dotnet/wpf · error · System.ArgumentException

SR.Collection_BadType

Error message

SR.Collection_BadType

What it means

This ArgumentException is thrown by the strongly-typed Cast(object) method that CollectionHelper generates for each typed collection in the WPF mcg codegen tool. IList.Add/IList.Insert receive System.Object, and the generator wraps them with a runtime type check so that only instances of the collection's element type are accepted. If the value is not assignable to [[type]], the collection rejects it rather than storing a wrongly-typed item.

Solutions

  1. Check the value's type before adding: only insert instances of the collection's declared element type.
  2. Convert or map the value to the expected element type prior to the Add/Insert call.
  3. If the collection type is wrong for your data, use a collection whose element type matches.

Example fix

// before
((IList)collection).Add(someObject); // throws if someObject is not [[type]]
// after
if (someObject is [[type]] typed)
{
    ((IList)collection).Add(typed);
}
Defensive patterns

Strategy: type-guard

Validate before calling

if (value == null) throw new ArgumentNullException(nameof(value));
if (!(value is [[type]]))
    throw new ArgumentException($"Expected [[type]], got {value.GetType().Name}");

Type guard

static bool Is acceptableItemType(object value) => value is [[type]];
// pattern-match before adding:
if (value is [[type]] typed) ((IList)collection).Add(typed);

Try / catch

try { ((IList)collection).Add(value); }
catch (ArgumentException ex) when (ex.Message.Contains("wrong type") || ex.Message.Contains("BadType")) { /* convert or log the type mismatch */ }

Prevention

When it happens

Trigger: Calling Add/Insert/Contains/Indexer-set on a generated typed collection through its non-generic IList interface with an object that is not of (or derived from) the declared element type, e.g. `((IList)stringCollection).Add(42)`.

Common situations: Reflection-based or XAML/BAML deserialization code inserting values of the wrong type; passing a base-type collection an incompatible item; API version changes that changed a collection's element type while caller code still adds old values.

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

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/7a53426daecbccac. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/WpfGfx/codegen/mcg/helpers/CollectionHelper.cs:1168

                    /// </summary>
                    internal [[type]] Internal_GetItem(int i)
                    {
                        return _collection[i];
                    }

                    [[onInheritanceContextChangedCoreMethod]]

                    #endregion

                    #region Private Helpers

                    private [[type]] Cast(object value)
                    {
                        ArgumentNullException.ThrowIfNull(value);

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

                        return ([[type]]) 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([[type]] value)
                    {
                        int index = AddWithoutFiringPublicEvents(value);

                        // AddAtWithoutFiringPublicEvents incremented the version

                        WritePostscript();

                        return index;
                    }

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