What causes "SR.NullPropertyIllegal" in dotnet/wpf?

Set Condition.Property (and Value) before the style/trigger is sealed

dotnet/wpf · error · InvalidOperationException

SR.NullPropertyIllegal

Error message

SR.NullPropertyIllegal

What it means

InvalidOperationException from Condition.Seal for trigger-style lookups: neither Property nor Binding was set on the Condition, so the sealed trigger condition has nothing to evaluate and is invalid (the NullPropertyIllegal sentinel naming the missing property).

Solutions

Set Condition.Property (and Value) before the style/trigger is sealed
Use a data trigger with Binding when no property condition is intended

Example fix

// before
var c = new Condition(null, true);
// after
var c = new Condition(Selector.IsSelectedProperty, true);

Defensive patterns

Strategy: validation

Validate before calling

if (condition.Property == null)
    throw new InvalidOperationException("Property-trigger Condition requires a Property");

Type guard

bool HasProperty(Condition c) => c.Property != null;

Try / catch

try { trigger.Conditions.Add(condition); style.Seal(); }
catch (InvalidOperationException ex) { /* supply missing Property */ }

Prevention

Always set Property when building property-trigger conditions.
Validate condition completeness before adding to triggers.
In XAML, include the Property attribute on every Condition in a Trigger.

When it happens

Trigger: Sealing a Condition whose Property was never set while lookup type is Trigger or PropertyTriggerResource (e.g. <Condition Value="True"/> without Property in XAML).

Common situations: XAML parse errors on incomplete Condition elements; code-built Conditions missing Property before adding to a Trigger's Conditions collection.

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/8938a4f2239ce883. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Condition.cs:193

        {
            if (_sealed)
            {
                return;
            }

            _sealed = true;

            // Ensure valid condition
            if (_property != null && _binding != null)
                throw new InvalidOperationException(SR.ConditionCannotUseBothPropertyAndBinding);

            switch (type)
            {
                case ValueLookupType.Trigger:
                case ValueLookupType.PropertyTriggerResource:
                    if (_property == null)
                    {
                        throw new InvalidOperationException(SR.Format(SR.NullPropertyIllegal, "Property"));
                    }

                    if (!_property.IsValidValue(_value))
                    {
                        throw new InvalidOperationException(SR.Format(SR.InvalidPropertyValue, _value, _property.Name));
                    }
                    break;

                case ValueLookupType.DataTrigger:
                case ValueLookupType.DataTriggerResource:
                    if (_binding == null)
                    {
                        throw new InvalidOperationException(SR.Format(SR.NullPropertyIllegal, "Binding"));
                    }
                    break;

                default:
                    throw new InvalidOperationException(SR.Format(SR.UnexpectedValueTypeForCondition, type));

View on GitHub (pinned to 81131a70a4)