unoplatform/uno · error · ArgumentNullException

instance

Error message

instance

What it means

Thrown by XamlMemberInvoker.GetValue/SetValue when the instance argument is null (after the unknown-member check passes). The invoker must reflect over a concrete target object to read or write the member, so a null instance is invalid.

Source

Thrown at src/SourceGenerators/System.Xaml/System.Xaml.Schema/XamlMemberInvoker.cs:68

		public MethodInfo UnderlyingGetter {
			get { return member != null ? member.UnderlyingGetter : null; }
		}

		public MethodInfo UnderlyingSetter {
			get { return member != null ? member.UnderlyingSetter : null; }
		}

		void ThrowIfUnknown ()
		{
			if (member == null)
				throw new NotSupportedException ("Current operation is invalid for unknown member.");
		}

		public virtual object GetValue (object instance)
		{
			ThrowIfUnknown ();
			if (instance == null)
				throw new ArgumentNullException ("instance");
			if (member is XamlDirective)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "not supported operation on directive member {0}", member));
			if (UnderlyingGetter == null)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Attempt to get value from write-only property or event {0}", member));
			return UnderlyingGetter.Invoke (instance, Array.Empty<object>());
		}
		public virtual void SetValue (object instance, object value)
		{
			ThrowIfUnknown ();
			if (instance == null)
				throw new ArgumentNullException ("instance");
			if (member is XamlDirective)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "not supported operation on directive member {0}", member));
			if (UnderlyingSetter == null)
				throw new NotSupportedException (String.Format (CultureInfo.InvariantCulture, "Attempt to set value from read-only property {0}", member));
			if (member.IsAttachable)
				UnderlyingSetter.Invoke (null, new object [] {instance, value});
			else

View on GitHub (pinned to 0418340488)

Solutions

1. Ensure the owning instance is non-null before invoking, e.g. skip or queue the operation until the instance is realized.
2. For attached members, confirm the target element was created in the visual/logical tree.
3. Add a null guard at the call site to fail with a domain-specific message.

Example fix

// before
invoker.SetValue(target, value); // throws if target null

// after
if (target != null)
    invoker.SetValue(target, value);
else
    _pendingSets.Enqueue((invoker, value));

if (instance == null)
    throw new InvalidOperationException("owning instance not realized");
invoker.SetValue(instance, value);

static bool HasInstance(object o) => o != null;

When it happens

Trigger: Calling invoker.GetValue(null) or invoker.SetValue(null, value) where the owning object was not supplied, e.g. processing an attached property against a null parent.

Common situations: XAML readers that emit SetValue calls before the parent object is constructed; tree-walking code that hits a null node; deferred loading where the instance is not yet realized.

Related errors

• member
• Current operation is invalid for unknown member.
• not supported operation on directive member {0}
• Attempt to get value from write-only property or event {0}
• typeName

AI-assisted analysis of unoplatform/uno@0418340488 (2026-08-13). Data as JSON: /api/errors/056c414727a0a7d9. Report an issue: GitHub.