HandyOrg/HandyControl · error · InvalidOperationException

Unable to combine two HRGNs.

Error message

Unable to combine two HRGNs.

What it means

WindowChromeWorker._CreateAndCombineRoundRectRgn builds a rounded-rectangle GDI region and OR-combines it into an existing HRGN using NativeMethods.CombineRgn. If CombineRgn returns CombineRgnResult.ERROR (a GDI failure), the worker throws InvalidOperationException because the window chrome region can no longer be constructed correctly. The original source HRGN is cleaned up safely before rethrowing.

Solutions

1. Check for GDI handle leaks (GDI Objects count in Task Manager) and ensure all HRGNs are deleted via DeleteObject
2. Verify the source hrgnSource handle is valid before combining
3. Catch the InvalidOperationException around window chrome setup and fall back to a non-chrome (standard) window presentation

Example fix

// before
worker.EnableChrome(); // throws InvalidOperationException under GDI pressure
// after
try { worker.EnableChrome(); }
catch (InvalidOperationException) { /* fall back: don't customize chrome */ }

if (hrgnSource == IntPtr.Zero || hrgnSource == hrgnInvalid) throw new InvalidOperationException("Source HRGN is invalid before CombineRgn");

bool IsValidRegion(IntPtr hrgn) => hrgn != IntPtr.Zero && NativeMethods.GetRegionData(hrgn, 0, IntPtr.Zero) != 0;

try { worker.EnableChrome(); } catch (InvalidOperationException ex) when (ex.Message == "Unable to combine two HRGNs.") { /* disable chrome customization, keep standard window frame */ }

When it happens

Trigger: CombineRgn returning ERROR during chrome region creation — typically when one of the HRGN handles is invalid/zero (previous _CreateRoundRectRgn failed under GDI resource pressure), or the target HRGN was already deleted.

Common situations: GDI handle exhaustion (many windows/regions leaked), window operations racing with handle destruction, running in Remote Desktop or low-memory sessions where GDI allocations fail.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• Win32Exception
• Win32Exception
• Unable to create a device context from the specified device…
• Unable to initialize GDI+
• The element must be a DependencyObject

AI-assisted analysis of HandyOrg/HandyControl@2c0875ebd6 (2026-09-14). Data as JSON: /api/errors/687c3bcc05cbf0e1. Report an issue: GitHub.

Appendix: source

Thrown at src/Shared/Microsoft.Windows.Shell/WindowChromeWorker.cs:630

    private static IntPtr _CreateRoundRectRgn(Rect region, double radius)
    {
        if (DoubleUtilities.AreClose(0.0, radius))
        {
            return NativeMethods.CreateRectRgn((int) Math.Floor(region.Left), (int) Math.Floor(region.Top), (int) Math.Ceiling(region.Right), (int) Math.Ceiling(region.Bottom));
        }
        return NativeMethods.CreateRoundRectRgn((int) Math.Floor(region.Left), (int) Math.Floor(region.Top), (int) Math.Ceiling(region.Right) + 1, (int) Math.Ceiling(region.Bottom) + 1, (int) Math.Ceiling(radius), (int) Math.Ceiling(radius));
    }

    [SuppressMessage("Microsoft.Naming", "CA2204:Literals should be spelled correctly", MessageId = "HRGNs")]
    private static void _CreateAndCombineRoundRectRgn(IntPtr hrgnSource, Rect region, double radius)
    {
        IntPtr hrgnSrc = IntPtr.Zero;
        try
        {
            hrgnSrc = WindowChromeWorker._CreateRoundRectRgn(region, radius);
            if (NativeMethods.CombineRgn(hrgnSource, hrgnSource, hrgnSrc, RGN.OR) == CombineRgnResult.ERROR)
            {
                throw new InvalidOperationException("Unable to combine two HRGNs.");
            }
        }
        catch
        {
            Utility.SafeDeleteObject(ref hrgnSrc);
            throw;
        }
    }

    private static bool _IsUniform(CornerRadius cornerRadius)
    {
        return DoubleUtilities.AreClose(cornerRadius.BottomLeft, cornerRadius.BottomRight) && DoubleUtilities.AreClose(cornerRadius.TopLeft, cornerRadius.TopRight) && DoubleUtilities.AreClose(cornerRadius.BottomLeft, cornerRadius.TopRight);
    }

    private void _ExtendGlassFrame()
    {
        if (!Utility.IsOSVistaOrNewer)
        {

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