What causes "ArgumentNullException(nameof(instance))" in dotnet/wpf?

Pass a non-null object instance to the XamlObjectEventArgs constructor

dotnet/wpf · error · ArgumentNullException

ArgumentNullException(nameof(instance))

Error message

ArgumentNullException(nameof(instance))

What it means

ArgumentNullException thrown by the public XamlObjectEventArgs constructor when the instance parameter is null. The library requires every event-args object to carry a non-null XAML object instance, so it validates immediately at construction.

Solutions

Pass a non-null object instance to the XamlObjectEventArgs constructor
Check for null before constructing the event args and skip raising the event if there is no instance
Use the internal overload path or a different notification mechanism when the object legitimately does not exist

Example fix

// before
var args = new XamlObjectEventArgs(obj); // obj may be null
// after
if (obj != null) { var args = new XamlObjectEventArgs(obj); handler(this, args); }

Defensive patterns

Strategy: type-guard

Validate before calling

if (instance == null) throw new InvalidOperationException("Cannot raise XamlObjectEventArgs without an instance");

Type guard

static bool IsValidXObjectArgs(object instance) => instance is not null;
// guard: if (!IsValidXObjectArgs(obj)) return; var args = new XamlObjectEventArgs(obj);

Try / catch

try { var args = new XamlObjectEventArgs(instance); handler(sender, args); }
catch (ArgumentNullException) { log.Error("XamlObjectEventArgs requires a non-null instance"); }

Prevention

Never construct event args for null objects — skip the event instead
Null-check object instances before raising XAML object lifecycle events
Keep the null check adjacent to event-raising code so refactors don't lose it

When it happens

Trigger: Calling new XamlObjectEventArgs(null) directly, or wiring XAML object events (e.g. XamlObjectWriter settings OnXObjectHandled/AfterProperties handlers) and constructing the args yourself with a null object.

Common situations: Custom XAML-writing pipelines that raise object lifecycle events for synthetic/placeholder objects that end up null; refactored event code where the object variable was not yet assigned when the event was raised.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a61376b9178db2a3. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/Events/XamlObjectEventArgs.cs:12

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

#nullable disable

namespace System.Xaml
{
    public class XamlObjectEventArgs : EventArgs
    {
        public XamlObjectEventArgs(object instance)
        {
            Instance = instance ?? throw new ArgumentNullException(nameof(instance));
        }

        internal XamlObjectEventArgs(object instance, Uri sourceBamlUri, int elementLineNumber, int elementLinePosition) :
            this(instance)
        {
            SourceBamlUri = sourceBamlUri;
            ElementLineNumber = elementLineNumber;
            ElementLinePosition = elementLinePosition;
        }

        public object Instance { get; private set; }

        public Uri SourceBamlUri { get; private set; }

        public int ElementLineNumber { get; private set; }

        public int ElementLinePosition { get; private set; }
    }

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