RuntimeHelpers.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

namespace System.Runtime.CompilerServices
{
    internal static class RuntimeHelpers
    {
        /// <summary>
        /// Slices the specified array using the specified range.
        /// </summary>
        public static T[] GetSubArray<T>(T[] array, Range range)
        {
            if (array == null)
            {
                throw new ArgumentNullException();
            }

            (int offset, int length) = range.GetOffsetAndLength(array.Length);

            if (default(T)! != null || typeof(T[]) == array.GetType()) // TODO-NULLABLE: default(T) == null warning (https://github.com/dotnet/roslyn/issues/34757)
            {
                // We know the type of the array to be exactly T[].

                if (length == 0)
                {
                    return Array.Empty<T>();
                }

                var dest = new T[length];
                Array.Copy(array, offset, dest, 0, length);
                return dest;
            }
            else
            {
                // The array is actually a U[] where U:T.
                T[] dest = (T[])Array.CreateInstance(array.GetType().GetElementType()!, length);
                Array.Copy(array, offset, dest, 0, length);
                return dest;
            }
        }
    }
}