Best way to remove items from a collection

Question

What is the best way to approach removing items from a collection in C   once the item is known  but not it s index   This is one way to do it  but it seems inelegant at best     Remove the existing role assignment for the user  int cnt   0  int assToDelete   0  foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName                assToDelete   cnt            cnt      workspace RoleAssignments Remove assToDelete     What I would really like to do is find the item to remove by property  in this case  name  without looping through the entire collection and using 2 additional variables

User · Answer

If RoleAssignments is a List lt T gt  you can use the following code   workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName

User · Answer

A lot of good responses here  I especially like the lambda expressions   very clean   I was remiss  however  in not specifying the type of Collection   This is a SPRoleAssignmentCollection  from MOSS  that only has Remove int  and Remove SPPrincipal   not the handy RemoveAll     So  I have settled on this  unless there is a better suggestion  foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName  continue      workspace RoleAssignments Remove  SPPrincipal spAssignment Member       break

User · Answer

If RoleAssignments is a List lt T gt  you can use the following code   workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName

User · Answer

smaclell asked why reverse iteration was more efficient in in a comment to  sambo99   Sometimes it s more efficient  Consider you have a list of people  and you want to remove or filter all customers with a credit rating  lt  1000   We have the following data   Bob  999  Mary  999  Ted  1000   If we were to iterate forward  we d soon get into trouble  for  int idx   0  idx  lt  list Count   idx           if  list idx  Rating  lt  1000                 list RemoveAt idx      whoops            At idx   0 we remove Bob  which then shifts all remaining elements left  The next time through the loop idx   1  but   list 1  is now Ted instead of Mary  We end up skipping Mary by mistake  We could use a while loop  and we could introduce more variables   Or  we just reverse iterate    for  int idx   list Count-1  idx  gt   0  idx--        if  list idx  Rating  lt  1000                list RemoveAt idx             All the indexes to the left of the removed item stay the same  so you don t skip any items   The same principle applies if you re given a list of indexes to remove from an array  In order to keep things straight you need to sort the list and then remove the items from highest index to lowest   Now you can just use Linq and declare what you re doing in a straightforward manner   list RemoveAll o   gt  o Rating  lt  1000       For this case of removing a single item  it s no more efficient iterating forwards or backwards  You could also use Linq for this   int removeIndex   list FindIndex o   gt  o Name     Ted    if  removeIndex    -1         list RemoveAt removeIndex

User · Answer

If RoleAssignments is a List lt T gt  you can use the following code   workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName

User · Answer

What type is the collection  If it s List  you can use the helpful  RemoveAll    int cnt   workspace RoleAssignments                        RemoveAll spa   gt  spa Member Name    shortName     This works in  NET 2 0  Of course  if you don t have the newer compiler  you ll have to use  delegate  SPRoleAssignment spa    return spa Member Name    shortName     instead of the nice lambda syntax    Another approach if it s not a List  but still an ICollection      var toRemove   workspace RoleAssignments                                FirstOrDefault spa   gt  spa Member Name    shortName     if  toRemove    null  workspace RoleAssignments Remove toRemove     This requires the Enumerable extension methods   You can copy the Mono ones in  if you are stuck on  NET 2 0   If it s some custom collection that cannot take an item  but MUST take an index  some of the other Enumerable methods  such as Select  pass in the integer index for you

User · Answer

A lot of good responses here  I especially like the lambda expressions   very clean   I was remiss  however  in not specifying the type of Collection   This is a SPRoleAssignmentCollection  from MOSS  that only has Remove int  and Remove SPPrincipal   not the handy RemoveAll     So  I have settled on this  unless there is a better suggestion  foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName  continue      workspace RoleAssignments Remove  SPPrincipal spAssignment Member       break

User · Answer

There is another approach you can take depending on how you re using your collection  If you re downloading the assignments one time  e g   when the app runs   you could translate the collection on the fly into a hashtable where   shortname    SPRoleAssignment  If you do this  then when you want to remove an item by short name  all you need to do is remove the item from the hashtable by key   Unfortunately  if you re loading these SPRoleAssignments a lot  that obviously isn t going to be any more cost efficient in terms of time  The suggestions other people made about using Linq would be good if you re using a new version of the  NET Framework  but otherwise  you ll have to stick to the method you re using

User · Answer

If you want to access members of the collection by one of their properties  you might consider using a Dictionary lt T gt  or KeyedCollection lt T gt  instead  This way you don t have to search for the item you re looking for   Otherwise  you could at least do this   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment           break

