dotnet/wpf · error · ArgumentException

SR.NameScopeInvalidIdentifierName

Error message

SR.NameScopeInvalidIdentifierName

What it means

NameScope.RegisterName throws ArgumentException(SR.NameScopeInvalidIdentifierName) when the name is non-empty but is not a valid XAML identifier (per NameValidationHelper.IsValidIdentifierName). XAML names must follow identifier rules (e.g. start with a letter or underscore, no invalid characters).

Solutions

  1. Sanitize the name with NameValidationHelper.IsValidIdentifierName (or equivalent regex like ^[A-Za-z_][\w]*$) before registering.
  2. Strip or replace invalid characters programmatically before calling RegisterName.
  3. Prefix with a letter/underscore when the name starts with a digit.
  4. Catch ArgumentException and surface a user-facing 'invalid name' message.

Example fix

// before
scope.RegisterName(userText, element); // userText = "my control"
// after
var safe = new string(userText.Where(char.IsLetterOrDigit).ToArray());
if (safe.Length > 0 && !char.IsDigit(safe[0]))
    scope.RegisterName(safe, element);
Defensive patterns

Strategy: validation

Validate before calling

bool valid = !string.IsNullOrEmpty(name) && System.Text.RegularExpressions.Regex.IsMatch(name, "^[A-Za-z_][A-Za-z0-9_]*$");

Type guard

static bool IsValidXamlName(string name) => !string.IsNullOrEmpty(name) && System.Text.RegularExpressions.Regex.IsMatch(name, "^[A-Za-z_][A-Za-z0-9_]*$");

Try / catch

try { scope.RegisterName(name, element); }
catch (ArgumentException ex) { /* name is not a valid XAML identifier */ }

Prevention

When it happens

Trigger: Calling RegisterName with names containing spaces, dots, digits at the start, or other non-identifier characters (e.g. "my control", "1stItem", "a-b").

Common situations: Using user-entered text or file names as element names; constructing names by concatenation with delimiters; localized strings used as names; porting WinForms control names that allow characters XAML disallows.

Understand the failure class

Background: "invalid id" errors: invalid identifier format — why libraries reject IDs before lookup, and how to fix them — this error's family across 37 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/cecdc55fcce3814b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/NameScope.cs:46

    {        
        #region INameScope
        
        /// <summary>
        /// Register Name-Object Map 
        /// </summary>
        /// <param name="name">name to be registered</param>
        /// <param name="scopedElement">object mapped to name</param>
        public void RegisterName(string name, object scopedElement)
        {
            ArgumentNullException.ThrowIfNull(name);
            ArgumentNullException.ThrowIfNull(scopedElement);

            if (name.Length == 0)
                throw new ArgumentException(SR.NameScopeNameNotEmptyString);

            if (!NameValidationHelper.IsValidIdentifierName(name))
            {
                throw new ArgumentException(SR.Format(SR.NameScopeInvalidIdentifierName, name));
            }

            if (_nameMap == null)
            {
                _nameMap = new HybridDictionary();
                _nameMap[name] = scopedElement;
            }
            else
            {
                object nameContext = _nameMap[name];
                // first time adding the Name, set it
                if (nameContext == null)
                {
                    _nameMap[name] = scopedElement;
                }
                else if (scopedElement != nameContext)
                {
                    throw new ArgumentException(SR.Format(SR.NameScopeDuplicateNamesNotAllowed, name));

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