2. Custom exceptions and exception hierarchies

// A small hierarchy for the application's expected failures
public abstract class AppException : Exception
{
    protected AppException(string message, Exception? inner = null) : base(message, inner) { }
    public abstract string ErrorCode { get; }
}

public sealed class NotFoundException(string entity, object key)
    : AppException($"{entity} '{key}' was not found.")
{
    public override string ErrorCode => "not_found";
}

public sealed class BusinessRuleException(string message, string code) : AppException(message)
{
    public override string ErrorCode => code;                // e.g. "credit_limit_exceeded"
}

public sealed class ConflictException(string message) : AppException(message)
{
    public override string ErrorCode => "conflict";
}

// Usage in a service
public async Task ApproveAsync(int orderId)
{
    var order = await db.Orders.FindAsync(orderId) ?? throw new NotFoundException("Order", orderId);
    if (order.Total > order.Customer.CreditLimit)
        throw new BusinessRuleException("Order exceeds the customer's credit limit.", "credit_limit_exceeded");
}