程式設計筆記byChris

ref :http://stackoverflow.com/questions/1175645/find-an-item-in-list-by-linq

 

To find an item in a list of strings

 Normally I use for loop or anonymous delegate to do it like this

int GetItemIndex(string search) 
{ 
   int found = -1; 
   if ( _list != null ) 
   { 
     foreach (string item in _list) // _list is an instance of List<string> 
     {  
        found++; 
        if ( string.Equals(search, item) ) 
        { 
           break; 
        } 
      } 
      /* use anonymous delegate 
      string foundItem = _list.Find( delegate(string item) { 
         found++; 
         return string.Equals(search, item); 
      }); 
      */ 
   } 
   return found; 
}

 

by other ways :

1) Single will return a single result, but will throw an exception if it finds none or more than one (which may or may not be what you want):

string search = "lookforme"; 
List<string> myList = new List<string>(); 
string result = myList.Single(s => s == search); 

2) Where will return all items which match your criteria, so you may get an IEnumerable with one element:

IEnumerable<string> results = myList.Where(s => s == search); 

3) First will return the first item which matches your criteria:

string result = myList.First(s => s == search); 


4)
If it really is a List<string> you don't need LINQ, just use:
int GetItemIndex(string search) 
{ 
    return _list == null ? -1 : _list.IndexOf(search); 
} 
 
5)
If you are looking for the item itself, try:
string GetItem(string search) 
{ 
    return _list == null ? null : _list.FirstOrDefault(s => s.Equals(search)); 
} 

6)
Other options
Do you want the item in the list or the actual item itself (would assume the item itself).
Here are a bunch of options for you:

string result = _list.First(s => s == search); 
 
string result = (from s in _list 
                 where s == search
                 select s).Single(); 
 
string result = _list.Find(search); 
 
int result = _list.IndexOf(search);

If you want the item:

// Throws if not found 
var item = list.First(item => item == search); 
// or 
var item = (from item in list 
            where item == search 
            select item).First(); 
 
// Null if not found 
var item = list.FirstOrDefault(item => item == search); 
// or 
var item = (from item in list 
            where item == search 
            select item).FirstOrDefault(); 

If you want to count the number of items that match:

int count = list.Count(item => item == search); 
// or 
int count = (from item in list 
            where item == search 
            select item).Count(); 

If you want all the items that match:

var items = list.Where(item => item == search); 
// or 
var items = from item in list 
            where item == search 
            select item;