What causes "SR.PathGeometry_InternalReadBackError" in dotnet/wpf?

Re-create the PathGeometry from trusted path data (PathGeometry.CreateFromGeometry or Geometry.Parse) instead of reusing the suspect instance

dotnet/wpf · error · InvalidOperationException

SR.PathGeometry_InternalReadBackError

Error message

SR.PathGeometry_InternalReadBackError

What it means

Thrown in PathGeometry.AddFigureToList while reading back geometry data from the unmanaged MIL layer: a line-segment run claims more points (pointIndex + sameSegCount) than the returned point buffer holds (pointCount). This is a buffer-consistency check on data produced by the core geometry read-back, indicating the segment/point counts returned by milcore do not line up. It signals corrupted or internally inconsistent geometry data rather than bad user arguments.

Solutions

Re-create the PathGeometry from trusted path data (PathGeometry.CreateFromGeometry or Geometry.Parse) instead of reusing the suspect instance
Update to a patched .NET/WPF version and current graphics drivers; search for known milcore geometry read-back fixes
Freeze the geometry before use and avoid mutating geometry that may be in use by the render thread
Catch InvalidOperationException around geometry read-back operations and fall back to a rebuilt geometry

Example fix

// before
var s = suspectGeometry.ToString(); // read-back may throw
// after
Geometry fresh = Geometry.Parse(suspectGeometry.ToString(System.Globalization.CultureInfo.InvariantCulture));
var s = fresh.ToString();

Defensive patterns

Strategy: try-catch

Validate before calling

if (geometry == null || geometry.IsEmpty()) return Geometry.Empty;

Try / catch

try { return pathGeometry.ToString(); }
catch (InvalidOperationException ex) { log(ex); return PathGeometry.CreateFromGeometry(pathGeometry).ToString(); }

Prevention

Rebuild geometries from path data instead of reusing instances that came from render read-backs
Freeze geometries after creation
Avoid cross-thread mutation of geometry in use by the renderer
Keep WPF runtime patched

When it happens

Trigger: Calling PathGeometry APIs that internally call AddFigureToList (e.g. reading back path data via PathGeometry.ToString/Parse or internal flattening) when the MIL read-back reports SegTypeLine runs whose point count exceeds the total pointCount.

Common situations: Encountered inside WPF during serialization, hit-testing or animation of PathGeometry when the render-tier data is inconsistent; often tied to runtime/driver bugs or corrupted geometry state rather than developer error.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/a8a1cd61be49c931. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/PathGeometry.cs:460

                        byte segType = (byte)(pSegTypes[segIndex] & (byte)MILCoreSegFlags.SegTypeMask);

                        sameSegCount = 1;

                        // Look for a run of same-type segments for a PolyXXXSegment.
                        while (((segIndex + sameSegCount) < segmentCount) &&
                            (pSegTypes[segIndex] == pSegTypes[segIndex+sameSegCount]))
                        {
                            sameSegCount++;
                        }

                        bool fStroked = (pSegTypes[segIndex] & (byte)MILCoreSegFlags.SegIsAGap) == (byte)0;
                        bool fSmooth = (pSegTypes[segIndex] & (byte)MILCoreSegFlags.SegSmoothJoin) != (byte)0;

                        if (segType == (byte)MILCoreSegFlags.SegTypeLine)
                        {
                            if (pointIndex+sameSegCount > pointCount)
                            {
                                throw new System.InvalidOperationException(SR.PathGeometry_InternalReadBackError);
                            }

                            if (sameSegCount>1)
                            {
                                PointCollection ptCollection = new PointCollection();
                                for (int i=0; i<sameSegCount; i++)
                                {
                                    ptCollection.Add(new Point(pPoints[pointIndex+i].X, pPoints[pointIndex+i].Y));
                                }
                                ptCollection.Freeze();

                                PolyLineSegment polySeg = new PolyLineSegment(ptCollection, fStroked, fSmooth);
                                polySeg.Freeze();

                                figure.Segments.Add(polySeg);
                            }
                            else
                            {

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