dotnet/wpf · error · ArgumentException

SR.MaxLengthExceedsBufferSize (count, textBuffer.Length…

Error message

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

What it means

TextPointer.GetTextInRun also checks that count does not exceed the space available in the destination buffer: count must be <= textBuffer.Length - startIndex. When it is larger, the copy would overflow the buffer, so it throws ArgumentException(SR.MaxLengthExceedsBufferSize) reporting count, buffer length, and start index.

Solutions

1. Clamp the count: count = Math.Min(count, textBuffer.Length - startIndex) before the call.
2. Ensure startIndex + count <= textBuffer.Length whenever sizing buffers.
3. If the run may be longer than the buffer, loop: call GetTextInRun repeatedly with the remaining buffer space and advance the pointer.

Example fix

// before
pointer.GetTextInRun(dir, buffer, start, runLength);
// after
int count = Math.Min(runLength, buffer.Length - start);
pointer.GetTextInRun(dir, buffer, start, count);

if (count > buffer.Length - startIndex)
    count = buffer.Length - startIndex;

static bool FitsBuffer(char[] buf, int start, int count) => start >= 0 && count >= 0 && start + count <= buf.Length;

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

When it happens

Trigger: Calling TextPointer.GetTextInRun(direction, textBuffer, startIndex, count) where count > textBuffer.Length - startIndex, e.g. buffer length 10, startIndex 5, count 10 (only 5 slots remain).

Common situations: Allocating a buffer for the full run length but passing a startIndex that shrinks the usable space; reusing a stale buffer size after the document text grew; off-by-one in start/end index arithmetic.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• ' ' is not a valid value for ' '.
• A read or write operation references a location outside the…
• ArgumentException(message, name)
• ArgumentNullException(name)
• ArgumentOutOfRangeException(authentication)

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/42c0a68874128235. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/System/Windows/Documents/TextPointer.cs:1764

            int skipCount;
            int finalCount;

            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));
            }
            Invariant.Assert(textNode != null, "textNode is expected to be non-null");

            textContainer.EmptyDeadPositionList();

            if (nodeOffset < 0)
            {
                skipCount = 0;
            }
            else
            {
                skipCount = (direction == LogicalDirection.Forward) ? nodeOffset : textNode.SymbolCount - nodeOffset;
                symbolOffset += nodeOffset;
            }
            finalCount = 0;

            // Loop and combine adjacent text nodes into a single run.
            // This isn't just a perf optimization.  Because text positions

View on GitHub (pinned to 81131a70a4)