(Guid id)
{
    var order = db.Orders.Find(id);
    return order is not null
        ? Result.Success(order)
        : Result.Failure<Order>($"Order {id} not found");
}

Don't Return Raw Error Strings from APIs

// BAD — inconsistent error format
return Results.BadRequest("Something went wrong");
return Results.BadRequest(new { error = "Invalid input" });

// GOOD — always ProblemDetails
return TypedResults.Problem(title: "Invalid input", statusCode: 400);
return TypedResults.ValidationProblem(validationResult.ToDictionary());

Don't Catch and Swallow Exceptions

// BAD — silently swallowing
try { await ProcessOrder(order); }
catch (Exception) { /* ignore */ }

// GOOD — log and handle appropriately
try { await ProcessOrder(order); }
catch (PaymentException ex)
{
    logger.LogWarning(ex, "Payment failed for order {OrderId}", order.Id);
    return Result.Failure<Order>("Payment processing failed");
}

Decision Guide

Scenario	Recommendation
Expected business failure	Result pattern
Input validation	FluentValidation with endpoint filter
Unexpected crash	Global exception handler → ProblemDetails
API error format	RFC 9457 ProblemDetails — always
Validation in handler	Return Result.Failure, don't throw
External service failure	Catch specific exception, return Result.Failure
Logging errors	Structured logging with correlation ID