dotnet/wpf · error · ArgumentException

SR.WrongSelectionType

Error message

SR.WrongSelectionType (anchor1: type = {anchor1.GetType()})

What it means

TextSelectionHelper.MergeSelections merges two annotation anchors into one. The anchor1 parameter must be either null or a TextAnchor; any other type throws this ArgumentException because only TextAnchor instances can be merged.

Solutions

  1. Pass a TextAnchor instance (or null) as anchor1; cast or convert the selection before calling.
  2. Check anchor1 is TextAnchor before calling; if it is a TextRange, convert it via the appropriate selection processor first.
  3. Catch ArgumentException and treat the merge as failed, returning a fallback anchor.

Example fix

// before
object merged;
TextSelectionHelper.MergeSelections(myTextRange, existingAnchor, out merged);
// after
if (myTextRange is TextAnchor first)
{
    object merged;
    TextSelectionHelper.MergeSelections(first, existingAnchor, out merged);
}
else
{
    merged = existingAnchor; // skip incompatible anchor
}
Defensive patterns

Strategy: type-guard

Validate before calling

if (anchor1 != null && !(anchor1 is TextAnchor)) throw new InvalidOperationException("anchor1 must be TextAnchor");

Type guard

static bool IsMergeableAnchor(object a) => a == null || a is TextAnchor;

Try / catch

try { ok = TextSelectionHelper.MergeSelections(a1, a2, out merged); }
catch (ArgumentException ex) when (ex.ParamName == null && ex.Message.StartsWith("anchor1")) { merged = a2; ok = merged != null; }

Prevention

When it happens

Trigger: Calling TextSelectionHelper.MergeSelections with an anchor1 that is non-null but not a TextAnchor — e.g. passing a TextRange, TextSelection, ContentLocator or a custom selection object as the first anchor.

Common situations: Storing anchors as object and losing track of their concrete type; mixing anchor types produced by different selection processors; deserializing anchors whose type got changed between versions.

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/fbac92859939dded. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Annotations/Anchoring/TextSelectionHelper.cs:72

        #region Public Methods

        /// <summary>
        ///     Merges the two anchors into one, if possible.
        /// </summary>
        /// <param name="anchor1">anchor to merge </param>
        /// <param name="anchor2">other anchor to merge </param>
        /// <param name="newAnchor">new anchor that contains the data from both 
        /// anchor1 and anchor2</param>
        /// <returns>true if the anchors were merged, false otherwise 
        /// </returns>
        /// <exception cref="ArgumentNullException">anchor1 or anchor2 are null</exception>
        public static bool MergeSelections(Object anchor1, Object anchor2, out Object newAnchor)
        {
            TextAnchor firstAnchor = anchor1 as TextAnchor;
            TextAnchor secondAnchor = anchor2 as TextAnchor;

            if ((anchor1 != null) && (firstAnchor == null))
                throw new ArgumentException(SR.WrongSelectionType, $"anchor1: type = {anchor1.GetType()}");

            if ((anchor2 != null) && (secondAnchor == null))
                throw new ArgumentException(SR.WrongSelectionType, $"Anchor2: type = {anchor2.GetType()}");

            if (firstAnchor == null)
            {
                newAnchor = secondAnchor;
                return newAnchor != null;
            }

            if (secondAnchor == null)
            {
                newAnchor = firstAnchor;
                return newAnchor != null;
            }

            newAnchor = TextAnchor.ExclusiveUnion(firstAnchor, secondAnchor);

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