dotnet/maui · error · InvalidOperationException

call GtkThemes.Init() before this

Error message

call GtkThemes.Init() before this

What it means

GtkThemes.LoadCustomTheme parses a GTK resource (.rc) file to re-skin the app. It requires GtkThemes.Init() to have run (IsInitialized flag) and throws InvalidOperationException('call GtkThemes.Init() before this') otherwise, because Rc.Parse depends on GTK's resource system being initialized.

Solutions

1. Call GtkThemes.Init() at startup, before any LoadCustomTheme call.
2. Keep theme loading after both Gtk.Application.Init() and GtkThemes.Init().
3. If unsure of order, guard: `if (!GtkThemes.IsInitialized) GtkThemes.Init();` before loading — but prefer explicit ordering.

Example fix

// before
GtkThemes.LoadCustomTheme("mytheme.gtkrc"); // throws
GtkThemes.Init();
// after
Gtk.Application.Init();
GtkThemes.Init();
GtkThemes.LoadCustomTheme("mytheme.gtkrc");

static void SafeLoadTheme(string path)
{
    if (!GtkThemes.IsInitialized) GtkThemes.Init();
    GtkThemes.LoadCustomTheme(path);
}

static bool ThemesReady => GtkThemes.IsInitialized;

When it happens

Trigger: Calling GtkThemes.LoadCustomTheme(path) before GtkThemes.Init(). The Init call sets IsInitialized and runs platform checks (e.g. CheckWindowsGtk on Windows).

Common situations: App startup ordering where a theme is loaded in a static constructor or before the Init call; refactored startup that moved LoadCustomTheme ahead of Init; copy-paste from a sample that omitted Init.

Related errors

• call Forms.Init() before this
• Glx entry point not found!
• InsertPageBefore is not supported globally on GTK, please…
• MaximumDate must be greater than MinimumDate
• MinimumDate must be lower than MaximumDate

AI-assisted analysis of dotnet/maui@f377ff1c5e (2026-08-13). Data as JSON: /api/errors/b3b59362f1a5d758. Report an issue: GitHub.

Appendix: source

Thrown at src/Compatibility/Core/src/GTK/GtkThemes.cs:34

		public static void Init()
		{
			if (IsInitialized)
				return;

			if (PlatformHelper.GetGTKPlatform() == GTKPlatform.Windows)
				CheckWindowsGtk();

			IsInitialized = true;
		}

		public static void LoadCustomTheme(string filename)
		{
			if (string.IsNullOrEmpty(filename))
				return;

			if (!IsInitialized)
				throw new InvalidOperationException("call GtkThemes.Init() before this");

			// GTK provides resource file mechanism for configuring various aspects of the operation of a GTK program at runtime. 
			// Parses resource information from a string to allow change the App appearance.
			Rc.Parse(filename);
		}

		private static bool CheckWindowsGtk()
		{
			string location = null;
			Version version = null;
			Version minVersion = new Version(2, 12, 22);

			using (var key = Microsoft.Win32.Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Xamarin\GtkSharp\InstallFolder"))
			{
				if (key != null)
					location = key.GetValue(null) as string;
			}
			using (var key = Microsoft.Win32.Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Xamarin\GtkSharp\Version"))

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