What causes "NotImplementedException" in dotnet/wpf?

Do not use this PropertyInfo for writes; set indexer values directly on the object: obj[key] = value.

dotnet/wpf · error · NotImplementedException

NotImplementedException

Error message

NotImplementedException

What it means

IndexerPropertyInfo is an internal PropertyInfo wrapper used by WPF data binding to represent an indexer; SetValue is deliberately unsupported and always throws NotImplementedException. Values are written through binding-internal paths (DynamicIndexerAccessor), not through reflection SetValue.

Solutions

Do not use this PropertyInfo for writes; set indexer values directly on the object: obj[key] = value.
Filter out indexer properties before bulk-writing: skip PropertyInfo objects that are IndexerPropertyInfo or have IndexParameters.Length > 0.
If generic reflection is needed, detect the type via the internal MS.Internal.Data.IndexerPropertyInfo type name and handle indexers specially.

Example fix

// before
prop.SetValue(item, newValue, null); // NotImplementedException for indexer

// after
if (prop.GetIndexParameters().Length == 0) prop.SetValue(item, newValue, null);

Defensive patterns

Strategy: validation

Validate before calling

bool isWritable = prop != null && prop.GetIndexParameters().Length == 0;

Type guard

static bool IsRealProperty(System.Reflection.PropertyInfo p) => p.GetIndexParameters().Length == 0;

Try / catch

try { prop.SetValue(item, value, null); }
catch (NotImplementedException) { /* indexer write unsupported; set via obj[key] = value */ }

Prevention

Check GetIndexParameters().Length before using a PropertyInfo for read/write.
Skip WPF's synthetic indexer (IndexerPropertyInfo) in generic reflection loops.
Write indexer values directly through the object's this[] operator instead of reflection.

When it happens

Trigger: Any code calling SetValue on the IndexerPropertyInfo instance obtained via TypeDescriptor/reflection from ItemProperties — e.g. generic reflection-based serialization, property-grid code, or test code trying to assign to an indexer via this PropertyInfo.

Common situations: Using a reflection utility or ORM-ish code that enumerates ItemProperties of a bound item and attempts to write every property including the indexer.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/ffc76867120b4de4. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Data/IndexerPropertyInfo.cs:77

        public override MethodInfo GetSetMethod(bool nonPublic)
        {
            return null;
        }

        public override object GetValue(object obj, BindingFlags invokeAttr, Binder binder, object[] index, System.Globalization.CultureInfo culture)
        {
            // This is the only interesting line.  GetValue returns the base object itself.
            return obj;
        }

        public override Type PropertyType
        {
            get { return typeof(Object); }
        }

        public override void SetValue(object obj, object value, BindingFlags invokeAttr, Binder binder, object[] index, System.Globalization.CultureInfo culture)
        {
            throw new NotImplementedException();
        }

        public override Type DeclaringType
        {
            get { throw new NotImplementedException(); }
        }

        public override object[] GetCustomAttributes(Type attributeType, bool inherit)
        {
            throw new NotImplementedException();
        }

        public override object[] GetCustomAttributes(bool inherit)
        {
            throw new NotImplementedException();
        }

        public override bool IsDefined(Type attributeType, bool inherit)

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