dotnet/wpf · error · ArgumentException

SR.Format(SR.NullPropertyIllegal, "Setter.Property")

Error message

SR.Format(SR.NullPropertyIllegal, "Setter.Property")

What it means

A Setter used inside a Style, ControlTemplate, or Trigger must declare which dependency property it sets. WPF throws this ArgumentException during Setter.Seal() because the Property property is null when the style is sealed. Styles are validated (sealed) when applied, so the failure surfaces when the style is used, not when the Setter object is constructed.

Solutions

  1. Add the Property attribute to every Setter: `<Setter Property="Background" Value="Red"/>`.
  2. In code, set setter.Property = SomeUIElement.SomeProperty before adding it to a style or applying the style.
  3. Validate all setters programmatically before applying the style: foreach (SetterBase s in style.Setters) if (s is Setter st && st.Property == null) throw/log.

Example fix

// before
var s = new Setter { Value = Brushes.Red };
style.Setters.Add(s);
// after
var s = new Setter { Property = Control.BackgroundProperty, Value = Brushes.Red };
style.Setters.Add(s);
Defensive patterns

Strategy: validation

Validate before calling

foreach (var s in style.Setters.OfType<Setter>())
    if (s.Property == null) throw new InvalidOperationException("Setter missing Property");

Type guard

bool HasProperty(Setter s) => s.Property != null;

Try / catch

try { element.Style = style; }
catch (ArgumentException ex) when (ex.Message.Contains("Setter.Property")) { /* log and fix style */ }

Prevention

When it happens

Trigger: A Setter element in XAML without a Property attribute, or a Setter created in code via `new Setter { Value = x }` without assigning Property, then the containing Style/Trigger is sealed (applied to an element).

Common situations: XAML typo like `<Setter Value="Red"/>` missing `Property="Background"`; dynamically building styles in code and forgetting to set the Property; data-bound styles built at runtime.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d55db936da13bc4f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Setter.cs:90

                throw new InvalidOperationException(SR.Format(SR.CannotHavePropertyInStyle, FrameworkElement.NameProperty.Name));
            }
        }

        /// <summary>
        ///     Seals this setter
        /// </summary>
        internal override void Seal()
        {

            // Do the validation that can't be done until we know all of the property
            // values.

            DependencyProperty dp = Property;
            object value = ValueInternal;

            if (dp == null)
            {
                throw new ArgumentException(SR.Format(SR.NullPropertyIllegal, "Setter.Property"));
            }

            if( String.IsNullOrEmpty(TargetName))
            {
                // Setter on container is not allowed to affect the StyleProperty.
                if (dp == FrameworkElement.StyleProperty)
                {
                    throw new ArgumentException(SR.StylePropertyInStyleNotAllowed);
                }
            }

            // Value needs to be valid for the DP, or a deferred reference, or one of the supported
            // markup extensions.

            if (!dp.IsValidValue(value))
            {
                // The only markup extensions supported by styles is resources and bindings.
                if (value is MarkupExtension)

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