dotnet/wpf · error · NotSupportedException

This collection is fixed size.

Error message

This collection is fixed size.

What it means

PartialArray<T> is a fixed-size internal collection (IList<T> over a backing array segment); Remove throws NotSupportedException with the message 'This collection is fixed size.' because elements cannot be deleted once the array is allocated.

Solutions

1. Do not call Remove; PartialArray is fixed-size by design — rebuild the collection without the item into a resizable List<T>.
2. Copy the elements to a List<T> first and remove from that copy.
3. Replace the data structure with List<T> if mutation is required.

Example fix

// before
partialArray.Remove(item);
// after
var list = new List<T>(partialArray);
list.Remove(item);

if (coll.IsReadOnly || coll is System.Collections.Generic.ICollection<T> c && c.IsReadOnly) { /* skip mutation */ }

bool IsFixedSizeCollection<T>(ICollection<T> c) => c.IsReadOnly || c.GetType().Name == "PartialArray`1";

try { list.Remove(item); }
catch (NotSupportedException) { list = new List<T>(list); ((List<T>)list).Remove(item); }

When it happens

Trigger: Calling Remove(item) — directly or via IList<T>/ICollection<T> interfaces — on any PartialArray instance.

Common situations: Generic code that treats any IList<T> as mutable and tries to remove an item, or internal WPF paths (e.g. glyph/character run collections) exposed through collection interfaces.

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

• CollectionIsFixedSize
• SR.CalendarCollection_MultiThreadedCollectionChangeNotSuppor…
• throw new NotSupportedException();
• throw new NotSupportedException() in IList
• By default, ToolTip property does not support ToolTip…

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/b59f7849f2ca3a5c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/MS/internal/PartialArray.cs:61

            }
        }

        public bool Contains(T item)
        {
            return IndexOf(item) >= 0;
        }

        public bool IsFixedSize
        {
            get
            {
                return true;
            }
        }

        public bool Remove(T item)
        {
            throw new NotSupportedException(SR.CollectionIsFixedSize);                           
        }

        public void RemoveAt(int index)
        {
            throw new NotSupportedException(SR.CollectionIsFixedSize);                           
        }

        public void Clear()
        {
            throw new NotSupportedException();                           
        }

        public void Add(T item)
        {
            throw new NotSupportedException(SR.CollectionIsFixedSize);                           
        }

        public void Insert(int index, T item)

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