error-handling

---
name: error-handling
description: >
license: MIT
---

# 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.

```csharp
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

```csharp
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:

```csharp
// 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

```csharp
// 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.

```csharp
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

```csharp
// 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

```csharp
// 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

```csharp
// 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 |