API Hawk: MemoryDomains specified with attribute [RequiredService] will throw NullReferenceException when loading external tool DLL with EmuHawk using argument --open-ext-tool-dll .

[tommai78101]

Summary

When developing a DLL external tool project for BizHawk, if the tool needs to use MemoryDomains, it will throw a null pointer reference exception when the DLL is loaded by passing the argument --open-ext-tool-dll to EmuHawk.exe.

I did make sure to include this in my WinForm class:

		[RequiredService]
		public IEmulator Emulator { get; set; }

		[RequiredService]
		public IMemoryDomains MemoryDomains { get; set; }

And I also added this in the this.Load event from MainForm to bind a System.EventHandler delegate to this method:

		public void OnBotLoad(object sender, EventArgs eventArgs) {
			if (!CurrentMovie.IsActive() && !Tools.IsLoaded<TAStudio>()) {
				DialogController.ShowMessageBox("In order to proceed to use this tool, TAStudio is required to be opened.");
				this.Close();
				DialogResult = DialogResult.Cancel;
				return;
			}

			if (Config!.OpposingDirPolicy is not OpposingDirPolicy.Allow) {
				DialogController.ShowMessageBox("In order to proceed to use this tool, please check if the U+D/L+R controller binds policy is set to 'Allow'.");
				this.Close();
				DialogResult = DialogResult.Cancel;
				return;
			}

			if (OSTailoredCode.IsUnixHost) {
				ClientSize = new(707, 587);
			}

			_previousInvisibleEmulation = InvisibleEmulationCheckBox.Checked = Settings.InvisibleEmulation;
			_previousDisplayMessage = Config.DisplayMessages;
		}

Repro

1. Either pull down the commit here (commit ID: 23f8f660), or download the attachment below.

tool_src.zip

2. Build the DLL.
3. Follow the instructions from the Quick Development guide (here) to load the DLL via EmuHawk.
4. You should be able to see an exception thrown mentioning the MemoryDomains encountering a null pointer reference.

If not, you can try "Debugging a DLL using external application" and attaching the debugger to the application, by following this instruction from Microsoft:

https://docs.microsoft.com/en-us/visualstudio/debugger/how-to-debug-from-a-dll-project?view=vs-2022#specify-a-calling-app-in-a-c-dll-project-net-core-net-5

Output

These exceptions all point to the same thing.

Pictures are only here to help:

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=GeneticAlgorithmBot
  StackTrace:
   at GeneticAlgorithmBot.GeneticAlgorithmBot.Restart() in E:\LargeGithubProjects\Bizhawk-GeneticAlgorithmBot\src\GeneticAlgorithmBot.cs:line 266

System.NullReferenceException: Object reference not set to an instance of an object.
   at GeneticAlgorithmBot.GeneticAlgorithmBot.Restart()
   at BizHawk.Client.EmuHawk.ToolManager.LoadExternalToolForm(String toolPath, String customFormTypeName, Boolean focus, Boolean skipExtToolWarning)
   at BizHawk.Client.EmuHawk.MainForm.MainForm_Load(Object sender, EventArgs e)
   at System.EventHandler.Invoke(Object sender, EventArgs e)
   at System.Windows.Forms.Form.OnLoad(EventArgs e)
   at BizHawk.Client.EmuHawk.FormBase.OnLoad(EventArgs e)
   at System.Windows.Forms.Form.OnCreateControl()
   at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
   at System.Windows.Forms.Control.CreateControl()
   at System.Windows.Forms.Control.WmShowWindow(Message& m)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.Form.WmShowWindow(Message& m)
   at BizHawk.Client.EmuHawk.MainForm.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

Host env.

• BizHawk dev build at 84886b7; Win10 Pro 21H1; Intel/NVIDIA

[YoshiRulz]

IToolForm.Restart is called from two places; on init, and when restarting all tools. Only the latter checks services are available.