dotnet/wpf · error · ArgumentException

SR.Format(SR.NegativeValue, "count")

Error message

SR.Format(SR.NegativeValue, "count")

What it means

GetTextInRun rejects a negative count with ArgumentException (NegativeValue resource). The count is the maximum number of characters to copy; a negative value has no meaning, so the method fails fast rather than treating it as zero.

Solutions

  1. Pass 0 (or a non-negative value) instead of negative counts; there is no -1 sentinel here.
  2. Fix the subtraction producing the negative remaining length.
  3. Clamp: count = Math.Max(0, count) before calling.

Example fix

// before
pointer.GetTextInRun(dir, buffer, start, end - start);
// after
var count = Math.Max(0, end - start);
pointer.GetTextInRun(dir, buffer, start, count);
Defensive patterns

Strategy: validation

Validate before calling

if (count < 0) count = 0; // or throw ArgumentOutOfRangeException(nameof(count))

Try / catch

try { pointer.GetTextInRun(dir, buffer, start, count); } catch (ArgumentException) { count = Math.Max(0, count); }

Prevention

When it happens

Trigger: Calling GetTextInRun(direction, textBuffer, startIndex, count) with count < 0, e.g. remaining-length arithmetic that went negative (buffer.Length - startIndex underflow handled separately, but expressions like end - start can yield negatives).

Common situations: Computing count as (someEnd - startIndex) where someEnd < startIndex; passing -1 as an 'unlimited' sentinel, which this API does not support.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/87e244f142f122ff. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/DocumentSequenceTextPointer.cs:666

        /// <see cref="ITextPointer.GetTextInRun(LogicalDirection,char[],int,int)"/>
        /// </summary>
        /// <remarks>Only reutrn uninterrupted runs of text</remarks>
        public static int GetTextInRun(DocumentSequenceTextPointer thisTp, LogicalDirection direction, char[] textBuffer, int startIndex, int count)
        {
            ValidationHelper.VerifyDirection(direction, "direction");

            ArgumentNullException.ThrowIfNull(textBuffer);
            if (startIndex < 0)
            {
                throw new ArgumentException(SR.Format(SR.NegativeValue, "startIndex"));
            }
            if (startIndex > textBuffer.Length)
            {
                throw new ArgumentException(SR.Format(SR.StartIndexExceedsBufferSize, startIndex, textBuffer.Length));
            }
            if (count < 0)
            {
                throw new ArgumentException(SR.Format(SR.NegativeValue, "count"));
            }
            if (count > textBuffer.Length - startIndex)
            {
                throw new ArgumentException(SR.Format(SR.MaxLengthExceedsBufferSize, count, textBuffer.Length, startIndex));
            }

            return thisTp.ChildPointer.GetTextInRun(direction, textBuffer, startIndex, count);
        }

        /// <summary>
        /// <see cref="ITextPointer.GetAdjacentElement"/>
        /// </summary>
        /// <remarks>Return null if the embedded object does not exist</remarks>
        public static object GetAdjacentElement(DocumentSequenceTextPointer thisTp, LogicalDirection direction)
        {
            ValidationHelper.VerifyDirection(direction, "direction");

            return xGapAwareGetEmbeddedElement(thisTp, direction);

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