dotnet/wpf · error · ArgumentException

SR.Collection_BadType

Error message

SR.Collection_BadType

What it means

PathSegmentCollection.Cast throws Collection_BadType when a value added to the collection is not a PathSegment. The generated collection validates every Add/Insert argument and rejects any object whose runtime type is not PathSegment, naming the expected type in the message.

Solutions

1. Only pass objects deriving from System.Windows.Media.PathSegment (LineSegment, BezierSegment, PolyLineSegment, etc.).
2. Verify you are using the correct collection type for your element (PathFigureCollection for PathFigures).
3. Check the value at runtime with `is PathSegment` before adding.
4. If working through IList, cast/validate the element explicitly before Add/Insert.

Example fix

// before
((ICollection)segments).Add(new PathFigure());
// after
segments.Add(new LineSegment(new Point(10,10), true));

if (value is not PathSegment) throw new ArgumentException($"Expected PathSegment, got {value?.GetType().Name}");

static bool IsPathSegment(object v) => v is PathSegment;

try { segments.Insert(0, (PathSegment)value); }
catch (ArgumentException ex) { log(ex); /* reject or convert value */ }

When it happens

Trigger: Calling Add, Insert (or the IList.Add path) on a PathSegmentCollection with an object that is not a PathSegment — e.g. adding a PathFigure, Point, or raw geometry object.

Common situations: Mistyping which WPF media collection to add to (PathFigureCollection vs PathSegmentCollection), boxing value types into an IList-typed reference, or data binding/serialization producing wrong element types.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• SR.CannotConvertType
• SR.Collection_BadDestArray
• SR.Collection_BadDestArray
• SR.Collection_BadDestArray
• SR.Collection_BadType (double)

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/57a6e87e46656113. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/Generated/PathSegmentCollection.cs:506

                DependencyObject inheritanceChild = _collection[i];
                if (inheritanceChild != null && inheritanceChild.InheritanceContext == this)
                {
                    inheritanceChild.OnInheritanceContextChanged(args);
                }
            }
        }

        #endregion

        #region Private Helpers

        private PathSegment Cast(object value)
        {
            ArgumentNullException.ThrowIfNull(value);

            if (!(value is PathSegment))
            {
                throw new System.ArgumentException(SR.Format(SR.Collection_BadType, this.GetType().Name, value.GetType().Name, "PathSegment"));
            }

            return (PathSegment) value;
        }

        // IList.Add returns int and IList<T>.Add does not. This
        // is called by both Adds and IList<T>'s just ignores the
        // integer
        private int AddHelper(PathSegment value)
        {
            int index = AddWithoutFiringPublicEvents(value);

            // AddAtWithoutFiringPublicEvents incremented the version

            WritePostscript();

            return index;
        }

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