ToArrayByIndex for LINQ

Enumerable.cs

#region Copyright (c) 2016 Atif Aziz. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// 
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
#endregion

static partial class Enumerable
{
    public static T[] ToArrayByIndex<T>(this IEnumerable<T> source, Func<T, int> indexer) =>
        ToArrayByIndex(source, 0, indexer);

    public static T[] ToArrayByIndex<T>(this IEnumerable<T> source, int size, Func<T, int> indexer)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (size < 0) throw new ArgumentOutOfRangeException(nameof(size));
        if (indexer == null) throw new ArgumentNullException(nameof(indexer));

        var array = new T[size];
        foreach (var e in source)
        {
            var i = indexer(e);
            if (size == 0 && i >= array.Length)
                Array.Resize(ref array, i + 1);
            array[i] = e;
        }
        return array;
    }
}