dotnet/wpf · error · ArgumentException

SR.Format(SR.MaxLengthExceedsBufferSize, count…

Error message

SR.Format(SR.MaxLengthExceedsBufferSize, count, textBuffer.Length, startIndex)

What it means

GetTextInRun throws ArgumentException when count exceeds the remaining space in textBuffer (count > textBuffer.Length - startIndex). The destination region starting at startIndex must be large enough for the requested character count; the resource reports count, buffer length and start index.

Solutions

Pass count = textBuffer.Length - startIndex (the remaining space) instead of buffer.Length.
Compute the request size against the current startIndex each iteration.
Grow the buffer or cap the request to the available space.

Example fix

// before
pointer.GetTextInRun(dir, buffer, start, buffer.Length);
// after
pointer.GetTextInRun(dir, buffer, start, buffer.Length - start);

Defensive patterns

Strategy: validation

Validate before calling

count = Math.Min(count, buffer.Length - startIndex);

Try / catch

try { pointer.GetTextInRun(dir, buffer, start, count); } catch (ArgumentException) { count = buffer.Length - start; }

Prevention

Always request at most buffer.Length - startIndex characters.
In loops, compute remaining space per iteration.
Write a wrapper that does the clamping once and use it everywhere.

When it happens

Trigger: Requesting more characters than textBuffer can hold from startIndex onward — e.g. passing the whole-buffer size as count while startIndex > 0, or underestimating how much of the buffer is already used.

Common situations: Loops that pass buffer.Length as count each iteration while advancing startIndex; pre-allocated buffers reused across calls after their size assumptions changed.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/bd92f6044913123c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/DocumentSequenceTextPointer.cs:670

        {
            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);
        }

        /// <summary>
        /// <see cref="ITextPointer.GetElementType"/>

View on GitHub (pinned to 81131a70a4)