dotnet/wpf · error · InvalidOperationException

SR.CantSetOwnerWhosHwndIsNotCreated

Error message

SR.CantSetOwnerWhosHwndIsNotCreated

What it means

Window.Owner setter throws InvalidOperationException when the candidate owner window is non-null, not disposed, but its HWND has not been created yet (IsSourceWindowNull). A window whose Win32 handle does not exist cannot be an owner; WPF requires a real source window. This happens when the owner window was constructed but never shown.

Solutions

  1. Call Show() (or ShowDialog()) on the owner window before assigning it as the Owner of another window.
  2. Subscribe to the owner's SourceInitialized event and set Owner after that.
  3. If the owner should never be visible, consider a hidden helper window that is explicitly shown with ShowInTaskbar=false and opacity 0 first.

Example fix

// before
var owner = new MainWindow();
dlg.Owner = owner; // HWND not created yet
// after
var owner = new MainWindow();
owner.Show();
dlg.Owner = owner;
Defensive patterns

Strategy: validation

Validate before calling

bool ownerReady(Window w) => w != null && w.IsLoaded; // IsLoaded implies source/HWND initialized after Show

Try / catch

try { dlg.Owner = owner; }
catch (InvalidOperationException) { owner.SourceInitialized += (s,e) => dlg.Owner = owner; }

Prevention

When it happens

Trigger: Assigning window.Owner to a window that was created but Show()/ShowDialog() has never been called, so its HWND is not yet created.

Common situations: Creating a parent window and a child dialog together and setting Owner before showing the parent; background threads constructing windows ahead of time.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0ead4742d3e1a689. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Window.cs:1243

                VerifyContextAndObjectState();
                if (value == this)
                {
                    throw new ArgumentException(SR.CannotSetOwnerToItself);
                }

                if (_showingAsDialog)
                {
                    throw new InvalidOperationException(SR.CantSetOwnerAfterDialogIsShown);
                }

                if (value != null && value.IsSourceWindowNull)
                {
                    // Try to be specific in the error message.
                    if (value._disposed)
                    {
                        throw new InvalidOperationException(SR.CantSetOwnerToClosedWindow);
                    }
                    throw new InvalidOperationException(SR.CantSetOwnerWhosHwndIsNotCreated);
                }

                if ( _ownerWindow == value )
                {
                    return;
                }

                if (!_disposed)
                {
                    // Check to see if value is already a child of this window.
                    // If yes, throw Exception
                    if (value != null)
                    {
                        WindowCollection ownedWindows = OwnedWindows;
                        for (int i = 0; i < ownedWindows.Count; i++)
                        {
                            if (ownedWindows[i] == value)
                            {

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