dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(loaderType))

Error message

ArgumentNullException(nameof(loaderType))

What it means

The XamlDeferLoadAttribute(string,string) constructor throws ArgumentNullException when the loaderType string is null. This attribute records deferred-loading type names as strings for the XAML parser; a null loader name is meaningless and rejected at construction.

Solutions

1. Pass a valid assembly-qualified or XAML-namespace type-name string for loaderType.
2. Validate the config/source value for null before constructing the attribute.
3. Use the (Type,Type) constructor overload with concrete types when available.
4. Catch ArgumentNullException and log/skip invalid attribute metadata.

Example fix

// before
var attr = new XamlDeferLoadAttribute(GetSetting("loader"), "MyContent"); // setting missing
// after
var loader = GetSetting("loader") ?? "MyNs.MyDeferredLoader";
var attr = new XamlDeferLoadAttribute(loader, "MyContent");

if (string.IsNullOrEmpty(loaderTypeName)) throw new ArgumentException("loaderType is required.");
var attr = new XamlDeferLoadAttribute(loaderTypeName, contentTypeName);

try { attr = new XamlDeferLoadAttribute(loader, content); }
catch (ArgumentNullException ex) { log.Error($"Bad defer-load metadata: {ex.ParamName}"); attr = null; }

When it happens

Trigger: Executing new XamlDeferLoadAttribute(null, "SomeContentType") or new XamlDeferLoadAttribute(null, null).

Common situations: Building the attribute via reflection/config-driven code where the loader type name comes from a missing setting; hand-writing attribute metadata and leaving the loader argument empty.

Related errors

• ArgumentNullException(nameof(contentType))
• ArgumentNullException(nameof(newNamespace))
• ArgumentNullException(nameof(oldNamespace))
• ArgumentNullException(nameof(clrNamespace))
• ArgumentNullException(nameof(prefix))

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/c142cd0b56f1ed4d. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Windows/Markup/XamlDeferLoadAttribute.cs:21

namespace System.Windows.Markup
{
    [AttributeUsage(AttributeTargets.Property | AttributeTargets.Class, AllowMultiple = false, Inherited = true)]
    public sealed class XamlDeferLoadAttribute : Attribute
    {
        public XamlDeferLoadAttribute(Type loaderType, Type contentType)
        {
            ArgumentNullException.ThrowIfNull(loaderType);
            ArgumentNullException.ThrowIfNull(contentType);
            LoaderTypeName = loaderType.AssemblyQualifiedName!;
            ContentTypeName = contentType.AssemblyQualifiedName!;
            LoaderType = loaderType;
            ContentType = contentType;
        }

        public XamlDeferLoadAttribute(string loaderType, string contentType)
        {
            LoaderTypeName = loaderType ?? throw new ArgumentNullException(nameof(loaderType));
            ContentTypeName = contentType ?? throw new ArgumentNullException(nameof(contentType));
        }

        public string LoaderTypeName { get; }

        public string ContentTypeName { get; }

        public Type? LoaderType { get; private set; }
        public Type? ContentType { get; private set; }
    }
}

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