How to use LINQ to select object with minimum or maximum property value

People.Aggregate((curMin, x) => (curMin == null || (x.DateOfBirth ?? DateTime.MaxValue) <
    curMin.DateOfBirth ? x : curMin))

Unfortunately there isn't a built-in method to do this, but it's easy enough to implement for yourself. Here are the guts of it:

public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
    Func<TSource, TKey> selector)
{
    return source.MinBy(selector, null);
}

public static TSource MinBy<TSource, TKey>(this IEnumerable<TSource> source,
    Func<TSource, TKey> selector, IComparer<TKey> comparer)
{
    if (source == null) throw new ArgumentNullException("source");
    if (selector == null) throw new ArgumentNullException("selector");
    comparer ??= Comparer<TKey>.Default;

    using (var sourceIterator = source.GetEnumerator())
    {
        if (!sourceIterator.MoveNext())
        {
            throw new InvalidOperationException("Sequence contains no elements");
        }
        var min = sourceIterator.Current;
        var minKey = selector(min);
        while (sourceIterator.MoveNext())
        {
            var candidate = sourceIterator.Current;
            var candidateProjected = selector(candidate);
            if (comparer.Compare(candidateProjected, minKey) < 0)
            {
                min = candidate;
                minKey = candidateProjected;
            }
        }
        return min;
    }
}

Example usage:

var firstBorn = People.MinBy(p => p.DateOfBirth ?? DateTime.MaxValue);

Note that this will throw an exception if the sequence is empty, and will return the first element with the minimal value if there's more than one.

Alternatively, you can use the implementation we've got in MoreLINQ, in MinBy.cs. (There's a corresponding MaxBy, of course.)

Install via package manager console:

PM> Install-Package morelinq