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Profiling my C# application indicated that significant time is spent in List<T>.AddRange . Using Reflector to look at the code in this method indicated that it calls List<T>.InsertRange which is implemented as such: public void InsertRange(int index, IEnumerable<T> collection) { if (collection == null) { ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection); } if (index > this._size) { ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index, ExceptionResource.ArgumentOutOfRange_Index); } ICollection<T> is2 = collection as ICollection<T>; if (is2 != null) { int count = is2.Count; if (count > 0) { this.EnsureCapacity(this._size + count); if (index < this._size) { Array.Copy(this._items, index, this._items, index + count, this._size - index); } if (this == is2) { Array.Copy(this._items, 0, this._items, index, index); Array.Copy(this._items, (int) (index + count), this._items, (int) (index * 2), (int) (this._size - index)); } else { T[] array = new T[count]; // (*) is2.CopyTo(array, 0); // (*) array.CopyTo(this._items, index); // (*) } this._size += count; } } else { using (IEnumerator<T> enumerator = collection.GetEnumerator()) { while (enumerator.MoveNext()) { this.Insert(index++, enumerator.Current); } } } this._version++; } private T[] _items; One can argue that the simplicity of the interface (only having one overload of InsertRange) justifies the performance overhead of runtime type cheching and casting. But

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