dotnet/wpf · error · InvalidOperationException

SR.Format(SR.GridCollection_CannotModifyReadOnly…

Error message

SR.Format(SR.GridCollection_CannotModifyReadOnly, "ColumnDefinitionCollection")

What it means

The collection is read-only (IsReadOnly is true), so any mutating operation is rejected with InvalidOperationException. WPF marks a ColumnDefinitionCollection read-only in certain internal states where modification is not permitted.

Solutions

Defer the modification until after layout completes, e.g. via Dispatcher.BeginInvoke or Loaded
Check grid.ColumnDefinitions.IsReadOnly before mutating and skip/queue the change
Modify definitions before the Grid enters layout (in the constructor or before Show)

Example fix

// before
grid.LayoutUpdated += (s,e) => grid.ColumnDefinitions.Add(new ColumnDefinition());
// after
grid.Loaded += (s,e) => { if (!grid.ColumnDefinitions.IsReadOnly) grid.ColumnDefinitions.Add(new ColumnDefinition()); };

Defensive patterns

Strategy: validation

Validate before calling

if (grid.ColumnDefinitions.IsReadOnly) throw new InvalidOperationException("ColumnDefinitions are read-only right now; defer the change.");

Try / catch

try { grid.ColumnDefinitions.Add(newCol); } catch (InvalidOperationException) { Dispatcher.BeginInvoke(() => grid.ColumnDefinitions.Add(newCol)); }

Prevention

Don't mutate definitions during measure/arrange or LayoutUpdated
Check IsReadOnly before mutating
Perform structural Grid changes before the window is shown or via Dispatcher

When it happens

Trigger: Calling Add, Clear, Insert, Remove, RemoveAt or RemoveRange on a ColumnDefinitionCollection whose IsReadOnly property returns true (typically during a Grid's internal layout/definition-locking phase).

Common situations: Modifying ColumnDefinitions while the Grid is being measured/arranged (e.g. from a LayoutUpdated handler or measure override); touching the collection from a different thread or reentrant callback during rendering.

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/9a5205cb9f711b93. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/ColumnDefinition.cs:578

        #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, "ColumnDefinitionCollection"));
            }
        }

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

            ColumnDefinition item = value as ColumnDefinition;

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

            if (item.Parent != null)

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