User · Answer

The best way to do it is by using linq    Example class    public class Product               public string Name   get  set            public string Price   get  set                  Linq query   var subCollection   collection1 RemoveAll w   gt  collection2 Any q   gt  q Name    w Name      This query will remove all elements from collection1 if Name match any element Name from collection2  Remember to use  using System Linq

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What type is the collection  If it s List  you can use the helpful  RemoveAll    int cnt   workspace RoleAssignments                        RemoveAll spa   gt  spa Member Name    shortName     This works in  NET 2 0  Of course  if you don t have the newer compiler  you ll have to use  delegate  SPRoleAssignment spa    return spa Member Name    shortName     instead of the nice lambda syntax    Another approach if it s not a List  but still an ICollection      var toRemove   workspace RoleAssignments                                FirstOrDefault spa   gt  spa Member Name    shortName     if  toRemove    null  workspace RoleAssignments Remove toRemove     This requires the Enumerable extension methods   You can copy the Mono ones in  if you are stuck on  NET 2 0   If it s some custom collection that cannot take an item  but MUST take an index  some of the other Enumerable methods  such as Select  pass in the integer index for you

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To do this while looping through the collection and not to get the modifying a collection exception  this is the approach I ve taken in the past  note the  ToList   at the end of the original collection  this creates another collection in memory  then you can modify the existing collection   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments ToList          if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment

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This is my generic solution  public static IEnumerable lt T gt  Remove lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  match                var list   items ToList            for  int idx   0  idx  lt  list Count    idx                          if  match list idx                                  list RemoveAt idx                   idx--     the list is 1 item shorter                                 return list AsEnumerable            It would look much simpler if extension methods support passing by reference   usage   var result   string    mike    john    ali   result   result Remove x   gt  x Username     mike   ToArray    Assert IsTrue result Length    2     EDIT  ensured that the list looping remains valid even when deleting items by decrementing the index  idx

User · Answer

If RoleAssignments is a List lt T gt  you can use the following code   workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName

User · Answer

Save your items first  than delete them   var itemsToDelete   Items Where x   gt     your condition     ToArray    for  int i   0  i  lt  itemsToDelete Length    i      Items Remove itemsToDelete i      You need to override GetHashCode   in your Item class

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A lot of good responses here  I especially like the lambda expressions   very clean   I was remiss  however  in not specifying the type of Collection   This is a SPRoleAssignmentCollection  from MOSS  that only has Remove int  and Remove SPPrincipal   not the handy RemoveAll     So  I have settled on this  unless there is a better suggestion  foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName  continue      workspace RoleAssignments Remove  SPPrincipal spAssignment Member       break

User · Answer

If you want to access members of the collection by one of their properties  you might consider using a Dictionary lt T gt  or KeyedCollection lt T gt  instead  This way you don t have to search for the item you re looking for   Otherwise  you could at least do this   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment           break

User · Answer

smaclell asked why reverse iteration was more efficient in in a comment to  sambo99   Sometimes it s more efficient  Consider you have a list of people  and you want to remove or filter all customers with a credit rating  lt  1000   We have the following data   Bob  999  Mary  999  Ted  1000   If we were to iterate forward  we d soon get into trouble  for  int idx   0  idx  lt  list Count   idx           if  list idx  Rating  lt  1000                 list RemoveAt idx      whoops            At idx   0 we remove Bob  which then shifts all remaining elements left  The next time through the loop idx   1  but   list 1  is now Ted instead of Mary  We end up skipping Mary by mistake  We could use a while loop  and we could introduce more variables   Or  we just reverse iterate    for  int idx   list Count-1  idx  gt   0  idx--        if  list idx  Rating  lt  1000                list RemoveAt idx             All the indexes to the left of the removed item stay the same  so you don t skip any items   The same principle applies if you re given a list of indexes to remove from an array  In order to keep things straight you need to sort the list and then remove the items from highest index to lowest   Now you can just use Linq and declare what you re doing in a straightforward manner   list RemoveAll o   gt  o Rating  lt  1000       For this case of removing a single item  it s no more efficient iterating forwards or backwards  You could also use Linq for this   int removeIndex   list FindIndex o   gt  o Name     Ted    if  removeIndex    -1         list RemoveAt removeIndex

