/// <summary>
/// Returns the maximal element of a sequence based on a specified projection and comparer for projected
/// values.
/// </summary>
/// <remarks>
/// If more than one element has the maximal projected value, the first one encountered will be returned. This
/// overload uses the default comparer for the projected type. This operator uses immediate execution, but only
/// buffers a single result (the current maximal element).
/// </remarks>
/// <typeparam name="TSource">The type of source sequence.</typeparam>
/// <typeparam name="TKey">The type of projected element.</typeparam>
/// <param name="source">The source sequence.</param>
/// <param name="selector">A transform function to apply to each element.</param>
/// <param name="comparer">A comparer used to compare projected values.</param>
/// <returns>The maximal element, according to the projection.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="selector"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="comparer"/> is <see langword="null"/>.</exception>
/// <exception cref="InvalidOperationException"><paramref name="source"/> is empty.</exception>
public static TSource MaxBy <TSource, TKey>(
this IEnumerable <TSource> source, Func <TSource, TKey> selector, IComparer <TKey> comparer)
{
source.AssertNotNull("source");
selector.AssertNotNull("selector");
comparer.AssertNotNull("comparer");
using (IEnumerator <TSource> sourceIterator = source.GetEnumerator())
{
if (!sourceIterator.MoveNext())
{
throw new InvalidOperationException("The sequence is empty.");
}
var max = sourceIterator.Current;
var maxKey = selector(max);
while (sourceIterator.MoveNext())
{
var candidate = sourceIterator.Current;
var candidateProjected = selector(candidate);
if (comparer.Compare(candidateProjected, maxKey) > 0)
{
max = candidate;
maxKey = candidateProjected;
}
}
return(max);
}
}