EnumerableExtensions.cs

// And same for IQueryable
public static class EnumerableExtensions
{
    public static T? SingleOrNull<T>(this IEnumerable<T> seq, Func<T, bool> predicate) where T : struct
    {
        var data = seq.Where(predicate).Take(2).ToArray();
        if (data.Length == 2)
            throw new InvalidOperationException();
        if (data.Length == 0)
            return null;
        return data[0];
    }
}