error-handlinglisted

# Error Handling Patterns Reference for structured error handling in .NET APIs. Used by `dnp-planner`, `dnp-api-scaffolder`, and `dnp-tdd-developer-hard`. ## Philosophy: When to Use Results vs Exceptions | Scenario | Approach | |----------|----------| | Domain rule violation (not found, invalid state, business constraint) | `Result<TValue, TError>` — expected failure path | | Infrastructure failure (DB timeout, network error, config missing) | Exception — unexpected, unrecoverable at call site | | Validation failure (bad input from HTTP layer) | `ValidationProblemDetails` via model binding / FluentValidation | | Unhandled exception escaping to HTTP | `GlobalExceptionHandler` → 500 ProblemDetails | Never throw exceptions for expected domain outcomes. Never swallow exceptions at call sites. ## Result Type Define in a shared location (e.g., `Common/Result.cs`): ```csharp namespace MyApp.Common; public readonly record struct Result<TValue, TError> { private readonly TValue? _value; private readonly TError? _error; public bool IsSuccess { get; } public bool IsFailure => !IsSuccess; private Result(TValue value) { _value = value; IsSuccess = true; } private Result(TError error) { _error = error; IsSuccess = false; } public static Result<TValue, TError> Success(TValue value) => new(value); public static Result<TValue, TError> Failure(TError error) => new(error); public TResult Match<TResult>( Func<TValue, TResult> onSuccess,