User · Answer

If it s an ICollection then you won t have a RemoveAll method  Here s an extension method that will do it       public static void RemoveAll lt T gt  this ICollection lt T gt  source                                       Func lt T  bool gt  predicate                if  source    null              throw new ArgumentNullException  source    source is null              if  predicate    null              throw new ArgumentNullException  predicate    predicate is null              source Where predicate  ToList   ForEach e   gt  source Remove e            Based on  http   phejndorf wordpress com 2011 03 09 a-removeall-extension-for-the-collection-class

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To do this while looping through the collection and not to get the modifying a collection exception  this is the approach I ve taken in the past  note the  ToList   at the end of the original collection  this creates another collection in memory  then you can modify the existing collection   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments ToList          if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment

User · Answer

Save your items first  than delete them   var itemsToDelete   Items Where x   gt     your condition     ToArray    for  int i   0  i  lt  itemsToDelete Length    i      Items Remove itemsToDelete i      You need to override GetHashCode   in your Item class

User · Answer

Here is a pretty good way to do it  http   support microsoft com kb 555972          System Collections ArrayList arr   new System Collections ArrayList            arr Add  1            arr Add  2            arr Add  3               This throws an exception         foreach  string s in arr                        arr Remove s                                   where as this works correctly         Console WriteLine arr Count           foreach  string s in new System Collections ArrayList arr                          arr Remove s                     Console WriteLine arr Count           Console ReadKey

User · Answer

The best way to do it is by using linq    Example class    public class Product               public string Name   get  set            public string Price   get  set                  Linq query   var subCollection   collection1 RemoveAll w   gt  collection2 Any q   gt  q Name    w Name      This query will remove all elements from collection1 if Name match any element Name from collection2  Remember to use  using System Linq

User · Answer

If you have got a List lt T gt   then List lt T gt  RemoveAll is your best bet  There can t be anything more efficient  Internally it does the array moving in one shot  not to mention it is O N   If all you got is an IList lt T gt  or an ICollection lt T gt  you got roughly these three options      public static void RemoveAll lt T gt  this IList lt T gt  ilist  Predicate lt T gt  predicate     O N 2                for  var index   ilist Count - 1  index  gt   0  index--                        var item   ilist index               if  predicate item                                 ilist RemoveAt index                                  or     public static void RemoveAll lt T gt  this ICollection lt T gt  icollection  Predicate lt T gt  predicate     O N                var nonMatchingItems   new List lt T gt                 Move all the items that do not match to another collection          foreach  var item in icollection                         if   predicate item                                 nonMatchingItems Add item                                       Clear the collection and then copy back the non-matched items          icollection Clear            foreach  var item in nonMatchingItems                        icollection Add item                    or     public static void RemoveAll lt T gt  this ICollection lt T gt  icollection  Func lt T  bool gt  predicate     O N 2                foreach  var item in icollection Where predicate  ToList                          icollection Remove item                    Go for either 1 or 2  1 is lighter on memory and faster if you have less deletes to perform  i e  predicate is false most of the times   2 is faster if you have more deletes to perform  3 is the cleanest code but performs poorly IMO  Again all that depends on input data  For some benchmarking details see https   github com dotnet BenchmarkDotNet issues 1505

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If you want to access members of the collection by one of their properties  you might consider using a Dictionary lt T gt  or KeyedCollection lt T gt  instead  This way you don t have to search for the item you re looking for   Otherwise  you could at least do this   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment           break

User · Answer

If you have got a List lt T gt   then List lt T gt  RemoveAll is your best bet  There can t be anything more efficient  Internally it does the array moving in one shot  not to mention it is O N   If all you got is an IList lt T gt  or an ICollection lt T gt  you got roughly these three options      public static void RemoveAll lt T gt  this IList lt T gt  ilist  Predicate lt T gt  predicate     O N 2                for  var index   ilist Count - 1  index  gt   0  index--                        var item   ilist index               if  predicate item                                 ilist RemoveAt index                                  or     public static void RemoveAll lt T gt  this ICollection lt T gt  icollection  Predicate lt T gt  predicate     O N                var nonMatchingItems   new List lt T gt                 Move all the items that do not match to another collection          foreach  var item in icollection                         if   predicate item                                 nonMatchingItems Add item                                       Clear the collection and then copy back the non-matched items          icollection Clear            foreach  var item in nonMatchingItems                        icollection Add item                    or     public static void RemoveAll lt T gt  this ICollection lt T gt  icollection  Func lt T  bool gt  predicate     O N 2                foreach  var item in icollection Where predicate  ToList                          icollection Remove item                    Go for either 1 or 2  1 is lighter on memory and faster if you have less deletes to perform  i e  predicate is false most of the times   2 is faster if you have more deletes to perform  3 is the cleanest code but performs poorly IMO  Again all that depends on input data  For some benchmarking details see https   github com dotnet BenchmarkDotNet issues 1505

