AvaloniaUI/Avalonia · error · NotSupportedException

TemplateBinding does not support OneTime or OneWayToSource b

Error message

TemplateBinding does not support OneTime or OneWayToSource bindings.

What it means

TemplateBinding only forwards values from the templated parent to template children and is inherently OneWay/TwoWay-ish. OneTime (no ongoing propagation) and OneWayToSource (writing back to the templated parent) are semantically incompatible, so CreateInstance throws NotSupportedException.

Source

Thrown at src/Avalonia.Base/Data/TemplateBinding.cs:60

        /// <summary>
        /// Gets or sets the binding mode.
        /// </summary>
        public BindingMode Mode { get; set; }

        /// <summary>
        /// Gets or sets the name of the source property on the templated parent.
        /// </summary>
        [ConstructorArgument("property")]
        [InheritDataTypeFrom(InheritDataTypeFromScopeKind.ControlTemplate)]
        public AvaloniaProperty? Property { get; set; }

        public BindingBase ProvideValue() => this;

        internal override BindingExpressionBase CreateInstance(AvaloniaObject target, AvaloniaProperty? targetProperty, object? anchor)
        {
            if (Mode is BindingMode.OneTime or BindingMode.OneWayToSource)
                throw new NotSupportedException("TemplateBinding does not support OneTime or OneWayToSource bindings.");

            return new TemplateBindingExpression(
                Property,
                Converter,
                ConverterCulture,
                ConverterParameter,
                Mode);
        }
    }
}

View on GitHub (pinned to 11c5427268)

Solutions

1. Use BindingMode.OneWay or TwoWay (or leave Mode unset for the default) with TemplateBinding.
2. If you truly need OneTime, use a regular `{Binding RelativeSource={RelativeSource TemplatedParent}}` instead of TemplateBinding.
3. If you need OneWayToSource, use a normal binding to the templated parent with explicit mode.

Example fix

// before (XAML):
<ContentPresenter Content="{TemplateBinding Content}" Mode="OneTime"/>

// after:
<ContentPresenter Content="{TemplateBinding Content}"/>
<!-- or for OneTime semantics: -->
<ContentPresenter Content="{Binding Content, RelativeSource={RelativeSource TemplatedParent}, Mode=OneTime}"/>

if (templateBinding.Mode is BindingMode.OneTime or BindingMode.OneWayToSource)
    throw new NotSupportedException("TemplateBinding needs OneWay/TwoWay.");

static bool TemplateModeOk(BindingMode m) => m is BindingMode.OneWay or BindingMode.TwoWay or BindingMode.Default;

When it happens

Trigger: Constructing a TemplateBinding with `Mode = BindingMode.OneTime` or `Mode = BindingMode.OneWayToSource`, then applying it so CreateInstance runs.

Common situations: XAML `<TemplateBinding Property="X" Mode="OneTime"/>`. Copying a normal Binding's Mode into a TemplateBinding. Programmatically setting Mode on a TemplateBinding.

Related errors

• Dictionary reset not supported
• BindingExpression has not been started.
• Cannot call AsObservable on a to binding expression which is
• BindingExpression was already attached.
• BindingExpression was already attached to a different proper

AI-assisted analysis of AvaloniaUI/Avalonia@11c5427268 (2026-08-13). Data as JSON: /api/errors/ef109bbddca12a83. Report an issue: GitHub.