Using LINQ to remove elements from a List T

Question

Say that I have LINQ query such as   var authors   from x in authorsList               where x firstname     Bob                select x    Given that authorsList is of type List lt Author gt   how can I delete the Author elements from authorsList that are returned by the query into authors   Or  put another way  how can I delete all of the firstname s equalling Bob from authorsList   Note  This is a simplified example for the purposes of the question

User · Answer

Below is the example to remove the element from the list    List lt int gt  items   new List lt int gt      2  2  3  4  2  7  3 3 3     var result   items Remove 2    Remove the first ocurence of matched elements and returns boolean value  var result1   items RemoveAll lst   gt  lst    3     Remove all the matched elements and returns count of removed element  items RemoveAt 3    Removes the elements at the specified index

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Well  it would be easier to exclude them in the first place   authorsList   authorsList Where x   gt  x FirstName     Bob   ToList      However  that would just change the value of authorsList instead of removing the authors from the previous collection  Alternatively  you can use RemoveAll   authorsList RemoveAll x   gt  x FirstName     Bob      If you really need to do it based on another collection  I d use a HashSet  RemoveAll and Contains   var setToRemove   new HashSet lt Author gt  authors   authorsList RemoveAll x   gt  setToRemove Contains x

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I was wondering  if there is any difference between RemoveAll and Except and the pros of using HashSet  so I have done quick performance check     using System  using System Collections Generic  using System Diagnostics  using System Linq  using System Text   namespace ListRemoveTest       class Program               private static Random random   new Random   int DateTime Now Ticks             static void Main  string   args                         Console WriteLine   Be patient  generating data                     List lt string gt  list   new List lt string gt                 List lt string gt  toRemove   new List lt string gt                 for  int x 0  x  lt  1000000  x                                   string randString   RandomString  random Next  100                      list Add  randString                    if  random Next  1000      0                       toRemove Insert  0  randString                               List lt string gt  l1   new List lt string gt   list                List lt string gt  l2   new List lt string gt   list                List lt string gt  l3   new List lt string gt   list                List lt string gt  l4   new List lt string gt   list                 Console WriteLine   Be patient  testing                     Stopwatch sw1   Stopwatch StartNew                l1 RemoveAll  toRemove Contains                sw1 Stop                 Stopwatch sw2   Stopwatch StartNew                l2 RemoveAll  new HashSet lt string gt   toRemove   Contains                sw2 Stop                 Stopwatch sw3   Stopwatch StartNew                l3   l3 Except  toRemove   ToList                sw3 Stop                 Stopwatch sw4   Stopwatch StartNew                l4   l4 Except  new HashSet lt string gt   toRemove     ToList                sw3 Stop                  Console WriteLine   L1 Len    0   Time taken   1 ms   l1 Count  sw1 Elapsed TotalMilliseconds                Console WriteLine   L2 Len    0   Time taken   1 ms   l1 Count  sw2 Elapsed TotalMilliseconds                Console WriteLine   L3 Len    0   Time taken   1 ms   l1 Count  sw3 Elapsed TotalMilliseconds                Console WriteLine   L4 Len    0   Time taken   1 ms   l1 Count  sw3 Elapsed TotalMilliseconds                 Console ReadKey                        private static string RandomString  int size                         StringBuilder builder   new StringBuilder                char ch              for  int i   0  i  lt  size  i                                   ch   Convert ToChar  Convert ToInt32  Math Floor  26   random NextDouble     65                        builder Append  ch                               return builder ToString                        Results below   Be patient  generating data    Be patient  testing    L1 Len   985263  Time taken  13411 8648ms L2 Len   985263  Time taken  76 4042ms L3 Len   985263  Time taken  340 6933ms L4 Len   985263  Time taken  340 6933ms   As we can see  best option in that case is to use RemoveAll HashSet

User · Answer

This is a very old question  but I found a really simple way to do this    authorsList   authorsList Except authors  ToList      Note that since the return variable authorsList is a List lt T gt   the IEnumerable lt T gt  returned by Except   must be converted to a List lt T gt

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To keep the code fluent  if code optimisation is not crucial  and you would need to do some further operations on the list   authorsList   authorsList Where x   gt  x FirstName     Bob    lt do some further Linq gt     or  authorsList   authorsList Where x   gt   setToRemove Contains x    lt do some further Linq gt

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LINQ has its origins in functional programming  which emphasises immutability of objects  so it doesn t provide a built-in way to update the original list in-place   Note on immutability  taken from another SO answer    Here is the definition of immutability from Wikipedia      In object-oriented and functional programming  an immutable object is   an object whose state cannot be modified after it is created

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I think you could do something like this      authorsList    from a in authorsList                   where  authors Contains a                    select a  ToList      Although I think the solutions already given solve the problem in a more readable way

User · Answer

Say that authorsToRemove is an IEnumerable lt T gt  that contains the elements you want to remove from authorsList   Then here is another very simple way to accomplish the removal task asked by the OP   authorsList RemoveAll authorsToRemove Contains

User · Answer

Simple solution   static void Main         List lt string gt  myList   new List lt string gt     Jason    Bob    Frank    Bob         myList RemoveAll x   gt  x     Bob         foreach  string s in myList

User · Answer

It d be better to use List lt T gt  RemoveAll to accomplish this   authorsList RemoveAll  x    gt  x firstname     Bob

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You cannot do this with standard LINQ operators because LINQ provides query  not update support   But you can generate a new list and replace the old one   var authorsList   GetAuthorList     authorsList   authorsList Where a   gt  a FirstName     Bob   ToList      Or you could remove all items in authors in a second pass   var authorsList   GetAuthorList     var authors   authorsList Where a   gt  a FirstName     Bob   ToList     foreach  var author in authors        authorList Remove author

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i think you just have to assign the items from Author list to a new list to take that effect     assume oldAuthor is the old list Author newAuthorList    select x from oldAuthor where x firstname   Bob  select x  ToList    oldAuthor   newAuthorList  newAuthorList   null

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If you really need to remove items then what about Except    You can remove based on a new list  or remove on-the-fly by nesting the Linq   var authorsList   new List lt Author gt          new Author  Firstname    Bob   Lastname    Smith         new Author  Firstname    Fred   Lastname    Jones         new Author  Firstname    Brian   Lastname    Brains         new Author  Firstname    Billy   Lastname    TheKid        var authors   authorsList Where a   gt  a Firstname     Bob    authorsList   authorsList Except authors  ToList    authorsList   authorsList Except authorsList Where a  gt a Firstname   Billy    ToList

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You can remove in two ways  var output   from x in authorsList              where x firstname     Bob               select x    or  var authors   from x in authorsList               where x firstname     Bob                select x   var output   from x in authorsList              where  authors Contains x                select x    I had same issue  if you want simple output based on your where condition   then first solution is better

[c#] Using LINQ to remove elements from a List<T>

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