Add command-line switch --open-ext-tool-dll

takes an absolute path; or a relative path with or without `.dll` (relative to
ExternalTools)

src/BizHawk.Client.EmuHawk/ArgParser.cs

@@ -39,6 +39,7 @@ namespace BizHawk.Client.EmuHawk
 		public HttpCommunication httpCommunication = null;
 		public SocketServer socketServer = null;
 		public MemoryMappedFiles memoryMappedFiles = null;
+		public string openExtToolDll;
 
 		/// <exception cref="ArgParserException"><c>--socket_ip</c> passed without specifying <c>--socket_port</c> or vice-versa</exception>
 		public void ParseArguments(string[] args, Func<byte[]> takeScreenshotCallback)
@@ -146,6 +147,13 @@ namespace BizHawk.Client.EmuHawk
 				{
 					audiosync = arg.Substring(arg.IndexOf('=') + 1) == "true";
 				}
+				else if (arg.StartsWith("--open-ext-tool-dll="))
+				{
+					// the first ext. tool from ExternalToolManager.ToolStripMenu which satisfies both of these will be opened:
+					// - available (no load errors, correct system/rom, etc.)
+					// - dll path matches given string; or dll filename matches given string with or without `.dll`
+					openExtToolDll = args[i].Substring(20);
+				}
 				else
 				{
 					cmdRom = args[i];

src/BizHawk.Client.EmuHawk/MainForm.cs

@@ -153,6 +153,26 @@ namespace BizHawk.Client.EmuHawk
 			};
 			UpdateChecker.GlobalConfig = Config;
 			UpdateChecker.BeginCheck(); // Won't actually check unless enabled by user
+
+			// open requested ext. tool
+			var requestedExtToolDll = _argParser.openExtToolDll;
+			if (requestedExtToolDll != null)
+			{
+				var enabled = ExternalToolManager.ToolStripMenu.Where(item => item.Enabled)
+					.Select(item => (ValueTuple<string, string>) item.Tag)
+					.ToList();
+				try
+				{
+					var found = enabled.First(tuple => tuple.Item1 == requestedExtToolDll
+						|| Path.GetFileName(tuple.Item1) == requestedExtToolDll
+						|| Path.GetFileNameWithoutExtension(tuple.Item1) == requestedExtToolDll);
+					Tools.LoadExternalToolForm(found.Item1, found.Item2);
+				}
+				catch (Exception)
+				{
+					Console.WriteLine($"requested ext. tool dll {requestedExtToolDll} could not be loaded");
+				}
+			}
 		}
 
 		static MainForm()
@@ -1552,6 +1572,8 @@ namespace BizHawk.Client.EmuHawk
 		// input state which has been destined for client hotkey consumption are colesced here
 		private readonly InputCoalescer _hotkeyCoalescer = new InputCoalescer();
 
+		private readonly (string, string)? _loadExtToolForm;
+
 		private readonly PresentationPanel PresentationPanel;
 
 		// countdown for saveram autoflushing