dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(assemblyPath))

Error message

ArgumentNullException(nameof(assemblyPath))

What it means

XamlTypeMapper.SetAssemblyPath throws ArgumentNullException when assemblyPath is null. The path is used to locate and load the assembly during XAML parsing; a null path would break resolution later, so the API rejects it up front. Note it also rejects the empty string as a separate error.

Solutions

1. Pass the full path to the assembly file as the second argument
2. Ensure the assembly actually exists on disk and its path was resolved before calling
3. Use Assembly.Location (checked for non-empty) to obtain the path

Example fix

// before
string path = settings.GetPath(assemblyName); // null when not configured
mapper.SetAssemblyPath(assemblyName, path);
// after
string path = settings.GetPath(assemblyName);
if (path == null) path = Assembly.ReflectionOnlyLoad(assemblyName).Location;
mapper.SetAssemblyPath(assemblyName, path);

if (assemblyPath == null || !File.Exists(assemblyPath))
    throw new FileNotFoundException("Assembly path missing", assemblyPath);
mapper.SetAssemblyPath(assemblyName, assemblyPath);

bool IsValidAssemblyPath(string path) => !string.IsNullOrEmpty(path) && File.Exists(path);

try
{
    mapper.SetAssemblyPath(assemblyName, assemblyPath);
}
catch (ArgumentNullException ex) when (ex.ParamName == "assemblyPath")
{
    log.LogError(ex, "No disk path registered for {Assembly}", assemblyName);
}

When it happens

Trigger: Calling SetAssemblyPath(assemblyName, null) — often when the path comes from a settings file, registry value, or probe that returned null because the assembly wasn't found.

Common situations: Custom designer/parse hosts mapping assemblies to disk paths; deployment changes where an assembly's registered path entry is missing; tests passing null placeholders.

Related errors

• ArgumentNullException(nameof(assemblyName))
• ArgumentNullException(nameof(assemblyNames))
• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(localName))
• ArgumentNullException(nameof(propertyName))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/fe2f1050b7e0267e. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Markup/XamlTypeMapper.cs:186

        ///     This allows specifying a path to use when loading the named assembly.
        /// </summary>
        /// <param name="assemblyName">
        /// The short name of the assembly, with no extension or path specified
        /// </param>
        /// <param name="assemblyPath">
        /// The file path of the assembly
        /// </param>
        public void SetAssemblyPath(
            string assemblyName,
            string assemblyPath)
        {
            if( null == assemblyName )
            {
                throw new ArgumentNullException(nameof(assemblyName));
            }
            if( null == assemblyPath )
            {
                throw new ArgumentNullException(nameof(assemblyPath));
            }
            if (assemblyPath == string.Empty)
            {
                _lineNumber = 0;  // Public API, so we don't know the line number.
                ThrowException(nameof(SR.ParserBadAssemblyPath));
            }
            if (assemblyName == string.Empty)
            {
                _lineNumber = 0;  // Public API, so we don't know the line number.
                ThrowException(nameof(SR.ParserBadAssemblyName));
            }

            string asmName = assemblyName.ToUpper(CultureInfo.InvariantCulture);
            lock (_assemblyPathTable)
            {
                _assemblyPathTable[asmName] = assemblyPath;
            }

View on GitHub (pinned to 81131a70a4)