Part of DKNet.Fw.Extensions. Use CollectionExtensions.AddRange when the destination is exposed as ICollection<T>.
using System;
using System.Collections.Generic;
using DKNet.Fw.Extensions.Collections;
ICollection<int> values = new List<int> { 1, 2 };
values.AddRange(new[] { 3, 4 });
Console.WriteLine(string.Join(",", values)); // 1,2,3,4
The helper is a foreach followed by one Add per item. Destination collection semantics determine duplicate handling and enumeration order.