dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(assemblyNames))

Error message

ArgumentNullException(nameof(assemblyNames))

What it means

The XamlTypeMapper(string[] assemblyNames) constructor validates its required argument and throws ArgumentNullException when assemblyNames is null. The mapper needs a list of assemblies to resolve XAML namespace/types, so a null list is rejected immediately at construction.

Solutions

1. Pass a non-null string array, e.g. new XamlTypeMapper(Array.Empty<string>()) if no assemblies are needed.
2. Null-check the value feeding the constructor and provide a default assembly list before calling it.
3. Fix the configuration/source so the assembly-names collection is populated.
4. Wrap construction in try/catch for ArgumentNullException to surface a clearer diagnostic in plug-in scenarios.

Example fix

// before
var mapper = new XamlTypeMapper(GetAssemblyNamesFromConfig()); // null -> ArgumentNullException
// after
string[] assemblies = GetAssemblyNamesFromConfig() ?? new[] { typeof(Window).Assembly.FullName };
var mapper = new XamlTypeMapper(assemblies);

if (assemblyNames is null)
    throw new ArgumentException("assemblyNames must not be null; pass an empty array if none.", nameof(assemblyNames));
var mapper = new System.Windows.Markup.XamlTypeMapper(assemblyNames);

static bool IsValidAssemblyNames(string[] names) => names != null;

try { var mapper = new XamlTypeMapper(assemblyNames); }
catch (ArgumentNullException ex) { /* log ex.ParamName and supply defaults */ }

When it happens

Trigger: Calling new XamlTypeMapper(null), typically when the assembly list is built dynamically (e.g. from configuration) and the source collection came back null. Source: XamlTypeMapper.cs:55-60.

Common situations: Config or app-settings lookups returning null for the assembly list; refactoring that drops a default array; dependency-injection wiring where a null collection is passed through; code paths compiled under the conditional #if block where the ctor is available.

Related errors

• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyPath))
• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(loaderType))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/cad1c2529bb62cd9. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:55

    internal class XamlTypeMapper
#else
    public partial class XamlTypeMapper
#endif
    {
#region Public

#region Methods

#if !PBTCOMPILER
        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="assemblyNames">Assemblies XamlTypeMapper should use when resolving XAML</param>
        public XamlTypeMapper(string[] assemblyNames)
        {
            if(null == assemblyNames)
            {
                throw new ArgumentNullException( nameof(assemblyNames));
            }

            _assemblyNames = assemblyNames;
            _namespaceMaps = null;
        }
#endif

        /// <summary>
        /// Constructor
        /// </summary>
        /// <param name="assemblyNames">Assemblies XamlTypeMapper should use when resolving XAML</param>
        /// <param name="namespaceMaps">NamespaceMap the XamlTypeMapper should use when resolving XAML</param>
        public XamlTypeMapper(
            string[] assemblyNames,
            NamespaceMapEntry[] namespaceMaps)
        {
            if(null == assemblyNames)
            {

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