dotnet/wpf · error · InvalidOperationException

SR.GridCollection_CannotModifyReadOnly

Error message

SR.GridCollection_CannotModifyReadOnly

What it means

PrivateVerifyWriteAccess throws InvalidOperationException(SR.GridCollection_CannotModifyReadOnly, "RowDefinitionCollection") when the collection's IList IsReadOnly is true. WPF marks row collections read-only during measurement/layout and serialization phases, so any structural change is rejected.

Solutions

1. Defer modifications until layout is complete (Dispatcher.BeginInvoke(DispatcherPriority.Loaded, ...) or postpone to Loaded event).
2. Perform collection changes before the Grid enters measure (e.g. in the constructor or before Show()).
3. Check grid.RowDefinitions.IsReadOnly before mutating and queue the change if true.

Example fix

// before
void OnSizeChanged(...) { grid.RowDefinitions.RemoveAt(0); }
// after
void OnSizeChanged(...) {
  Dispatcher.BeginInvoke(DispatcherPriority.Loaded, new Action(() =>
    grid.RowDefinitions.RemoveAt(0)));
}

public static void SafeClearRows(Grid g, Action mutate) {
  if (g.RowDefinitions.IsReadOnly)
      g.Dispatcher.BeginInvoke(DispatcherPriority.Loaded, mutate);
  else
      mutate();
}

bool CanMutateRows(Grid g) => !g.RowDefinitions.IsReadOnly;

try { grid.RowDefinitions.RemoveAt(0); } catch (InvalidOperationException) { /* collection is read-only; defer to after layout */ }

When it happens

Trigger: Calling Add, Clear, Insert, Remove, RemoveAt or RemoveRange on grid.RowDefinitions while it is in read-only state (e.g. during Grid.MeasureOverride/layout pass or from a frozen/serialized context).

Common situations: Mutating row definitions inside a SizeChanged/LayoutUpdated handler or MeasureOverride override; modifying the collection from a background thread during layout.

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

• SR.GridCollection_InOtherCollection
• 0x80040206
• ArgumentNullException: child
• Exception of type 'System.InvalidOperationException' was…
• NotSupportedException

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/766c02f79f61eeda. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/RowDefinition.cs:573

        #endregion Internal Properties

        //------------------------------------------------------
        //
        //  Private Methods
        //
        //------------------------------------------------------

        #region Private Methods

        /// <summary>
        ///     Throws if the collection is in readonly state.
        /// </summary>
        private void PrivateVerifyWriteAccess()
        {
            if (this.IsReadOnly)
            {
                throw new InvalidOperationException(SR.Format(SR.GridCollection_CannotModifyReadOnly, "RowDefinitionCollection"));
            }
        }

        /// <summary>
        ///     Throws if value is not something the collection expects.
        /// </summary>
        private void PrivateValidateValueForAddition(object value)
        {
            ArgumentNullException.ThrowIfNull(value);

            RowDefinition item = value as RowDefinition;

            if (item == null)
            {
                throw new ArgumentException(SR.Format(SR.GridCollection_MustBeCertainType, "RowDefinitionCollection", "RowDefinition"));
            }

            if (item.Parent != null)

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