lepoco/wpfui · error · InvalidOperationException

Only one ContentDialogHost instance is allowed per Window.

Error message

Only one ContentDialogHost instance is allowed per Window.

What it means

Thrown by ContentDialogHost.RegisterHost when a second, distinct ContentDialogHost instance tries to register itself for the same Window. The library keeps a single-slot-per-window map (WindowHosts) because only one host can own the overlay layer for a window; registering a different instance is treated as a programming error rather than silently replacing the first.

Source

Thrown at src/Wpf.Ui/Controls/ContentDialog/ContentDialogHost.cs:220

    private void RegisterHostForWindow()
    {
        var window = Window.GetWindow(this);
        if (window != null)
        {
            RegisterHost(window);
        }
    }

    private void RegisterHost(Window window)
    {
        lock (WindowHostsLock)
        {
            if (WindowHosts.TryGetValue(window, out var existing))
            {
                if (!ReferenceEquals(existing, this))
                {
                    throw new InvalidOperationException(
                        "Only one ContentDialogHost instance is allowed per Window."
                    );
                }

                // already registered for this window and it's this instance
                return;
            }

            WindowHosts.Add(window, this);
        }
    }
}

#pragma warning restore IDE0008 // Use explicit type instead of 'var'

View on GitHub (pinned to ffebacd610)

Solutions

Keep exactly one ContentDialogHost per Window — typically at the root of the window's content.
If you need hosts in multiple user controls, remove the duplicates and let them share the window-level host.
Ensure the previous host is properly unloaded (removed from the visual tree) before adding a new one.
Check WindowHosts registrations during debugging if you suspect a stale entry.

Example fix

<!-- before -->
<Window>
  <StackPanel>
    <ui:ContentDialogHost x:Name="host1"/>
    <ui:ContentDialogHost x:Name="host2"/> <!-- throws on load -->
  </StackPanel>
</Window>

<!-- after -->
<Window>
  <ui:ContentDialogHost x:Name="host"/>
</Window>

Defensive patterns

Strategy: validation

Validate before calling

// In a window, ensure only one host exists
var hosts = FindVisualChildren<ContentDialogHost>(this).ToList();
Debug.Assert(hosts.Count <= 1, $"{hosts.Count} ContentDialogHosts in window");

Prevention

Place a single ContentDialogHost at the window content root.
Do not ship ContentDialogHost inside reusable user controls that get composed into the same window.
Audit XAML for duplicate hosts when merging user controls.

When it happens

Trigger: Placing two <ui:ContentDialogHost> elements in the same Window's visual tree; adding a second host dynamically after one is already loaded; re-hosting on a window that still holds a previously-loaded host that was never unloaded.

Common situations: Migrating an app to WPF UI and leaving an old ContentDialogHost in XAML while adding another in a user control that lives in the same window; merging two user controls each of which ships its own host into one window; a host whose Unloaded handler did not fire (e.g. window closed via ShutdownMode) leaving a stale registration.

Related errors

AI-assisted analysis of lepoco/wpfui@ffebacd610 (2026-08-13). Data as JSON: /api/errors/227ae3fd6d172f65. Report an issue: GitHub.