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TRLS005: Consider using MatchError for error type discrimination

Cause

Using switch statements or if/else chains to discriminate between error types when MatchError provides a better, type-safe alternative.

Rule Description

When you need to handle different error types differently, MatchError provides type-safe pattern matching with compile-time checking and a required fallback for unhandled types.

How to Fix Violations

Replace switch/if statements with MatchError:

// ❌ Less ideal - Manual switch
if (result.IsFailure)
{
    switch (result.Error)
    {
        case ValidationError ve:
            return BadRequest(ve.Detail);
        case NotFoundError nfe:
            return NotFound(nfe.Detail);
        default:
            return StatusCode(500, result.Error.Detail);
    }
}

// ✅ Better - Type-safe MatchError
return result.MatchError(
    onValidationError: ve => BadRequest(ve.Detail),
    onNotFoundError: nfe => NotFound(nfe.Detail),
    onOtherError: error => StatusCode(500, error.Detail));

Benefits of MatchError

  1. Type Safety: Each error handler gets the correctly typed error
  2. Exhaustiveness: Fallback handler is required
  3. Readability: Clear intent for error handling
  4. Maintainability: Easy to add new error type handlers

Example

public IActionResult UpdateCustomer(Guid id, UpdateCustomerDto dto)
{
    return customerService.UpdateCustomer(id, dto)
        .Match(
            onSuccess: customer => Ok(customer.ToDto()),
            onFailure: error => error.ToHttpResult());
}

public static class ErrorExtensions
{
    public static IActionResult ToHttpResult(this Error error) =>
        error.MatchError(
            onValidationError: ve => new BadRequestObjectResult(ve.Detail),
            onNotFoundError: nfe => new NotFoundObjectResult(nfe.Detail),
            onUnauthorizedError: ue => new UnauthorizedObjectResult(ue.Detail),
            onOtherError: e => new ObjectResult(e.Detail) { StatusCode = 500 });
}

When to Suppress Warnings

This is a suggestion-level diagnostic (Info severity). Suppress it if:

  • You prefer explicit switch statements
  • You're only checking for one specific error type
  • The error handling logic is complex and doesn't fit the MatchError pattern
  • TRLS004 - Unsafe access to Result.Error
  • TRLS011 - Use specific error type instead of base Error class
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