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

Assign a whole new Rect instead of mutating: rect = new Rect(point, size).

dotnet/wpf · error · InvalidOperationException

SR.Rect_CannotModifyEmptyRect

Error message

SR.Rect_CannotModifyEmptyRect

What it means

Rect.Location's setter throws InvalidOperationException when the rect is the special Empty rect. The empty rect is a singleton (all fields NaN) with no defined position, so modifying its location is meaningless and would corrupt the sentinel representation. WPF exposes this as an invalid-state error rather than silently assigning.

Solutions

Assign a whole new Rect instead of mutating: rect = new Rect(point, size).
Check rect.IsEmpty before setting Location and handle the empty case explicitly.
Initialize fields with a real rect (e.g. Rect(0, 0, w, h)) instead of Rect.Empty/default.

Example fix

// before
if (hitRect.IsEmpty) { hitRect.Location = origin; } // throws
// after
if (!hitRect.IsEmpty) { hitRect.Location = origin; } else { hitRect = new Rect(origin, hitSize); }

Defensive patterns

Strategy: validation

Validate before calling

if (!rect.IsEmpty) { rect.Location = new Point(x, y); }

Type guard

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

Try / catch

try { rect.Location = p; }
catch (InvalidOperationException) { rect = new Rect(p, rect.Size == default ? new Size(0,0) : rect.Size); }

Prevention

Check IsEmpty before any per-field mutation of a Rect.
Avoid default(Rect) and Rect.Empty as mutable placeholders; initialize with real geometry.
Prefer constructing a new Rect over mutating Location/X/Y on shared values.

When it happens

Trigger: Assigning rect.Location = new Point(...) while rect == Rect.Empty, typically after a default Rect field or an Intersection/Union that produced Empty.

Common situations: Storing a Rect in a class field initialized to default(Rect) or Rect.Empty and later setting Location directly; hit-test results returning Empty when nothing was hit; layout code that forgot to initialize a rect.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/f1867c926989462a. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WindowsBase/System/Windows/Rect.cs:148

                return _width < 0;
            }
        }

        /// <summary>
        /// Location - The Point representing the origin of the Rectangle
        /// </summary>
        public Point Location
        {
            get
            {
                return new Point(_x, _y);
            }
            set
            {
                if (IsEmpty)
                {
                    throw new System.InvalidOperationException(SR.Rect_CannotModifyEmptyRect);
                }
                
                _x = value._x;
                _y = value._y;
            }
        }

        /// <summary>
        /// Size - The Size representing the area of the Rectangle
        /// </summary>
        public Size Size
        {
            get
            {
                if (IsEmpty)
                    return Size.Empty;
                return new Size(_width, _height);
            }

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