dotnet/wpf · error · InvalidOperationException

SR.Format(SR.MemberNotAllowedForView…

Error message

SR.Format(SR.MemberNotAllowedForView, "NewItemPlaceholderPosition")

What it means

Setting ItemCollection.NewItemPlaceholderPosition throws InvalidOperationException(SR.MemberNotAllowedForView, "NewItemPlaceholderPosition") when the current view does not implement IEditableCollectionView (ecv == null). The member only exists when the underlying view supports transactional item editing.

Solutions

Bind ItemsSource to a type whose view implements IEditableCollectionView (e.g. ObservableCollection<T> with a ListCollectionView).
Check `items.CollectionView is IEditableCollectionView` before setting the property.
Configure the view explicitly (CollectionViewSource.GetDefaultView(source) as IEditableCollectionView) and set the property on the view.

Example fix

// before
items.NewItemPlaceholderPosition = NewItemPlaceholderPosition.AtBeginning;
// after
if (items.CollectionView is IEditableCollectionView ecv)
    ecv.NewItemPlaceholderPosition = NewItemPlaceholderPosition.AtBeginning;

Defensive patterns

Strategy: validation

Validate before calling

if (items.CollectionView is IEditableCollectionView ecv)
    ecv.NewItemPlaceholderPosition = NewItemPlaceholderPosition.AtBeginning;

Type guard

static bool SupportsEditing(ItemCollection c) => c.CollectionView is IEditableCollectionView;

Try / catch

try { items.NewItemPlaceholderPosition = pos; }
catch (InvalidOperationException) { /* view not editable; skip placeholder config */ }

Prevention

Check CanGroup/IEditableCollectionView capability flags before view-specific members
Bind ItemsSource to types with editable default views
Configure view features via CollectionViewSource where possible

When it happens

Trigger: Setting items.NewItemPlaceholderPosition on a view backed by a collection view that does not implement IEditableCollectionView (e.g. a plain ListCollectionView-less source or a custom view).

Common situations: Assigning ItemsSource to a plain IEnumerable/Collection<T> whose default view lacks editing support; code written for an ObservableCollection-based view reused against other sources.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/358dceaa5be4536b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ItemCollection.cs:887

                if (ecv != null)
                {
                    return ecv.NewItemPlaceholderPosition;
                }
                else
                {
                    return NewItemPlaceholderPosition.None;
                }
            }
            set
            {
                IEditableCollectionView ecv = _collectionView as IEditableCollectionView;
                if (ecv != null)
                {
                    ecv.NewItemPlaceholderPosition = value;
                }
                else
                {
                    throw new InvalidOperationException(SR.Format(SR.MemberNotAllowedForView, "NewItemPlaceholderPosition"));
                }
            }
        }

        /// <summary>
        /// Return true if the view supports <seealso cref="IEditableCollectionView.AddNew"/>.
        /// </summary>
        bool    IEditableCollectionView.CanAddNew
        {
            get
            {
                IEditableCollectionView ecv = _collectionView as IEditableCollectionView;
                if (ecv != null)
                {
                    return ecv.CanAddNew;
                }
                else
                {

View on GitHub (pinned to 81131a70a4)