dotnet/wpf · error · ArgumentException

SR.ConditionValueOfExpressionNotSupported

Error message

SR.ConditionValueOfExpressionNotSupported

What it means

Setting the Value of a WPF trigger Condition to an Expression (the result of a dynamic markup extension such as {Binding}) is not supported; conditions must have static, sealable values. The Condition class throws ArgumentException because dynamic expressions cannot be evaluated during style/trigger sealing.

Solutions

Use a static literal value for the condition Value (e.g. "True", a fixed enum value or color).
If dynamic behavior is needed, use a DataTrigger with a Binding in the Condition's Binding property rather than an Expression as Value.
Compute the desired value before assigning and assign the concrete result, not the expression.

Example fix

// before
new Condition(Selector.IsSelectedProperty, someBinding.Expression);
// after
new Condition(Selector.IsSelectedProperty, true);

Defensive patterns

Strategy: validation

Validate before calling

if (condition.Value is Expression)
    throw new InvalidOperationException("Condition.Value must be a static value, not an Expression");

Type guard

bool IsValidConditionValue(object v) => v is not Expression;

Prevention

Never bind a trigger condition Value with {Binding}; use static literals.
Use DataTrigger Binding conditions for dynamic data-driven behavior.
Check the value with 'is Expression' before assigning in code.

When it happens

Trigger: Assigning a value produced by a markup extension that returns an Expression (e.g. condition.Value = someBindingExpression) in code, or XAML where a Binding is used as the condition value.

Common situations: Developers try to write <Condition Property="IsSelected" Value="{Binding ...}" /> or set Value in code to a dynamic value, expecting triggers to re-evaluate dynamically.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/602cead8daf6a24a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Condition.cs:140

        public object Value
        {
            get { return _value; }
            set
            {
                if (_sealed)
                {
                    throw new InvalidOperationException(SR.Format(SR.CannotChangeAfterSealed, "Condition"));
                }

                if (value is MarkupExtension)
                {
                    throw new ArgumentException(SR.Format(SR.ConditionValueOfMarkupExtensionNotSupported,
                                                       value.GetType().Name));
                }

                if(value is Expression)
                {
                    throw new ArgumentException(SR.ConditionValueOfExpressionNotSupported);
                }

                _value = value;
            }
        }

        /// <summary>
        /// The x:Name of the object whose property shall
        /// trigger the associated setters to be applied.
        /// If null, then this is the object being Styled
        /// and not anything under its Template Tree.
        /// </summary>
        [DefaultValue(null)]
        public string SourceName
        {
            get
            {
                return _sourceName;

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