Cleanup usage of Config.PreferredPlatformsForExtensions

src/BizHawk.Client.Common/RomLoader.cs

@@ -191,16 +191,6 @@ namespace BizHawk.Client.Common
 			OnLoadError?.Invoke(this, new RomErrorArgs(message, systemId, path, det, type));
 		}
 
-		private bool PreferredPlatformIsDefined(string extension)
-		{
-			if (_config.PreferredPlatformsForExtensions.ContainsKey(extension))
-			{
-				return !string.IsNullOrEmpty(_config.PreferredPlatformsForExtensions[extension]);
-			}
-
-			return false;
-		}
-
 		public IOpenAdvanced OpenAdvanced { get; set; }
 
 		private bool HandleArchiveBinding(HawkFile file)
@@ -290,9 +280,7 @@ namespace BizHawk.Client.Common
 					case DiscType.AudioDisc:
 					case DiscType.UnknownCDFS:
 					case DiscType.UnknownFormat:
-						game.System = PreferredPlatformIsDefined(ext)
+						game.System = _config.TryGetChosenSystemForFileExt(ext, out var sysID) ? sysID : "NULL";
-							? _config.PreferredPlatformsForExtensions[ext]
-							: "NULL";
 						break;
 
 					default: //"for an unknown disc, default to psx instead of pce-cd, since that is far more likely to be what they are attempting to open" [5e07ab3ec3b8b8de9eae71b489b55d23a3909f55, year 2015]
@@ -439,9 +427,9 @@ namespace BizHawk.Client.Common
 			if (string.IsNullOrEmpty(rom.GameInfo.System))
 			{
 				// Has the user picked a preference for this extension?
-				if (PreferredPlatformIsDefined(rom.Extension.ToLowerInvariant()))
+				if (_config.TryGetChosenSystemForFileExt(rom.Extension.ToLowerInvariant(), out var systemID))
 				{
-					rom.GameInfo.System = _config.PreferredPlatformsForExtensions[rom.Extension.ToLowerInvariant()];
+					rom.GameInfo.System = systemID;
 				}
 				else if (ChoosePlatform != null)
 				{

src/BizHawk.Client.Common/config/Config.cs

@@ -38,14 +38,18 @@ namespace BizHawk.Client.Common
 			PathEntries.RefreshTempPath();
 		}
 
-		// Core preference for generic file extension, key: file extension, value: a systemID or empty if no preference
+		/// <summary>
+		/// Used to determine the system a rom is classified as (and thus which core to use) for the times when our romloading autodetect magic can't determine a system.
+		/// Keys are file extensions, include the leading period in each, and use lowercase;
+		/// values are system IDs, use <see cref="string.Empty"/> for unset (though <see langword="null"/> should also work, omitting will remove from UI).
+		/// </summary>
 		public Dictionary<string, string> PreferredPlatformsForExtensions { get; set; } = new Dictionary<string, string>
 		{
 			[".bin"] = "",
-			[".rom"] =  "",
+			[".cue"] = "",
-			[".iso"] =  "",
 			[".img"] = "",
-			[".cue"] =  ""
+			[".iso"] = "",
+			[".rom"] = "",
 		};
 
 		public PathEntryCollection PathEntries { get; set; } = new PathEntryCollection();

src/BizHawk.Client.Common/config/ConfigExtensions.cs

@@ -135,5 +135,19 @@ namespace BizHawk.Client.Common
 		{
 			config.PutCoreSyncSettings(o, typeof(TCore));
 		}
+
+		/// <param name="fileExt">file extension, including the leading period and in lowercase</param>
+		/// <remarks><paramref name="systemID"/> will be <see langword="null"/> if returned value is <see langword="false"/></remarks>
+		public static bool TryGetChosenSystemForFileExt(this Config config, string fileExt, out string systemID)
+		{
+			var b = config.PreferredPlatformsForExtensions.TryGetValue(fileExt, out var v);
+			if (b && !string.IsNullOrEmpty(v))
+			{
+				systemID = v;
+				return true;
+			}
+			systemID = null;
+			return false;
+		}
 	}
 }