User · Answer

For a simple List structure the most efficient way seems to be using the Predicate RemoveAll implementation    Eg    workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName     The reasons are     The Predicate Linq RemoveAll method is implemented in List and has access to the internal array storing the actual data  It will shift the data and resize the internal array  The RemoveAt method implementation is quite slow  and will copy the entire underlying array of data into a new array  This means reverse iteration is useless for List    If you are stuck implementing this in a the pre c  3 0 era  You have 2 options     The easily maintainable option  Copy all the matching items into a new list and and swap the underlying list     Eg    List lt int gt  list2   new List lt int gt       foreach  int i in GetList          if    i   2    0                 list2 Add i           list2   list2    Or    The tricky slightly faster option  which involves shifting all the data in the list down when it does not match and then resizing the array     If you are removing stuff really frequently from a list  perhaps another structure like a HashTable   net 1 1  or a Dictionary   net 2 0  or a HashSet   net 3 5  are better suited for this purpose

User · Answer

What type is the collection  If it s List  you can use the helpful  RemoveAll    int cnt   workspace RoleAssignments                        RemoveAll spa   gt  spa Member Name    shortName     This works in  NET 2 0  Of course  if you don t have the newer compiler  you ll have to use  delegate  SPRoleAssignment spa    return spa Member Name    shortName     instead of the nice lambda syntax    Another approach if it s not a List  but still an ICollection      var toRemove   workspace RoleAssignments                                FirstOrDefault spa   gt  spa Member Name    shortName     if  toRemove    null  workspace RoleAssignments Remove toRemove     This requires the Enumerable extension methods   You can copy the Mono ones in  if you are stuck on  NET 2 0   If it s some custom collection that cannot take an item  but MUST take an index  some of the other Enumerable methods  such as Select  pass in the integer index for you

User · Answer

If you want to access members of the collection by one of their properties  you might consider using a Dictionary lt T gt  or KeyedCollection lt T gt  instead  This way you don t have to search for the item you re looking for   Otherwise  you could at least do this   foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName                workspace RoleAssignments Remove spAssignment           break

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Similar to Dictionary Collection point of view  I have done this   Dictionary lt string  bool gt  sourceDict   new Dictionary lt string  bool gt     sourceDict Add  Sai   true   sourceDict Add  Sri   false   sourceDict Add  SaiSri   true   sourceDict Add  SaiSriMahi   true    var itemsToDelete   sourceDict Where DictItem   gt  DictItem Value    false    foreach  var item in itemsToDelete        sourceDict Remove item Key       Note   Above code will fail in  Net Client Profile  3 5 and 4 5  also some viewers mentioned it is  Failing for them in  Net4 0 as well not sure which settings are causing the problem   So replace with below code   ToList    for Where statement  to avoid that error     Collection was modified  enumeration operation may not execute      var itemsToDelete   sourceDict Where DictItem   gt  DictItem Value    false  ToList      Per MSDN From  Net4 5 onwards Client Profile are discontinued  http   msdn microsoft com en-us library cc656912 v vs 110  aspx

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If it s an ICollection then you won t have a RemoveAll method  Here s an extension method that will do it       public static void RemoveAll lt T gt  this ICollection lt T gt  source                                       Func lt T  bool gt  predicate                if  source    null              throw new ArgumentNullException  source    source is null              if  predicate    null              throw new ArgumentNullException  predicate    predicate is null              source Where predicate  ToList   ForEach e   gt  source Remove e            Based on  http   phejndorf wordpress com 2011 03 09 a-removeall-extension-for-the-collection-class

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smaclell asked why reverse iteration was more efficient in in a comment to  sambo99   Sometimes it s more efficient  Consider you have a list of people  and you want to remove or filter all customers with a credit rating  lt  1000   We have the following data   Bob  999  Mary  999  Ted  1000   If we were to iterate forward  we d soon get into trouble  for  int idx   0  idx  lt  list Count   idx           if  list idx  Rating  lt  1000                 list RemoveAt idx      whoops            At idx   0 we remove Bob  which then shifts all remaining elements left  The next time through the loop idx   1  but   list 1  is now Ted instead of Mary  We end up skipping Mary by mistake  We could use a while loop  and we could introduce more variables   Or  we just reverse iterate    for  int idx   list Count-1  idx  gt   0  idx--        if  list idx  Rating  lt  1000                list RemoveAt idx             All the indexes to the left of the removed item stay the same  so you don t skip any items   The same principle applies if you re given a list of indexes to remove from an array  In order to keep things straight you need to sort the list and then remove the items from highest index to lowest   Now you can just use Linq and declare what you re doing in a straightforward manner   list RemoveAll o   gt  o Rating  lt  1000       For this case of removing a single item  it s no more efficient iterating forwards or backwards  You could also use Linq for this   int removeIndex   list FindIndex o   gt  o Name     Ted    if  removeIndex    -1         list RemoveAt removeIndex

