dotnet/wpf · error · InvalidOperationException

SR.RichTextBox_CantSetDocumentInsideChangeBlock

Error message

SR.RichTextBox_CantSetDocumentInsideChangeBlock

What it means

While an editing change block is open (e.g. inside TextEditor.ChangeBlock, used by undo units and programmatic edits), RichTextBox.Document cannot be replaced. The setter throws InvalidOperationException(SR.RichTextBox_CantSetDocumentInsideChangeBlock) because swapping the text container mid-change would corrupt the undo stack and active selection.

Solutions

Defer the assignment out of the change block, e.g. Dispatcher.BeginInvoke to run after editing completes.
Close the change block (EndChange/Dispose) before assigning Document.
Restructure so Document replacement is not triggered by editing events of the same control.

Example fix

// before
void OnTextChanged(...) { richTextBox.Document = newDoc; }
// after
void OnTextChanged(...) { Dispatcher.BeginInvoke(() => richTextBox.Document = newDoc); }

Defensive patterns

Strategy: try-catch

Validate before calling

if (richTextBox.TextSelectionInternal?.ChangeBlockLevel > 0)
    Dispatcher.BeginInvoke(() => richTextBox.Document = newDoc);
else
    richTextBox.Document = newDoc;

Type guard

bool OutsideChangeBlock(RichTextBox rtb) => rtb.TextSelectionInternal == null || rtb.TextSelectionInternal.ChangeBlockLevel == 0;

Try / catch

try { richTextBox.Document = newDoc; }
catch (InvalidOperationException) { Dispatcher.BeginInvoke(() => richTextBox.Document = newDoc); }

Prevention

Never mutate Document inside TextChanged/selection handlers of the same control.
Ensure BeginChange scopes are closed (using/dispose) before swapping documents.
Marshal document swaps to the dispatcher queue when they originate from editing events.

When it happens

Trigger: Setting Document from within an open change block: inside BeginChange/EndChange scopes, editing-event handlers such as TextChanged or selection events fired during a change block, or re-entrancy from a TextChanged handler that resets the Document.

Common situations: Reacting to TextChanged by replacing the Document; undo/redo callbacks that swap documents; custom commands wrapping edits in BeginChange and then assigning Document.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/d56be563dcbe3aca. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/RichTextBox.cs:365

            {
                Invariant.Assert(_document != null);
                return _document;
            }

            set
            {
                ArgumentNullException.ThrowIfNull(value);

                if (value != _document &&
                    value.StructuralCache != null && value.StructuralCache.TextContainer != null && 
                    value.StructuralCache.TextContainer.TextSelection != null)
                {
                    throw new ArgumentException(SR.RichTextBox_DocumentBelongsToAnotherRichTextBoxAlready);
                }

                if (_document != null && this.TextSelectionInternal.ChangeBlockLevel > 0)
                {
                    throw new InvalidOperationException(SR.RichTextBox_CantSetDocumentInsideChangeBlock);
                }

                if (value == _document)
                {
                    // Same document nothing to do.
                    return;
                }

                // Identify the case for the _document initialization
                bool initialSetting = _document == null;
                
                // Detach existing FlowDocument
                if (_document != null)
                {
                    // Detach PageSize change listener
                    _document.PageSizeChanged -= new EventHandler(this.OnPageSizeChangedHandler);

                    // Remove the document from the logical tree

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