dotnet/wpf · error · ArgumentException

SR.RichTextBox_PointerNotInSameDocument

Error message

SR.RichTextBox_PointerNotInSameDocument

What it means

RichTextBox.CaretPosition assigns the caret to a TextPointer, and the pointer must belong to the same text container (document) as the selection. If Selection.Start.IsInSameDocument(value) is false the setter throws ArgumentException(SR.RichTextBox_PointerNotInSameDocument), preventing cross-document TextPointer use.

Solutions

  1. Obtain the TextPointer from the same RichTextBox's Document/Selection before assigning CaretPosition.
  2. After replacing the Document, recompute the pointer from the new document instead of reusing the old one.
  3. Convert positions to an offset (e.g. via TextPointer offsets) and re-create the pointer against the target document.

Example fix

// before
rtb.CaretPosition = pointerFromOldDocument;
// after
var newPointer = rtb.Document.ContentStart.GetPositionAtOffset(savedOffset);
rtb.CaretPosition = newPointer;
Defensive patterns

Strategy: type-guard

Validate before calling

if (pointer == null || !richTextBox.Selection.Start.IsInSameDocument(pointer))
    pointer = richTextBox.Document.ContentStart.GetPositionAtOffset(savedOffset) ?? richTextBox.Document.ContentStart;
richTextBox.CaretPosition = pointer;

Type guard

bool IsPointerInDocument(RichTextBox rtb, TextPointer p) =>
    p != null && rtb.Selection.Start.IsInSameDocument(p);

Try / catch

try { richTextBox.CaretPosition = pointer; }
catch (ArgumentException) { richTextBox.CaretPosition = richTextBox.Document.ContentStart; }

Prevention

When it happens

Trigger: Assigning CaretPosition with a TextPointer obtained from a different FlowDocument or another RichTextBox — e.g. caching pointers across document swaps, or setting the caret from a pointer captured before Document was replaced.

Common situations: Restoring saved caret positions after loading new content; copying positions between two RichTextBox instances; using pointers from a FlowDocumentScrollViewer or TextPointer from a stale document instance.

Understand the failure class

Background: "must be a positive integer", "cannot be empty", "invalid argument": how invalid-argument errors work across open-source libraries — this error's family across 33 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9bf6c82429fab68b. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Controls/RichTextBox.cs:576

        }

        /// <summary>
        /// Position of the caret.
        /// </summary>
        [DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public TextPointer CaretPosition
        {
            get
            {
                return Selection.MovingPosition;
            }

            set
            {
                ArgumentNullException.ThrowIfNull(value);
                if (!Selection.Start.IsInSameDocument(value))
                {
                    throw new ArgumentException(SR.RichTextBox_PointerNotInSameDocument, nameof(value));
                }
                Selection.SetCaretToPosition(value, value.LogicalDirection, /*allowStopAtLineEnd:*/true, /*allowStopNearSpace:*/false);
            }
        }
        
        #endregion Content Accessing Properties

        #endregion Public Properties

        //------------------------------------------------------
        //
        //  Internal Methods
        //
        //------------------------------------------------------

        #region Internal Methods

        // Returns the DependencyObjectType for the registered ThemeStyleKey's default

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