dotnet/wpf · error · ArgumentException

SR.BadFixedTextPosition

Error message

SR.BadFixedTextPosition

What it means

FixedTextContainer.VerifyPosition also requires the ITextPointer to be a FixedTextPointer (the container's concrete pointer type). Passing a valid ITextPointer from another implementation throws ArgumentException(SR.BadFixedTextPosition) because only FixedTextPointer instances can be used with this container.

Solutions

  1. Create pointers via the FixedTextContainer (CreatePointerAtOffset/CreateStaticPointer) instead of other text stacks.
  2. Cast/convert the argument to FixedTextPointer within the owning container before use.
  3. Add an assertion/guard that the pointer type matches before calling.
  4. Catch ArgumentException and fall back to creating a container-native pointer.

Example fix

// before
ITextPointer p = flowDoc.ContentStart; // wrong implementation
fixedContainer.VerifyPosition(p); // throws
// after
FixedTextPointer p = fixedContainer.CreatePointerAtOffset(0, LogicalDirection.Forward);
fixedContainer.VerifyPosition(p);
Defensive patterns

Strategy: type-guard

Validate before calling

bool IsFixedPointer(ITextPointer p) => p is FixedTextPointer;

Type guard

FixedTextPointer AsFixedPointer(ITextPointer p) => p as FixedTextPointer; // null check before use

Try / catch

try { container.VerifyPosition(pointer); }
catch (ArgumentException) { pointer = container.CreatePointerAtOffset(offset, direction); }

Prevention

When it happens

Trigger: Calling a FixedTextContainer API with an ITextPointer implemented by another text stack (e.g. a FlowDocument's TextPointer or a custom ITextPointer).

Common situations: Code written against ITextContainer/ITextPointer abstractions that mixes FlowDocument and FixedDocument pointers; unit tests passing mock pointers; refactoring that changed pointer types.

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/379597b320abf59f. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/FixedTextContainer.cs:320

        #region Internal Methods

        //--------------------------------------------------------------------
        // Utility Method
        //---------------------------------------------------------------------
        internal FixedTextPointer VerifyPosition(ITextPointer position)
        {
            ArgumentNullException.ThrowIfNull(position);

            if (position.TextContainer != this)
            {
                throw new ArgumentException(SR.Format(SR.NotInAssociatedContainer, "position"));
            }

            FixedTextPointer ftp = position as FixedTextPointer;

            if (ftp == null)
            {
                throw new ArgumentException(SR.Format(SR.BadFixedTextPosition, "position"));
            }

            return ftp;
        }


        internal int GetPageNumber(ITextPointer  textPointer)
        {
            FixedTextPointer fixedTextPointer = textPointer as FixedTextPointer;
            int  pageNumber = int.MaxValue;

            if (fixedTextPointer != null)
            {
                if (fixedTextPointer.CompareTo(((ITextContainer)this).Start) == 0)
                {
                    pageNumber = 0;
                }
                else if (fixedTextPointer.CompareTo(((ITextContainer)this).End) == 0)

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