dotnet/wpf · error · ArgumentException

SR.Format(SR.NameScopeDuplicateNamesNotAllowed, childName)

Error message

SR.Format(SR.NameScopeDuplicateNamesNotAllowed, childName)

What it means

When registering a template child name (AddTemplatedChildName-style registration), the name already exists in the template's childIndexFromChildName map. Duplicate x:Name values within one template are illegal, so ArgumentException (NameScopeDuplicateNamesNotAllowed) is thrown — mirroring NameScope duplicate-name semantics.

Solutions

Make every x:Name unique within the template (e.g. prefix per element)
Remove the duplicate element or its name attribute if the name isn't referenced
Uniquify names programmatically before registering: check childIndexFromChildName.Contains first
Run an XAML linter to detect duplicate names in templates

Example fix

<!-- before -->
<Border x:Name="panel"/><TextBlock x:Name="panel"/>
<!-- after -->
<Border x:Name="panelBorder"/><TextBlock x:Name="panelText"/>

Defensive patterns

Strategy: validation

Validate before calling

var seen = new HashSet<string>();
foreach (var name in CollectXNames(template))
    if (!seen.Add(name)) throw new Exception("Duplicate x:Name in template: " + name);

Try / catch

try { ApplyTemplate(); } catch (ArgumentException ex) when (ex.Message.Contains("name")) { Log("Duplicate template name: " + ex.Message); }

Prevention

Enforce unique x:Name per template in code review/lint
Uniquify generated names with prefixes or counters
Search template XAML for duplicated Name attributes before build

When it happens

Trigger: Two elements in the same ControlTemplate/DataTemplate sharing the same x:Name; re-registering an already-registered child name during template optimization/BAML processing; merging template fragments that reuse names.

Common situations: Copy-pasted XAML fragments inside one template with repeated names; programmatic template building calling the registration API twice for the same name; tooling-generated XAML that doesn't deduplicate names.

Understand the failure class

Background: "already exists" / EEXIST / FileAlreadyExistsException: what the 'file already exists' error means and how to fix it — this error's family across 37 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b28324b480ad98fd. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/StyleHelper.cs:5316

        internal static int CreateChildIndexFromChildName(
            string              childName,
            FrameworkTemplate   frameworkTemplate)
        {
            Debug.Assert(frameworkTemplate != null );

            // Lock must be made by caller

            HybridDictionary childIndexFromChildName;
            int lastChildIndex;

            Debug.Assert(frameworkTemplate != null);

            childIndexFromChildName = frameworkTemplate.ChildIndexFromChildName;
            lastChildIndex = frameworkTemplate.LastChildIndex;

            if (childIndexFromChildName.Contains(childName))
            {
                throw new ArgumentException(SR.Format(SR.NameScopeDuplicateNamesNotAllowed, childName));
            }

            // Normal templated child check
            // If we're about to give out an index that we can't support, throw.
            if (lastChildIndex >= 0xFFFF)
            {
                throw new InvalidOperationException(SR.StyleHasTooManyElements);
            }

            // No index found, allocate
            object value = lastChildIndex;

            childIndexFromChildName[childName] = value;

            Interlocked.Increment(ref lastChildIndex);

            Debug.Assert(frameworkTemplate != null);
            frameworkTemplate.LastChildIndex = lastChildIndex;

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