MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

Multiple viable DialogHosts. Specify a unique Identifier on

Error message

Multiple viable DialogHosts. Specify a unique Identifier on each DialogHost, especially where multiple Windows are a concern.

What it means

If more than one loaded `DialogHost` has an `Identifier` matching the supplied `dialogIdentifier`, `GetInstance` cannot pick one and throws `InvalidOperationException("Multiple viable DialogHosts. Specify a unique Identifier...")`. Identifiers must be unique among simultaneously loaded hosts.

Source

Thrown at src/MaterialDesignThemes.Wpf/DialogHost.cs:253

                {
                    // Retrieve the identifier using the Dispatcher on the owning thread (effectively replaces the VerifyAccess() call previously used)
                    identifier = dialogInstance.Dispatcher.Invoke(() => dialogInstance.Identifier);
                }
                if (Equals(dialogIdentifier, identifier))
                {
                    targets.Add(dialogInstance);
                }
            }
            else
            {
                LoadedInstances.Remove(instance);
            }
        }

        if (targets.Count == 0)
            throw new InvalidOperationException($"No loaded DialogHost have an {nameof(Identifier)} property matching {nameof(dialogIdentifier)} ('{dialogIdentifier}') argument.");
        if (targets.Count > 1)
            throw new InvalidOperationException("Multiple viable DialogHosts. Specify a unique Identifier on each DialogHost, especially where multiple Windows are a concern.");

        return targets[0];
    }

    internal async Task<object?> ShowInternal(object content, DialogOpenedEventHandler? openedEventHandler, DialogClosingEventHandler? closingEventHandler, DialogClosedEventHandler? closedEventHandler)
    {
        if (IsOpen)
            throw new InvalidOperationException("DialogHost is already open.");

        _dialogTaskCompletionSource = new TaskCompletionSource<object?>();

        AssertTargetableContent();

        if (content != null)
            DialogContent = content;

        _asyncShowOpenedEventHandler = openedEventHandler;
        _asyncShowClosingEventHandler = closingEventHandler;

View on GitHub (pinned to 98edec3a0b)

Solutions

Give each concurrently loaded `DialogHost` a unique `Identifier`.
When opening dialogs from a specific window, scope the identifier to that window.
Unregister/close other hosts with the same identifier, or pass a window-specific identifier.
If only one host is ever needed, ensure stray duplicate hosts (e.g. in another open window) are closed.

Example fix

<!-- before: two hosts, same identifier -->
<materialDesign:DialogHost Identifier="RootDialog" /> <!-- window A -->
<materialDesign:DialogHost Identifier="RootDialog" /> <!-- window B -->
await DialogHost.Show(vm, "RootDialog"); // throws: multiple viable

<!-- after -->
<materialDesign:DialogHost Identifier="MainDialog" />  <!-- window A -->
<materialDesign:DialogHost Identifier="ToolsDialog" />  <!-- window B -->

Defensive patterns

Strategy: validation

Validate before calling

// Give each concurrently loaded DialogHost a distinct Identifier.
// Open with the specific identifier for the target window:
await DialogHost.Show(content, windowScopedId);

Try / catch

try { await DialogHost.Show(content, id); }
catch (InvalidOperationException ex) when (ex.Message.Contains("Multiple viable"))
{ /* disambiguate: close other hosts or use a unique id */ }

Prevention

Assign unique Identifiers to concurrently loaded DialogHosts.
Scope identifiers per window when multiple windows can be open.
Avoid leaving stray duplicate hosts mounted.

When it happens

Trigger: Two or more `DialogHost` instances in the visual tree share the same `Identifier` (or both have null identifiers), and you call Show/Close with that identifier.

Common situations: Multiple windows each with a `DialogHost` using the same identifier; a DialogHost in a main window plus another in a user control with the same identifier; reusing a well-known identifier like "RootDialog" across views that are loaded at the same time; all hosts left with null identifiers while several are loaded.

Related errors

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/441184166a41b28b. Report an issue: GitHub.