如何在列表中查找特定元素

Question

I want to filter my List and if an item already exists , I don't want to add that item to that List . That's why I try to filter by the item property.

if(!(PolyLineList.Contains(PolyLineList.Find(x => x.item == item))))
{
    cPolyline currentPolyLine = new cPolyline(pointlist,item);
    PolyLineList.Add(currentPolyLine);
}

When an item already exists in PolyLineList , that is equal to item , it should skip the if -statement.

Answer 1

Use Linq's Any method:

if (!PolyLineList.Any(x => x.item == item))
{
    ...
}

Answer 2

You may want to change List<T> to HashSet<T> and just Add (providing that PolyLine implements Equals and GetHashCode methods for PolyLine instances to be compared):

 //TODO: you, probably, want to rename PolyLineList into PolyLineSet or something 
 HashSet<cPolyline> PolyLineList = new HashSet<cPolyline>();

 ...

 PolyLineList.Add(currentPolyLine);

Answer 3

Assuming the type of item is ItemType :

var itemSet = new HashSet<ItemType>();

...

if (itemSet.Add(item))
{ // item is unique
    PolyLineList.Add(new cPolyline(pointlist,item));
}