dotnet/wpf · error · ArgumentException

SR.Format(SR.ConditionValueOfMarkupExtensionNotSupported…

Error message

SR.Format(SR.ConditionValueOfMarkupExtensionNotSupported, value.GetType().Name)

What it means

Trigger.Value cannot be a MarkupExtension; trigger conditions are compared against a literal value, and extensions are resolved at XAML load time as values, not as condition operands. Setting a MarkupExtension instance as the value throws ArgumentException naming the extension type.

Solutions

Use a literal value for Trigger.Value (e.g. true, a color, an enum constant).
If the condition depends on data, use a DataTrigger whose Binding does the comparison instead of a MarkupExtension value.
For resource-based values, resolve the resource first (FindResource) and assign the resolved object.

Example fix

// before
trigger.Value = new StaticResourceExtension("MyBrush"); // throws
// after
trigger.Value = someWindow.FindResource("MyBrush");

Defensive patterns

Strategy: type-guard

Validate before calling

if (value is MarkupExtension) throw new ArgumentException("Trigger.Value cannot be a MarkupExtension; use a literal value or DataTrigger");

Type guard

static bool IsValidConditionValue(object v) => v is not MarkupExtension;

Try / catch

try { trigger.Value = value; }
catch (ArgumentException ex) { log.Error(ex.Message); trigger.Value = ResolveLiteral(value); }

Prevention

Never assign StaticResource/DynamicResource extensions directly to Trigger.Value in code
Resolve resources with FindResource before assigning
Use DataTrigger.Binding for data-dependent conditions

When it happens

Trigger: In code: trigger.Value = new StaticResourceExtension(...) or new Binding() on a Trigger (as opposed to MultiTrigger.Condition). In XAML this is blocked earlier by the parser.

Common situations: Trying to use Binding or DynamicResource as a trigger condition value; confusing triggers (value-based, no binding) with DataTriggers on bindings.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ebf2017377271aa5. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Trigger.cs:79

            }
            set
            {
                // Verify Context Access
                VerifyAccess();

                if (IsSealed)
                {
                    throw new InvalidOperationException(SR.Format(SR.CannotChangeAfterSealed, "Trigger"));
                }

                if (value is NullExtension)
                {
                    value = null;
                }

                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>

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