SR.Format(SR.TableCollectionElementTypeExpected…

Error message

SR.Format(SR.TableCollectionElementTypeExpected, typeof(TItem).Name)

What it means

IList.Insert validates that the boxed value is assignable to TItem; otherwise it throws ArgumentException with SR.Format(SR.TableCollectionElementTypeExpected, typeof(TItem).Name). The weakly-typed IList entry point must reject elements of the wrong type because the internal storage is TItem[].

Solutions

Pass only instances of the declared element type TItem
Check 'value is TItem' before calling the IList member
Use the strongly typed generic API instead of the IList non-generic members

Example fix

// before
((IList)collection).Insert(0, someObject);
// after
if (someObject is TItem typed) ((IList)collection).Insert(0, typed);

Defensive patterns

Strategy: type-guard

Validate before calling

if (value is TItem) ((IList)collection).Insert(index, value);

Type guard

bool IsExpectedElement(object value) => value is TItem;

Try / catch

try { ((IList)collection).Insert(index, value); } catch (ArgumentException) { /* value is not a TItem; reject or convert */ }

Prevention

Only pass TItem instances through the IList interface
Prefer the strongly typed generic API over IList
Guard reflection/late-bound calls with 'is TItem' checks

When it happens

Trigger: Calling IList.Insert(index, value) (or IList.Add) with an object that is not an instance of the collection's TItem type, e.g. inserting a string into a TableRowCollection.

Common situations: Late-bound/reflection code or XAML/scripting interop passing loosely typed values into the non-generic IList interface; deserializers adding placeholder objects of the wrong type.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7f8a1c1be17bf38a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/documents/ContentElementCollection.cs:455

            TItem item = value as TItem;

            if (item == null)
            {
                return -1;
            }

            return this.IndexOf(item);
        }

        void IList.Insert(int index, object value)
        {
            ArgumentNullException.ThrowIfNull(value);

            TItem newItem = value as TItem;

            if (newItem == null)
            {
                throw new ArgumentException(SR.Format(SR.TableCollectionElementTypeExpected, typeof(TItem).Name), nameof(value));
            }

            this.Insert(index, newItem);
        }

        bool IList.IsFixedSize
        {
            get
            {
                return false;
            }
        }

        bool IList.IsReadOnly
        {
            get
            {
                return this.IsReadOnly;

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