unoplatform/uno · error · ArgumentNullException

descriptor

Error message

descriptor

What it means

Thrown by ValueSerializer.GetSerializerFor(PropertyDescriptor, IValueSerializerContext) when the descriptor argument is null. The descriptor is used to obtain the TypeConverter and to delegate to the context, so it must be present.

Solutions

1. Obtain the PropertyDescriptor via a non-failing lookup (e.g. TypeDescriptor.GetProperties(type)[name]) and verify non-null before calling.
2. Use the GetSerializerFor(Type, context) overload when only a type is available.
3. Null-check the descriptor at the call site and handle the missing case explicitly.

Example fix

// before
var desc = TypeDescriptor.GetProperties(obj)["BadName"];
var ser = ValueSerializer.GetSerializerFor(desc, ctx); // throws

// after
var desc = TypeDescriptor.GetProperties(obj)["RealName"];
if (desc != null)
    var ser = ValueSerializer.GetSerializerFor(desc, ctx);

var desc = TypeDescriptor.GetProperties(obj)[memberName];
if (desc == null)
    throw new InvalidOperationException($"No descriptor for {memberName}");
var ser = ValueSerializer.GetSerializerFor(desc, ctx);

static bool HasDescriptor(PropertyDescriptor d) => d != null;

When it happens

Trigger: Calling GetSerializerFor((PropertyDescriptor)null, context), typically from a custom XAML serialization pipeline that passed a descriptor obtained from a failed TypeDescriptor.GetDescriptor lookup.

Common situations: Custom serializers that obtain PropertyDescriptor dynamically and do not null-check; serialization of members without an associated descriptor; reflection-based binding failures.

Related errors

• type
• assembly
• assemblyName
• assemblyQualifiedTypeName
• instance

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/eeb6d9f4d475657c. Report an issue: GitHub.

Appendix: source

Thrown at src/SourceGenerators/System.Xaml/System.Windows.Markup/ValueSerializer.cs:53

	// [System.Runtime.CompilerServices.TypeForwardedFrom (Consts.AssemblyWindowsBase)]
	public abstract class ValueSerializer
	{
		public static ValueSerializer GetSerializerFor (PropertyDescriptor descriptor)
		{
			return GetSerializerFor (descriptor, null);
		}

		public static ValueSerializer GetSerializerFor (Type type)
		{
			return GetSerializerFor (type, null);
		}

		// untested
		public static ValueSerializer GetSerializerFor (PropertyDescriptor descriptor, IValueSerializerContext context)
		{
			if (descriptor == null)
			{
				throw new ArgumentNullException ("descriptor");
			}

			if (context != null)
			{
				return context.GetValueSerializerFor (descriptor);
			}

			var tc = descriptor.Converter;
			if (tc != null && tc.GetType () != typeof (TypeConverter))
			{
				return new TypeConverterValueSerializer (tc);
			}

			return null;
		}

		public static ValueSerializer GetSerializerFor (Type type, IValueSerializerContext context)
		{

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