MaterialDesignInXAML/MaterialDesignInXamlToolkit · error · InvalidOperationException

No loaded DialogHost have an {nameof(Identifier)} property m

Error message

No loaded DialogHost have an {nameof(Identifier)} property matching {nameof(dialogIdentifier)} ('{dialogIdentifier}') argument.

What it means

After confirming at least one `DialogHost` is loaded, `GetInstance` filters by matching `Equals(dialogIdentifier, instance.Identifier)`. If none match it throws `InvalidOperationException` reporting that no loaded host has an `Identifier` matching the supplied `dialogIdentifier`. Note a null identifier matches hosts whose `Identifier` is also null.

Source

Thrown at src/MaterialDesignThemes.Wpf/DialogHost.cs:251

                }
                else
                {
                    // 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;

View on GitHub (pinned to 98edec3a0b)

Solutions

Use the exact same `Identifier` value in XAML and in `Show`/`Close` calls (define it once as a const/shared key).
If you set no identifier in XAML, call Show/Close with `null`.
Ensure the identifier type's `Equals`/`GetHashCode` behave as you expect (strings, enums, records are fine; custom objects may not).
Double-check spelling/casing of string identifiers.

Example fix

<!-- before -->
<materialDesign:DialogHost Identifier="root" />
await DialogHost.Show(vm, "RootDialog"); // mismatch -> throws

<!-- after (single source of truth) -->
<materialDesign:DialogHost Identifier="{x:Static local:DialogIds.Root}" />
await DialogHost.Show(vm, DialogIds.Root);

Defensive patterns

Strategy: validation

Validate before calling

object id = DialogIds.Root; // shared with XAML Identifier
await DialogHost.Show(content, id);

Type guard

static bool IdentifierIsMounted(object id)
    => DialogHost.GetDialogSession(id) is not null || DialogHost.IsDialogOpen(id) || /* loaded */ true;

Try / catch

try { await DialogHost.Show(content, id); }
catch (InvalidOperationException ex) when (ex.Message.Contains("matching"))
{ /* identifier typo: log expected id */ }

Prevention

Define the identifier once and reference it from both XAML and code.
Use null consistently if no identifier is configured.
Ensure identifier types have correct Equals/GetHashCode semantics.

When it happens

Trigger: Calling Show/Close with an identifier that does not equal any loaded host's `Identifier` (typo, different casing won't matter since it's `Equals`, but wrong value will), or with a non-null identifier when hosts are configured with null identifiers.

Common situations: Identifier mismatch between XAML (`Identifier="root"`) and code (`Show(content, "RootDialog")`); renaming the identifier in one place only; using an enum/object identifier that doesn't `Equals` the configured one; passing an identifier when the host has none.

Related errors

AI-assisted analysis of MaterialDesignInXAML/MaterialDesignInXamlToolkit@98edec3a0b (2026-08-13). Data as JSON: /api/errors/faf492b0fae3cf68. Report an issue: GitHub.