<PackageReference Include="System.Configuration.ConfigurationManager" Version="10.0.0-rc.1.25451.107" />

System.Configuration.SettingElementCollection

Contains a collection of SettingElement objects. This class cannot be inherited.
namespace System.Configuration { public sealed class SettingElementCollection : ConfigurationElementCollection { public SettingElement Get(string elementKey); public void Add(SettingElement element); public void Remove(SettingElement element); public void Clear(); public SettingElementCollection(); } }