<PackageReference Include="System.ServiceModel.Primitives" Version="6.1.0" />

System.ServiceModel.ExtensionCollection<T>

namespace System.ServiceModel { public sealed class ExtensionCollection<T> : SynchronizedCollection<IExtension<T>>, IExtensionCollection<T>, ICollection<IExtension<T>>, IEnumerable<IExtension<T>>, IEnumerable where T : IExtensibleObject<T> { public ExtensionCollection(T owner); public ExtensionCollection(T owner, object syncRoot); public E Find<E>(); public Collection<E> FindAll<E>(); } }