What causes "The method or operation is not implemented." in dotnet/reactive?

Use the generic IQueryProvider.CreateQuery (Expression) path instead (via ToQueryable )

dotnet/reactive · error · NotImplementedException

The method or operation is not implemented.

Error message

The method or operation is not implemented.

What it means

The non-generic IQueryProvider.CreateQuery(Expression) overload of the ObservableQuery provider is deliberately unimplemented — the bridge only supports the generic CreateQuery<T> path used by ToQueryable. Calling the non-generic overload always throws NotImplementedException.

Solutions

Use the generic IQueryProvider.CreateQuery<T>(Expression) path instead (via ToQueryable<T>)
Cast the queryable to its typed IQueryable<T> interface before creating queries
Wrap the call and surface a clearer error explaining only the generic provider API is supported

Example fix

// before
IQueryable q = ((IQueryProvider)provider).CreateQuery(expression);
// after
IQueryable<T> q = ((IQueryProvider)provider).CreateQuery<T>(expression);

Defensive patterns

Strategy: try-catch

Validate before calling

var typed = queryable as IQueryable<T>; if (typed == null) throw new NotSupportedException("Use the generic CreateQuery<T> path");

Type guard

static bool SupportsGenericCreateQuery(IQueryProvider p) => p is IQueryProvider && typeof(IQueryable<>).IsAssignableFrom(p.GetType());

Try / catch

try { return provider.CreateQuery(expression); } catch (NotImplementedException) { throw new NotSupportedException("Non-generic CreateQuery is not supported by the ObservableQuery bridge; use CreateQuery<T>."); }

Prevention

Always work with the typed IQueryable<T> interface
Avoid reflection/late-binding paths that resolve the non-generic provider API
Prefer consuming IQbservable directly instead of the bridge

When it happens

Trigger: Calling IQueryProvider.CreateQuery (non-generic) on the ObservableQuery provider, e.g. via late-bound reflection code or non-generic IQueryable usage patterns.

Common situations: Framework code that resolves the provider through the non-generic IQueryable interface, data-binding or LINQ infrastructure that only has an Expression and no compile-time T.

Related errors

AI-assisted analysis of dotnet/reactive@94b5d5ab91 (2026-09-15). Data as JSON: /api/errors/be1dc0f7174c0d34. Report an issue: GitHub.

Appendix: source

Thrown at Rx.NET/Source/src/System.Reactive/ObservableQuery.cs:84

                        arg0
                    )
                );

            //
            // Queryable operator calls should be taken care of by the provider for
            // LINQ to Objects. So we compile and get the resulting IQueryable<T>
            // back to hand it out.
            //
            return Expression.Lambda<Func<IQueryable<TElement>>>(res).Compile()();
        }

        private static MethodInfo? _staticAsQueryable;

        private static MethodInfo AsQueryable => _staticAsQueryable ??=  Qbservable.InfoOf<object>(() => Queryable.AsQueryable<object>(null!)).GetGenericMethodDefinition();

        IQueryable IQueryProvider.CreateQuery(Expression expression)
        {
            throw new NotImplementedException();
        }

        TResult IQueryProvider.Execute<TResult>(Expression expression)
        {
            throw new NotImplementedException();
        }

        object IQueryProvider.Execute(Expression expression)
        {
            throw new NotImplementedException();
        }
    }

#if HAS_TRIMMABILITY_ATTRIBUTES
    [RequiresUnreferencedCode(Constants_Core.AsQueryableTrimIncompatibilityMessage)]
#endif
    internal class ObservableQuery
    {

View on GitHub (pinned to 94b5d5ab91)