dotnet/wpf · error · ArgumentException

SR.Format(SR.Collection_BadType, this.GetType().Name…

Error message

SR.Format(SR.Collection_BadType, this.GetType().Name, value.GetType().Name, "Material")

What it means

The private Cast(object value) helper validates items added through the non-generic IList.Add/Insert paths. If value is not an instance of Material, it throws ArgumentException(SR.Collection_BadType) naming the collection type, the offending value's type, and the expected type "Material". This keeps the strongly-typed collection safe when accessed via weakly-typed interfaces.

Solutions

Add only Material instances (DiffuseMaterial, SpecularMaterial, EmissiveMaterial, MaterialGroup).
Wrap the value in an appropriate Material, e.g. new DiffuseMaterial(brush) if you have a Brush.
Type-check before adding: if (value is Material m) ((IList)materials).Add(m);

Example fix

// before
((IList)group.Materials).Add(myBrush);
// after
((IList)group.Materials).Add(new DiffuseMaterial(myBrush));

Defensive patterns

Strategy: type-guard

Validate before calling

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

Type guard

static bool IsMaterial(object value) => value is Material;

Try / catch

try { ((IList)materials).Add(value); }
catch (ArgumentException ex) { /* value was not a Material */ }

Prevention

Prefer the generic Add(Material) over the non-generic IList.Add
Pattern-match with 'is Material m' before weakly-typed inserts
Convert Brushes to DiffuseMaterial explicitly instead of adding them raw

When it happens

Trigger: Calling ((IList)materials).Add(someObject) or Insert(i, someObject) where someObject is not a Material (e.g. a string, a Brush, or a MaterialGroup typed as object).

Common situations: Reflection or XAML/shader code that builds collections from untyped data; porting code from ArrayList; UI toolkits passing heterogeneous object lists; passing a Brush where a Material was intended.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/872f1e03a57c8194. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media3D/Generated/MaterialCollection.cs:516

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

        #endregion

        #region Private Helpers

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

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

            return (Material) 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(Material value)
        {
            int index = AddWithoutFiringPublicEvents(value);

            // AddAtWithoutFiringPublicEvents incremented the version

            WritePostscript();

            return index;
        }

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