dotnet/wpf · error · ArgumentException

SR.NotInAssociatedTree

Error message

SR.NotInAssociatedTree

What it means

ITextRange.Contains throws ArgumentException when the supplied TextPointer belongs to a different TextContainer than the range itself. Ranges can only reason about positions within their own text tree; cross-tree pointers are meaningless. The message name SR.NotInAssociatedTree conveys the pointer is not in the range's associated tree.

Solutions

  1. Ensure the TextPointer comes from the same document/container as the range
  2. Compare within the correct TextSelection of the same RichTextBox
  3. If cross-document checks are needed, compare textual content or convert positions explicitly rather than passing pointers across trees

Example fix

// before
bool has = otherDoc.Selection.Contains(pointerFromDocA);
// after
bool has = (pointerFromDocA.TextContainer == range.Start.TextContainer) && range.Contains(pointerFromDocA);
Defensive patterns

Strategy: validation

Validate before calling

if (pointer == null || range.Start == null || pointer.TextContainer != range.Start.TextContainer) throw new ArgumentException("pointer from different tree");

Type guard

bool SameTree(ITextRange r, TextPointer p) => r.Start != null && p.TextContainer == r.Start.TextContainer;

Try / catch

try { bool c = range.Contains(pointer); } catch (ArgumentException ex) { /* cross-document pointer */ }

Prevention

When it happens

Trigger: Calling Contains(textPointer) on a TextRange/TextSelection whose Start.TextContainer differs from textPointer.TextContainer — e.g. testing whether a selection contains a pointer from another RichTextBox or FlowDocument.

Common situations: Multiple RichTextBox/FlowDocument instances in one window; comparing caret positions across documents after a paste or drag-drop between editors; caching TextPointers from one document and testing against a range in another.

Understand the failure class

Background: "is not a compatible type" / "cannot merge" errors: when a value's type doesn't match what the library requires — this error's family across 65 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/0368dd5321c2af67. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/TextRangeBase.cs:65

        //......................................................
        //
        // Selection Building
        //
        //......................................................

        /// <summary>
        /// </summary>
        // need to accound for position.LogicalDirection. -- see TextSegment.Contains implementation
        internal static bool Contains(ITextRange thisRange, ITextPointer textPointer)
        {
            NormalizeRange(thisRange);

            ArgumentNullException.ThrowIfNull(textPointer);

            if (textPointer.TextContainer != thisRange.Start.TextContainer)
            {
                throw new ArgumentException(SR.NotInAssociatedTree, nameof(textPointer));
            }

            // Correct position normalization on range boundary so that
            // our test would not depend on what side of formatting tags
            // pointer is located.
            if (textPointer.CompareTo(thisRange.Start) < 0)
            {
                textPointer = textPointer.GetFormatNormalizedPosition(LogicalDirection.Forward);
            }
            else if (textPointer.CompareTo(thisRange.End) > 0)
            {
                textPointer = textPointer.GetFormatNormalizedPosition(LogicalDirection.Backward);
            }

            // Check if at least one segment contains this position.
            for (int i = 0; i < thisRange._TextSegments.Count; i++)
            {
                if (thisRange._TextSegments[i].Contains(textPointer))

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