dotnet/wpf · error · InvalidOperationException

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

Error message

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

What it means

ItemCollection.AddNewItem adds a new item via the IEditableCollectionViewAddNewItem interface; it only works when the active view implements that interface. Otherwise it throws InvalidOperationException with MemberNotAllowedForView("AddNewItem").

Solutions

Check CanAddNewItem before calling AddNewItem
Use ItemCollection.Add() for simple insertion into an internal ItemsCollection instead
Bind to a source whose default view supports AddNewItem (e.g. IBindingList / ObservableCollection with an editable view)

Example fix

// before
items.AddNewItem(newItem);
// after
if (items.CanAddNewItem) { items.AddNewItem(newItem); } else { items.Add(newItem); }

Defensive patterns

Strategy: validation

Validate before calling

if (items.CanAddNewItem) items.AddNewItem(newItem); else items.Add(newItem);

Type guard

bool canAddNew = items.CollectionView is IEditableCollectionViewAddNewItem ani && ani.CanAddNewItem;

Try / catch

try { items.AddNewItem(newItem); } catch (InvalidOperationException ex) { items.Add(newItem); }

Prevention

Check CanAddNewItem first
Prefer plain Add() unless the view is IEditableCollectionViewAddNewItem
Don't confuse AddNewItem with Add

When it happens

Trigger: Calling ItemCollection.AddNewItem(newItem) when the underlying view does not implement IEditableCollectionViewAddNewItem (CanAddNewItem is false), e.g. a plain read-only or non-add-capable source view.

Common situations: Adding items programmatically to a DataGrid/ItemsControl bound to sources like LINQ results or custom views that don't support AddNew; confusing AddNewItem with the plain Add() method.

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/de1d7be5ee4be20a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ItemCollection.cs:1224

            }
        }

        /// <summary>
        /// Add a new item to the underlying collection.  Returns the new item.
        /// After calling AddNewItem and changing the new item as desired, either
        /// <seealso cref="IEditableCollectionView.CommitNew"/> or <seealso cref="IEditableCollectionView.CancelNew"/> should be
        /// called to complete the transaction.
        /// </summary>
        object  IEditableCollectionViewAddNewItem.AddNewItem(object newItem)
        {
            IEditableCollectionViewAddNewItem ani = _collectionView as IEditableCollectionViewAddNewItem;
            if (ani != null)
            {
                return ani.AddNewItem(newItem);
            }
            else
            {
                throw new InvalidOperationException(SR.Format(SR.MemberNotAllowedForView, "AddNewItem"));
            }
        }

        #endregion IEditableCollectionViewAddNewItem

        #region ICollectionViewLiveShaping

        ///<summary>
        /// Gets a value that indicates whether this view supports turning live sorting on or off.
        ///</summary>
        public bool CanChangeLiveSorting
        {
            get
            {
                ICollectionViewLiveShaping cvls = _collectionView as ICollectionViewLiveShaping;
                return (cvls != null) ? cvls.CanChangeLiveSorting : false;
            }
        }

View on GitHub (pinned to 81131a70a4)