[AI Refactoring] DownloadErrorFactory.ThrowException default 분기 누락 → 미매핑 에러 코드 silent failure (호출부 45곳) + switch expression 전환 [Scope: src/Tizen.Content.Download] (2026-07-26)

[JoonghyunCho]

[Type: Refactoring]
[Scope: src/Tizen.Content.Download]
[Priority: 🔴 Critical]
[Lens: Coding Guidelines, Modernization]

Observation

DownloadErrorFactory.ThrowException (src/Tizen.Content.Download/Tizen.Content.Download/DownloadErrorFactory.cs:58-100) is the single error-to-exception funnel for the whole module. Its switch enumerates 22 DownloadError cases — and has no default branch:

internal static void ThrowException(int errorCode, string errorMessage = null, string paramName = null)
{
    ...
    switch ((DownloadError)errorCode)
    {
        case DownloadError.InvalidParameter:
        ...
        case DownloadError.IoError: throw new InvalidOperationException(message);
        case DownloadError.NotSupported: throw new NotSupportedException(message);
        case DownloadError.PermissionDenied: throw new UnauthorizedAccessException(message);
    }
    // unmapped code -> falls through and RETURNS NORMALLY
}

All 45 call sites in the module (Request.cs 31, CacheManager.cs 8, Notification.cs 6) follow the pattern if (ret != (int)DownloadError.None) DownloadErrorFactory.ThrowException(ret, ...); and rely on the call never returning.

Problem

1. Silent failure at 45 call sites (bug risk — Coding Guidelines). If the native url_download layer ever returns a code outside the 22 mapped values (a newly added platform error, or a generic Tizen error not aliased into DownloadError), ThrowException returns normally and the caller proceeds as if the interop call succeeded. Concrete consequences: CacheManager.MaxCacheSize returns an uninitialized out value; the Request constructors keep building state on a broken _downloadId; Start() reports success while nothing was started. This is the identical defect class already confirmed in ConnectionErrorFactory.ThrowConnectionException ([AI Refactoring] ConnectionErrorFactory.ThrowConnectionException default 분기 누락 → switch expression 으로 silent failure 차단 [Scope: src/Tizen.Network.Connection] (2026-04-26) #7591).
2. 22-case switch statement with per-case throw (Modernization). The mapping is a pure value-to-exception translation — the canonical shape for a C# 8 switch expression with or patterns, which makes fall-through structurally impossible and cuts the method to a third of its size.

Proposed Improvement

Convert to a throw ... switch expression whose _ arm covers every unmapped code.

Before

switch ((DownloadError)errorCode)
{
    case DownloadError.InvalidParameter:
    case DownloadError.InvalidUrl:
    case DownloadError.InvalidDestination:
    case DownloadError.InvalidNetworkType: throw new ArgumentException(message, paramName);
    case DownloadError.OutOfMemory:
    case DownloadError.NetworkUnreachable:
    // ... 15 more cases ...
    case DownloadError.IoError: throw new InvalidOperationException(message);
    case DownloadError.NotSupported: throw new NotSupportedException(message);
    case DownloadError.PermissionDenied: throw new UnauthorizedAccessException(message);
}

After

internal static void ThrowException(int errorCode, string errorMessage = null, string paramName = null)
{
    DownloadError err = (DownloadError)errorCode;
    string message = String.IsNullOrEmpty(errorMessage) ? err.ToString() : errorMessage;

    throw err switch
    {
        DownloadError.InvalidParameter or DownloadError.InvalidUrl or
        DownloadError.InvalidDestination or DownloadError.InvalidNetworkType
            => new ArgumentException(message, paramName),
        DownloadError.NotSupported => new NotSupportedException(message),
        DownloadError.PermissionDenied => new UnauthorizedAccessException(message),
        _ => new InvalidOperationException(message),
    };
}

(The 19 codes previously mapped to InvalidOperationException collapse into the _ arm together with all future unmapped codes.)

Target Files

• src/Tizen.Content.Download/Tizen.Content.Download/DownloadErrorFactory.cs

Expected Impact (Quantitative Metrics)

• Call sites exposed to silent fall-through: 45 -> 0 (the method now provably always throws).
• Method body: ~30 LOC -> ~12 LOC; case labels 22 -> 6 pattern arms.
• Property getters that could return uninitialized out values after a swallowed error: eliminated (e.g. CacheManager.MaxCacheSize, CachePath, Request.DownloadedPath).

API Compatibility Check

• Public API signature change: none (internal factory; public surface untouched).
• Behavior change: previously-unmapped native error codes now throw InvalidOperationException instead of being silently ignored. This matches the <exception cref="InvalidOperationException"> contract already documented on every public member of the module. All 45 call sites are guarded by ret != None (or pass fixed non-None codes), so no success path ever reaches the throw.
• Tizen API Level floor: maintained (switch expression is C# 8 syntax — no new runtime API).

Impact Scope

• 1 internal method, single assembly. Measured via rg: 45 call sites, all inside src/Tizen.Content.Download (no cross-assembly callers of the internal factory).