dotnet/wpf · error · ArgumentException

SR.Collection_NoNull

Error message

SR.Collection_NoNull

What it means

UIElementCollection.Cast (used by Add, Insert and the constructor's IList paths) throws ArgumentException when a null value is supplied, because the collection does not permit null children. The message names SR.Collection_NoNull with the collection type.

Solutions

1. Null-check the element before adding it to the collection.
2. Fix the factory/loader that returned null instead of a UIElement.
3. Use Children.Remove on placeholder elements rather than adding nulls.

Example fix

// before
panel.Children.Add(BuildElement()); // may return null
// after
var e = BuildElement();
if (e != null) panel.Children.Add(e);

if (element != null) panel.Children.Add(element);

static bool IsAddable(UIElement e) => e != null;

try { panel.Children.Add(value); }
catch (ArgumentException ex) { /* null or wrong type; log and skip */ }

When it happens

Trigger: panel.Children.Add(null), panel.Children.Insert(0, null), or adding through an IList reference where the item is null.

Common situations: Data binding or factory methods that can return null; building UI from deserialized models where an element failed to instantiate; loop-generated children where a creation step returned null.

Related errors

• SR.Collection_BadType
• ArgumentOutOfRangeException (timeout was Duration.Automatic)
• Buffer address passed to GetText cannot be NULL.
• Collection_BadRank
• Collection_CopyTo_ArrayCannotBeMultidimensional

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/2f0e6fe583d7dcee. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/UIElementCollection.cs:386

                _visualParent.InvalidateMeasure();
            }
        }


        /// <summary>
        /// Method that forwards to VisualCollection.Move
        /// </summary>
        /// <param name="visual"></param>
        /// <param name="destination"></param>
        internal void MoveVisualChild(Visual visual, Visual destination)
        {
            _visualChildren.Move(visual, destination);
        }
		
        private UIElement Cast(object value)
        {
            if (value == null)
                throw new System.ArgumentException(SR.Format(SR.Collection_NoNull, "UIElementCollection"));

            UIElement element = value as UIElement;

            if (element == null)
                throw new System.ArgumentException(SR.Format(SR.Collection_BadType, "UIElementCollection", value.GetType().Name, "UIElement"));

            return element;
        }
		
        #region IList Members

        /// <summary>
        /// Adds an element to the UIElementCollection
        /// </summary>
        int IList.Add(object value)
        {
            return Add(Cast(value));
        }

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