What causes "SR.HwndTarget_WindowAlreadyHasContent" in dotnet/wpf?

Use a fresh child HWND (e.g. via HwndHost) for the new HwndSource instead of the window that already has content

dotnet/wpf · error · InvalidOperationException

SR.HwndTarget_WindowAlreadyHasContent

Error message

SR.HwndTarget_WindowAlreadyHasContent

What it means

After VisualTarget_AttachToHwnd returns E_ACCESSDENIED (0x80070005), HwndTarget translates that HRESULT into InvalidOperationException (SR.HwndTarget_WindowAlreadyHasContent). It means the Win32 window already has another rendering target attached, and a window can host only one HwndTarget's composition content.

Solutions

Use a fresh child HWND (e.g. via HwndHost) for the new HwndSource instead of the window that already has content
Add the content to the existing WPF visual tree rather than interop-attaching to the same hwnd
Ensure the previous HwndSource on that hwnd is disposed before creating a new one

Example fix

// before
var s2 = HwndSource.FromHwnd(existingWpfWindowHwnd); // E_ACCESSDENIED -> throws
// after
var host = new HwndHost();
existingPanel.Children.Add(host); // host renders into its own child hwnd

Defensive patterns

Strategy: validation

Validate before calling

if (HwndSource.FromHwnd(hwnd) != null) throw new InvalidOperationException("window already hosts WPF content");

Try / catch

try { new HwndSource(...); } catch (InvalidOperationException) { /* reuse existing source or attach to a child hwnd */ }

Prevention

One HwndSource per HWND — track created sources in a map
Add WPF content via the visual tree instead of re-attaching to an existing window's hwnd
Dispose old HwndSource before creating a replacement

When it happens

Trigger: Creating a second HwndSource (or calling HwndSource.FromHwnd) on an HWND that already has WPF content; attaching to a WPF Window's own hwnd from custom code.

Common situations: Trying to host WPF content inside an existing WPF window via interop instead of adding it to the visual tree; re-parenting an HwndSource onto a window that already has one.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a22578ac31940246. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/InterOp/HwndTarget.cs:550

                    SR.HwndTarget_InvalidWindowProcess,
                    nameof(hwnd)
                    );
            }
            else if (threadId != SafeNativeMethods.GetCurrentThreadId())
            {
                throw new ArgumentException(
                    SR.HwndTarget_InvalidWindowThread,
                    nameof(hwnd)
                    );
            }

            int hr = VisualTarget_AttachToHwnd(hwnd);

            if (HRESULT.Failed(hr))
            {
                if (hr == unchecked((int)0x80070005)) // E_ACCESSDENIED
                {
                    throw new InvalidOperationException(
                        SR.HwndTarget_WindowAlreadyHasContent
                        );
                }
                else
                {
                    HRESULT.Check(hr);
                }
            }

            EnsureNotificationWindow();
            _notificationWindowHelper.AttachHwndTarget(this);
            UnsafeNativeMethods.WTSRegisterSessionNotification(hwnd, NativeMethods.NOTIFY_FOR_THIS_SESSION);
        }

        [DllImport(DllImport.MilCore, EntryPoint = "MilVisualTarget_AttachToHwnd")]
        internal static extern int VisualTarget_AttachToHwnd(
            IntPtr hwnd
            );

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