Алгоритм поиска 2-х ближайших точек из массива элементов Point [] points к заданной точке Point p.

internal class Program
{
    private static void Main()
    {
        List<Point> points = new List<Point>();
        points.Add(new Point(2, 2));
        points.Add(new Point(3, 2));
        points.Add(new Point(1, 3));
 
        Point nearest = GetNearestPoint(new Point(0, 0), points);
        Console.WriteLine("Ближайшая точка: X = {0}, Y = {1}", nearest.X, nearest.Y);
    }
 
    private static Point GetNearestPoint(Point point, List<Point> points)
    {
        if (points.Count == 0)
            throw new ArgumentException("Нет точек.", "points");
 
        Point nearest = points[0];
        double minDistance = nearest.DistanceTo(points[0]);
        for (int i = 1; i < points.Count; i++)
        {
            double temp = nearest.DistanceTo(points[i]);
            if (temp < minDistance)
            {
                nearest = points[i];
                minDistance = temp;
            }
        }
 
        return nearest;
    }
}
 
public class Point
{
    public Point(int x, int y)
    {
        X = x;
        Y = y;
    }
 
    public int X { get; set; }
    public int Y { get; set; }
 
    public double DistanceTo(Point other)
    {
        return Math.Sqrt(Math.Pow(X - other.X, 2) + Math.Pow(Y - other.Y, 2));
    }
}

double minDistance = nearest.DistanceTo(points[0]);

double minDistance = point.DistanceTo(points[0])

private static Point GetNearestPoint(Point point, List<Point> points)
{
    if (points.Count == 0)
        throw new ArgumentException("Нет точек.", "points");
 
    Point nearest = points[0];
    double minDistance = point.DistanceTo(points[0]);
    for (int i = 1; i < points.Count; i++)
    {
        double temp = point.DistanceTo(points[i]);
        if (temp < minDistance)
        {
            nearest = points[i];
            minDistance = temp;
        }
    }
 
    return nearest;
}