List<Point> mydatalist = new List<Point>();
...
int xToFind = 4;
int i = myDataList.FindIndex(delegate(Point pt) { return pt.X == xToFind;});

class XSearcher
{
 private int x;

 public XSearcher(int x)
 {
  this.x = x;
 }

 public int X
 {
   get {return x;}
   set {x = value; }
 }

 public bool Condition(Point pt)
 {
  return pt.X == x;
 }
}
 ....

List<Point> mydatalist = new List<Point>();
...
int xToFind = 4;
int i = myDataList.FindIndex(new XSearcher(xToFind).Condition);