AvaloniaUI/Avalonia · error · ArgumentException

UpdateSourceTrigger cannot be Default.

Error message

UpdateSourceTrigger cannot be Default.

What it means

BindingExpression's constructor requires a concrete UpdateSourceTrigger. UpdateSourceTrigger.Default is a sentinel meaning 'resolve from metadata'; it must be resolved to PropertyChanged, LostFocus, or Explicit before the expression is built. Passing Default through means the resolution step was skipped.

Source

Thrown at src/Avalonia.Base/Data/Core/BindingExpression.cs:82

        object? fallbackValue,
        TimeSpan delay = default,
        IValueConverter? converter = null,
        CultureInfo? converterCulture = null,
        object? converterParameter = null,
        bool enableDataValidation = false,
        BindingMode mode = BindingMode.OneWay,
        BindingPriority priority = BindingPriority.LocalValue,
        string? stringFormat = null,
        object? targetNullValue = null,
        AvaloniaProperty? targetProperty = null,
        TargetTypeConverter? targetTypeConverter = null,
        UpdateSourceTrigger updateSourceTrigger = UpdateSourceTrigger.PropertyChanged)
            : base(priority, targetProperty, enableDataValidation)
    {
        if (mode == BindingMode.Default)
            throw new ArgumentException("Binding mode cannot be Default.", nameof(mode));
        if (updateSourceTrigger == UpdateSourceTrigger.Default)
            throw new ArgumentException("UpdateSourceTrigger cannot be Default.", nameof(updateSourceTrigger));

        if (source == AvaloniaProperty.UnsetValue)
            source = null;

        _source = new(source);
        _mode = mode;
        _nodes = nodes ?? s_emptyExpressionNodes;
        _targetTypeConverter = targetTypeConverter;
        _shouldUpdateOneTimeBindingTarget = _mode == BindingMode.OneTime;

        if (delay != default ||
            converter is not null ||
            converterCulture is not null ||
            converterParameter is not null ||
            fallbackValue != AvaloniaProperty.UnsetValue ||
            !string.IsNullOrWhiteSpace(stringFormat) ||
            (targetNullValue is not null && targetNullValue != AvaloniaProperty.UnsetValue) ||
            updateSourceTrigger is not UpdateSourceTrigger.PropertyChanged)

View on GitHub (pinned to 11c5427268)

Solutions

1. Resolve Default to a concrete value (typically UpdateSourceTrigger.PropertyChanged) before construction.
2. Pass an explicit UpdateSourceTrigger.PropertyChanged (or LostFocus/Explicit) at the call site.
3. Use the standard Binding object initialization path so the sentinel is resolved upstream.

Example fix

// before
var expr = new BindingExpression(..., updateSourceTrigger: UpdateSourceTrigger.Default, ...);
// after
var resolved = trigger == UpdateSourceTrigger.Default ? UpdateSourceTrigger.PropertyChanged : trigger;
var expr = new BindingExpression(..., updateSourceTrigger: resolved, ...);

// Resolve Default before constructing a BindingExpression.
UpdateSourceTrigger ResolveTrigger(UpdateSourceTrigger t) =>
    t == UpdateSourceTrigger.Default ? UpdateSourceTrigger.PropertyChanged : t;

static bool IsConcreteTrigger(UpdateSourceTrigger t) => t != UpdateSourceTrigger.Default;

When it happens

Trigger: Directly constructing a BindingExpression with updateSourceTrigger == UpdateSourceTrigger.Default, or a binding builder that forwards the unresolved sentinel.

Common situations: A custom binding wrapper or compiled-binding helper that does not run the trigger-resolution step; framework version change altering where Default is resolved.

Related errors

• Binding mode cannot be Default.
• {this} already has an owner.
• Expected '{open}'.
• StorageItem is not a file
• StorageItem is not a writeable file

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/aae350b77f8458ba. Report an issue: GitHub.