<PackageReference Include="System.Configuration.ConfigurationManager" Version="6.0.1" />

KeyValueInternalCollection

using System.Collections.Specialized; namespace System.Configuration { internal sealed class KeyValueInternalCollection : NameValueCollection { private readonly AppSettingsSection _root; public KeyValueInternalCollection(AppSettingsSection root) { _root = root; foreach (KeyValueConfigurationElement setting in _root.Settings) { base.Add(setting.Key, setting.Value); } } public override void Add(string key, string value) { _root.Settings.Add(new KeyValueConfigurationElement(key, value)); base.Add(key, value); } public override void Clear() { _root.Settings.Clear(); base.Clear(); } public override void Remove(string key) { _root.Settings.Remove(key); base.Remove(key); } } }