dotnet/wpf · error · ArgumentNullException

ArgumentNullException(paramName)

Error message

ArgumentNullException(paramName)

What it means

ValidationHelper.VerifyPosition throws ArgumentNullException(paramName) when a position argument passed alongside an ITextContainer is null. This guard exists so text APIs fail fast with the correct parameter name instead of NullReferenceException deep in text-tree code.

Solutions

1. Null-check TextPointer properties (CaretPosition, Selection.Start/End) before use and re-query after document load.
2. Wait for Loaded/TextChanged before capturing positions.
3. Catch ArgumentNullException and treat the pointer as invalid — re-acquire from the container.

Example fix

// before
container.InsertText(cachedPosition, text); // cachedPosition may be null
// after
var pos = cachedPosition ?? textBox.CaretPosition;
if (pos != null && pos.TextContainer == container) container.InsertText(pos, text);

if (position == null) { reAcquireFromContainer(); return; }

bool IsValidPointer(ITextContainer c, ITextPointer p) => p != null && p.TextContainer == c;

try { UsePosition(pos); }
catch (ArgumentNullException) { pos = control.CaretPosition; UsePosition(pos); }

When it happens

Trigger: Calling any ITextContainer/TextEditor API that validates a position (e.g. TextContainer.InsertText(position,...), selection APIs) with position == null.

Common situations: CaretPosition/Selection captured before the control was initialized; storing a TextPointer after the document was replaced (it becomes invalid/null); async code reading TextEditor.Selection.Start before load.

Related errors

• ArgumentNullException: child
• ArgumentNullException: hitTestParameters
• ArgumentNullException(nameof(scriptName))
• ArgumentNullException(nameof(text))
• block

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/83a6e1db17980743. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/ValidationHelper.cs:39

        #region Internal Methods

        // Verifies a TextPointer is non-null and
        // is associated with a given TextContainer.
        //
        // Throws an appropriate exception if a test fails.
        internal static void VerifyPosition(ITextContainer tree, ITextPointer position)
        {
            VerifyPosition(tree, position, nameof(position));
        }
        
        // Verifies a TextPointer is non-null and is associated with a given TextContainer.
        //
        // Throws an appropriate exception if a test fails.
        internal static void VerifyPosition(ITextContainer container, ITextPointer position, string paramName)
        {
            if (position == null)
            {
                throw new ArgumentNullException(paramName);
            }

            if (position.TextContainer != container)
            {
                throw new ArgumentException(SR.Format(SR.NotInAssociatedTree, paramName));
            }
        }

        // Verifies two positions are safe to use as a logical text span.
        //
        // Throws ArgumentNullException if startPosition == null || endPosition == null
        //        ArgumentException if startPosition.TextContainer != endPosition.TextContainer or
        //                             startPosition > endPosition
        internal static void VerifyPositionPair(ITextPointer startPosition, ITextPointer endPosition)
        {
            ArgumentNullException.ThrowIfNull(startPosition);
            ArgumentNullException.ThrowIfNull(endPosition);
            if (startPosition.TextContainer != endPosition.TextContainer)

View on GitHub (pinned to 81131a70a4)