dotnet/wpf · error · InvalidOperationException

SR.Rect_CannotCallMethod

Error message

SR.Rect_CannotCallMethod

What it means

Rect.Offset(Vector) throws InvalidOperationException when the rect is Empty. There is no defined position to offset for the NaN-based empty sentinel, and Offset would overwrite X/Y with NaN-derived values, so WPF declares the operation illegal on Empty (the XML docs state 'If this is Empty, this method is illegal').

Solutions

1. Guard with if (!rect.IsEmpty) before calling Offset.
2. If the rect should exist, initialize it with a real Rect(x, y, w, h) instead of Rect.Empty/default.
3. Assign a new rect at the target position: rect = new Rect(newPoint, rect.Size) once non-empty.

Example fix

// before
viewport.Offset(panVector); // InvalidOperationException when viewport is Empty
// after
if (!viewport.IsEmpty) { viewport.Offset(panVector); } else { viewport = new Rect(panVector, initialSize); }

if (!rect.IsEmpty) { rect.Offset(offsetVector); }

static bool CanOffset(Rect r) => !r.IsEmpty;

try { rect.Offset(v); }
catch (InvalidOperationException) { rect = new Rect((Point)((Vector)v), new Size(0, 0)); }

When it happens

Trigger: Calling rect.Offset(vector) or rect.Offset(dx, dy) when rect == Rect.Empty — e.g. panning/moving a rect that came from an empty hit-test or intersection result.

Common situations: Viewport panning code moving a selection rect that was never set; animation updating offset each frame while the rect starts as Empty; chaining Offset after Intersect without checking overlap.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

• SR.Rect_CannotModifyEmptyRect
• Current DocumentSequence, FixedDocument, or FixedPage not…
• IAmbientProvider
• InvalidOperationException()
• IXamlSchemaContextProvider

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/e06b2a6850d3774c. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Rect.cs:582

        /// <summary>
        /// Union - Return the result of the union of rect and point.
        /// </summary>
        public static Rect Union(Rect rect, Point point)
        {
            rect.Union(new Rect(point, point));
            return rect;
        }

        /// <summary>
        /// Offset - translate the Location by the offset provided.
        /// If this is Empty, this method is illegal.
        /// </summary>
        public void Offset(Vector offsetVector)
        {
            if (IsEmpty)
            {
                throw new System.InvalidOperationException(SR.Rect_CannotCallMethod);
            }

            _x += offsetVector._x;
            _y += offsetVector._y;
        }

        /// <summary>
        /// Offset - translate the Location by the offset provided
        /// If this is Empty, this method is illegal.
        /// </summary>
        public void Offset(double offsetX, double offsetY)
        {
            if (IsEmpty)
            {
                throw new System.InvalidOperationException(SR.Rect_CannotCallMethod);
            }

            _x += offsetX;

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