dotnet/wpf · error · ArgumentException

SR.NameScopeInvalidIdentifierName

Error message

SR.NameScopeInvalidIdentifierName

What it means

NameScope.RegisterName throws ArgumentException(SR.NameScopeInvalidIdentifierName) when the name is not a valid identifier per NameValidationHelper (e.g. contains spaces, starts with a digit, or has invalid characters). XAML names must be legal identifiers to be referenced in markup and generated code.

Solutions

  1. Sanitize the name to a valid identifier (letters/digits/underscore, not starting with a digit) before registering.
  2. Use a helper that generates safe names (e.g. prefix + counter).
  3. Validate with the same rules the library uses before calling RegisterName.

Example fix

// before
scope.RegisterName(userInput, element);
// after
string safe = System.Text.RegularExpressions.Regex.Replace(userInput, "[^A-Za-z0-9_]", "_");
if (safe.Length > 0 && char.IsDigit(safe[0])) safe = "_" + safe;
scope.RegisterName(safe, element);
Defensive patterns

Strategy: validation

Validate before calling

static bool IsValidXamlName(string n) => !string.IsNullOrEmpty(n) && !char.IsDigit(n[0]) && n.All(c => char.IsLetterOrDigit(c) || c == '_');

Type guard

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

Try / catch

try { nameScope.RegisterName(name, element); } catch (ArgumentException ex) when (ex.Message.Contains("identifier")) { /* sanitize and retry */ }

Prevention

When it happens

Trigger: Calling RegisterName with names like "my name", "1st", "a-b"; any name failing NameValidationHelper.IsValidIdentifierName.

Common situations: Generating names from user input or file names, localizing names with spaces, converting element titles directly into names.

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/12d838d69899fbd6. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/NameScope.cs:38

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

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