dotnet/wpf · error · ArgumentException

SR.ChildNameMustBeNonEmpty

Error message

SR.ChildNameMustBeNonEmpty

What it means

TriggerBase.ProcessParametersVisualTreeChild throws ArgumentException with SR.ChildNameMustBeNonEmpty when the TargetName given to a trigger setter/action is an empty string. TargetName must reference a named element (via x:Name / Name) within the template; an empty string is meaningless and rejected after the null check.

Solutions

  1. Provide a valid TargetName matching an x:Name in the template, or omit TargetName to target the templated control
  2. Validate the name string is non-empty before calling the API
  3. Fix the source of the empty string (binding, config, concatenation)

Example fix

// before
setter.TargetName = string.Empty;
// after
if (!string.IsNullOrEmpty(name)) { setter.TargetName = name; }
else { /* omit TargetName or fix name source */ }
Defensive patterns

Strategy: validation

Validate before calling

if (string.IsNullOrEmpty(targetName))
    throw new ArgumentException("TargetName must be non-empty.");

Type guard

static bool IsValidTargetName(string name) => !string.IsNullOrEmpty(name);

Try / catch

try { setter.TargetName = name; }
catch (ArgumentException ex) when (ex.Message.Contains("non-empty") || ex.Message.Contains("empty")) { /* fix name */ }

Prevention

When it happens

Trigger: Setting Setter.TargetName="" or TriggerAction/TargetName to an empty string in XAML or via code (SetXxx(dp, "")), e.g. <Setter Property="Background" TargetName="" Value="Red"/>.

Common situations: Data-bound or string-concatenated TargetName that evaluated to empty; typos leaving TargetName=""; dynamic target names computed from config.

Understand the failure class

Background: "must not be empty", "cannot be empty" — required-field validation errors across open-source libraries — this error's family across 41 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b9c9b863b3244f6c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/TriggerBase.cs:191

        {
            // Not allowed to use Style to affect the StyleProperty.
            if (dp == FrameworkElement.StyleProperty)
            {
                throw new ArgumentException(SR.StylePropertyInStyleNotAllowed);
            }
        }

        /// <summary>
        ///     Parameter validation work common to the SetXXXX methods that deal
        /// with visual tree child nodes.
        /// </summary>
        internal string ProcessParametersVisualTreeChild(DependencyProperty dp, string target)
        {
            ArgumentNullException.ThrowIfNull(target);

            if (target.Length == 0)
            {
                throw new ArgumentException(SR.ChildNameMustBeNonEmpty);
            }

            return String.Intern(target);
        }

        /// <summary>
        ///     After the parameters have been validated, store it in the
        /// PropertyValues collection.
        /// </summary>
        /// <remarks>
        ///     All these will be looked at again (and processed into runtime
        /// data structures) by Style.Seal(). We keep them around even after
        /// that point should we need to serialize this data back out.
        /// </remarks>
        internal void AddToPropertyValues(string childName, DependencyProperty dp, object value, PropertyValueType valueType)
        {
            // Store original data
            PropertyValue propertyValue = new PropertyValue

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