Find an item in List by LINQ

Question

Here I have a simple example 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 lt string gt                  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      LINQ is new for me  I am curious if I can use LINQ to find item in list  How if possible

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You want to search an object in object list   This will help you in getting the first or default value in your Linq List search   var item   list FirstOrDefault items   gt   items Reference    ent BackToBackExternalReferenceId     or  var item    from items in list     where items Reference    ent BackToBackExternalReferenceId     select items  FirstOrDefault

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I used to use a Dictionary which is some sort of an indexed list which will give me exactly what I want when I want it   Dictionary lt string  int gt  margins   new Dictionary lt string  int gt     margins Add  left   10   margins Add  right   10   margins Add  top   20   margins Add  bottom   30     Whenever I wish to access my margins values  for instance  I address my dictionary   int xStartPos   margins  left    int xLimitPos   margins  right    int yStartPos   margins  top    int yLimitPos   margins  bottom      So  depending on what you re doing  a dictionary can be useful

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Try this code     return context EntitytableName AsEnumerable   Find p   gt  p LoginID Equals loginID   amp  amp  p Password Equals password   Select p   gt  new ModelTableName  FirstName   p FirstName  UserID   p UserID

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If you want the index of the element  this will do it   int index   list Select  item  i    gt  new   Item   item  Index   i                     First x   gt  x Item    search  Index      or var tagged   list Select  item  i    gt  new   Item   item  Index   i     int index    from pair in tagged             where pair Item    search             select pair Index  First      You can t get rid of the lambda in the first pass   Note that this will throw if the item doesn t exist  This solves the problem by resorting to nullable ints   var tagged   list Select  item  i    gt  new   Item   item  Index    int  i     int  index    from pair in tagged             where pair Item    search             select pair Index  FirstOrDefault      If you want the item      Throws if not found var item   list First item   gt  item    search      or var item    from item in list             where item    search             select item  First        Null if not found var item   list FirstOrDefault item   gt  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   gt  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   gt  item    search      or var items   from item in list             where item    search             select item    And don t forget to check the list for null in any of these cases   Or use  list    Enumerable Empty lt string gt     instead of list   Thanks to Pavel for helping out in the comments

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You can use FirstOfDefault with the Where Linq extension to get a MessageAction class from the IEnumerable   Reme   var action   Message Actions Where e    e targetByName    className  FirstOrDefault      where  List Actions   get  set

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If it really is a List lt string gt  you don t need LINQ  just use   int GetItemIndex string search        return  list    null   -1    list IndexOf search       If you are looking for the item itself  try   string GetItem string search        return  list    null   null    list FirstOrDefault s   gt  s Equals search

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This method is easier and safer  var lOrders   new List lt string gt       bool insertOrderNew   lOrders Find r   gt  r     1234      null   true   false

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One more way to check existence of an element in a List var result   myList Exists users   gt  users Equals  quot Vijai quot

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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   gt  s    search    string result    from s in  list                  where s    search                  select s  Single     string result    list Find search    int result    list IndexOf search

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There s a few ways  note this is not a complete list    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 lt string gt  myList   new List lt string gt     string result   myList Single s   gt  s    search     Note SingleOrDefault   will behave the same  except it will return null for reference types  or the default value for value types  instead of throwing an exception   2  Where will return all items which match your criteria  so you may get an IEnumerable with one element   IEnumerable lt string gt  results   myList Where s   gt  s    search     3  First will return the first item which matches your criteria   string result   myList First s   gt  s    search     Note FirstOrDefault   will behave the same  except it will return null for reference types  or the default value for value types  instead of throwing an exception

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This will help you in getting the first or default value in your Linq List search  var results    List Where item   gt  item    search  FirstOrDefault      This search will find the first or default value it will return

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If we need to find an element from the list  then we can use the Find and FindAll extensions method  but there is a slight difference between them  Here is an example    List lt int gt  items   new List lt int gt      10  9  8  4  8  7  8          It will return only one 8 as Find returns only the first occurrence of matched elements       var result   items Find ls   gt  ls    8             this will returns three  8 8 8  as FindAll returns all the matched elements        var result1   items FindAll ls   gt  ls    8

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How about IndexOf       Searches for the specified object and returns the index of the first occurrence within the list   For example   gt  var boys   new List lt string gt   Harry    Ron    Neville       gt  boys IndexOf  Neville     2  gt  boys 2      Neville  True   Note that it returns -1 if the value doesn t occur in the list    gt  boys IndexOf  Hermione     -1

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Here is one way to rewrite your method to use LINQ   public static int GetItemIndex string search        List lt string gt   list   new List lt string gt       one    two    three          var result    list Select  Value  Index    gt  new   Value  Index                 SingleOrDefault l   gt  l Value    search        return result    null   -1   result Index      Thus  calling it with  GetItemIndex  two   will return 1   and  GetItemIndex  notthere   will return -1   Reference  linqsamples com

[c#] Find an item in List by LINQ?

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