AvaloniaUI/Avalonia · error · ArgumentException

Binding mode cannot be Default.

Error message

Binding mode cannot be Default.

What it means

BindingExpression's constructor requires a concrete BindingMode. BindingMode.Default is a sentinel meaning 'resolve later from target/property metadata'; it must be resolved to a real mode (OneWay/TwoWay/OneTime/OneWayToSource) before the expression is built. Passing Default through indicates the binding infrastructure skipped resolution.

Source

Thrown at src/Avalonia.Base/Data/Core/BindingExpression.cs:80

        object? source,
        List<ExpressionNode>? nodes,
        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) ||

View on GitHub (pinned to 11c5427268)

Solutions

1. Resolve Default to a concrete mode before constructing the expression: use the target property's default mode or fall back to BindingMode.OneWay.
2. If you control the call site, pass an explicit BindingMode.OneWay (or the appropriate mode) instead of relying on Default.
3. When building from a Binding object, use Avalonia's Binding.Init / the standard resolution path so Default is converted before reaching BindingExpression.

Example fix

// before
var expr = new BindingExpression(..., mode: BindingMode.Default, ...);
// after
var resolved = mode == BindingMode.Default ? BindingMode.OneWay : mode;
var expr = new BindingExpression(..., mode: resolved, ...);

// Resolve Default before constructing a BindingExpression.
BindingMode ResolveMode(BindingMode mode) =>
    mode == BindingMode.Default ? BindingMode.OneWay : mode;

// usage:
var resolved = ResolveMode(mode);
if (resolved == BindingMode.Default) throw new ArgumentException("mode");

static bool IsConcreteMode(BindingMode m) => m != BindingMode.Default;

When it happens

Trigger: Directly constructing a BindingExpression (or a lower-level binding descriptor) with mode == BindingMode.Default. This normally only happens inside custom binding builders or reflection-based binding setup that forwards an unresolved Default.

Common situations: Writing a custom IBinding/BindingExpression wrapper that forgets to call the mode-resolution step; hand-rolling a compiled-binding pipeline that passes through the enum verbatim; a framework upgrade where Default is no longer auto-resolved at an earlier stage.

Related errors

• UpdateSourceTrigger 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/2c7b318efb81f7b6. Report an issue: GitHub.