I have two points (x1,y1) and (x2,y2). I want to know whether the points are within 5 meters of one another.
If you are using System.Windows.Point data type to represent a point, you can use
// assuming p1 and p2 data types
Point p1, p2;
// distanc can be calculated as follows
double distance = Point.Subtract(p2, p1).Length;
Update 2017-01-08:
Point.Subtract is System.Windows.Vector and it has also property LengthSquared to save one sqrt calculation if you just need to compare distance.WindowsBase assembly may be needed in your projectExample with LengthSquared and operators
// assuming p1 and p2 data types
Point p1, p2;
// distanc can be calculated as follows
double distanceSquared = (p2 - p1).LengthSquared;