dotnet/wpf · error · ArgumentException

SR.WrongSelectionType

Error message

SR.WrongSelectionType

What it means

TextSelectionProcessor.GenerateLocatorParts converts a selection into a ContentLocatorPart. After CheckSelection succeeds, it requires the resolved start pointer to be a TextPointer (the WPF text stack implementation); a different ITextPointer implementation cannot be used to compute element offsets and throws this ArgumentException.

Solutions

  1. Use anchors/selections created from WPF text content (FlowDocument, TextBox, RichTextBox) so the start pointer is a real TextPointer.
  2. Remove custom ITextPointer implementations from the anchor's backing container.
  3. Catch ArgumentException and skip locator-part generation for non-WPF-backed anchors.

Example fix

// before
var part = textSelectionProcessor.GenerateLocatorParts(anchor, documentPageView, startNode);
// after
if (anchor.Start is TextPointer)
{
    var part = textSelectionProcessor.GenerateLocatorParts(anchor, documentPageView, startNode);
}
else
{
    part = null; // not backed by the WPF text stack
}
Defensive patterns

Strategy: validation

Validate before calling

bool wpfBacked = anchor.Start is TextPointer;
if (!wpfBacked) throw new InvalidOperationException("Anchor must be backed by WPF TextPointer");

Type guard

static bool HasWpfPointers(TextAnchor a) => a is { Start: TextPointer, End: TextPointer };

Try / catch

try { part = processor.GenerateLocatorParts(selection, node, pageView); }
catch (ArgumentException ex) when (ex.Message.Contains(nameof(selection))) { part = null; }

Prevention

When it happens

Trigger: Calling GenerateLocatorParts with a TextAnchor whose Start pointer is not a TextPointer — e.g. an anchor built over a custom/non-WPF ITextContainer or a mock/custom ITextPointer implementation.

Common situations: Custom text edit controls implementing ITextPointer but not backed by the WPF TextPointer class; unit tests injecting fake text pointers; mixing anchors created against other frameworks.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/618fc848c30f837c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Annotations/Anchoring/TextSelectionProcessor.cs:135

        /// <param name="startNode">the node the locator parts should be in the
        /// context of</param>
        /// <returns>one or more locator parts representing the portion of 'startNode' spanned
        /// by 'selection'</returns>
        /// <exception cref="ArgumentNullException">startNode or selection is null</exception>
        /// <exception cref="ArgumentException">selection is of the wrong type</exception>
        public override IList<ContentLocatorPart>
            GenerateLocatorParts(Object selection, DependencyObject startNode)
        {
            ArgumentNullException.ThrowIfNull(startNode);
            ArgumentNullException.ThrowIfNull(selection);

            ITextPointer start;
            ITextPointer end;
            IList<TextSegment> textSegments = null;

            TextSelectionHelper.CheckSelection(selection, out start, out end, out textSegments);
            if (!(start is TextPointer))
                throw new ArgumentException(SR.WrongSelectionType, nameof(selection));

            ITextPointer elementStart;
            ITextPointer elementEnd;

            // If we can't get the start/end of the node then we can't generate a locator part
            if (!GetNodesStartAndEnd(startNode, out elementStart, out elementEnd))
                return null;

            if (elementStart.CompareTo(end) > 0)
                throw new ArgumentException(SR.InvalidStartNodeForTextSelection, nameof(startNode));

            if (elementEnd.CompareTo(start) < 0)
                throw new ArgumentException(SR.InvalidStartNodeForTextSelection, nameof(startNode));

            ContentLocatorPart part = new ContentLocatorPart(CharacterRangeElementName);

            int startOffset = 0;
            int endOffset = 0;

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