User · Answer

Similar to Dictionary Collection point of view  I have done this   Dictionary lt string  bool gt  sourceDict   new Dictionary lt string  bool gt     sourceDict Add  Sai   true   sourceDict Add  Sri   false   sourceDict Add  SaiSri   true   sourceDict Add  SaiSriMahi   true    var itemsToDelete   sourceDict Where DictItem   gt  DictItem Value    false    foreach  var item in itemsToDelete        sourceDict Remove item Key       Note   Above code will fail in  Net Client Profile  3 5 and 4 5  also some viewers mentioned it is  Failing for them in  Net4 0 as well not sure which settings are causing the problem   So replace with below code   ToList    for Where statement  to avoid that error     Collection was modified  enumeration operation may not execute      var itemsToDelete   sourceDict Where DictItem   gt  DictItem Value    false  ToList      Per MSDN From  Net4 5 onwards Client Profile are discontinued  http   msdn microsoft com en-us library cc656912 v vs 110  aspx

User · Answer

What type is the collection  If it s List  you can use the helpful  RemoveAll    int cnt   workspace RoleAssignments                        RemoveAll spa   gt  spa Member Name    shortName     This works in  NET 2 0  Of course  if you don t have the newer compiler  you ll have to use  delegate  SPRoleAssignment spa    return spa Member Name    shortName     instead of the nice lambda syntax    Another approach if it s not a List  but still an ICollection      var toRemove   workspace RoleAssignments                                FirstOrDefault spa   gt  spa Member Name    shortName     if  toRemove    null  workspace RoleAssignments Remove toRemove     This requires the Enumerable extension methods   You can copy the Mono ones in  if you are stuck on  NET 2 0   If it s some custom collection that cannot take an item  but MUST take an index  some of the other Enumerable methods  such as Select  pass in the integer index for you

User · Answer

A lot of good responses here  I especially like the lambda expressions   very clean   I was remiss  however  in not specifying the type of Collection   This is a SPRoleAssignmentCollection  from MOSS  that only has Remove int  and Remove SPPrincipal   not the handy RemoveAll     So  I have settled on this  unless there is a better suggestion  foreach  SPRoleAssignment spAssignment in workspace RoleAssignments        if  spAssignment Member Name    shortName  continue      workspace RoleAssignments Remove  SPPrincipal spAssignment Member       break

User · Answer

smaclell asked why reverse iteration was more efficient in in a comment to  sambo99   Sometimes it s more efficient  Consider you have a list of people  and you want to remove or filter all customers with a credit rating  lt  1000   We have the following data   Bob  999  Mary  999  Ted  1000   If we were to iterate forward  we d soon get into trouble  for  int idx   0  idx  lt  list Count   idx           if  list idx  Rating  lt  1000                 list RemoveAt idx      whoops            At idx   0 we remove Bob  which then shifts all remaining elements left  The next time through the loop idx   1  but   list 1  is now Ted instead of Mary  We end up skipping Mary by mistake  We could use a while loop  and we could introduce more variables   Or  we just reverse iterate    for  int idx   list Count-1  idx  gt   0  idx--        if  list idx  Rating  lt  1000                list RemoveAt idx             All the indexes to the left of the removed item stay the same  so you don t skip any items   The same principle applies if you re given a list of indexes to remove from an array  In order to keep things straight you need to sort the list and then remove the items from highest index to lowest   Now you can just use Linq and declare what you re doing in a straightforward manner   list RemoveAll o   gt  o Rating  lt  1000       For this case of removing a single item  it s no more efficient iterating forwards or backwards  You could also use Linq for this   int removeIndex   list FindIndex o   gt  o Name     Ted    if  removeIndex    -1         list RemoveAt removeIndex

User · Answer

This is my generic solution  public static IEnumerable lt T gt  Remove lt T gt  this IEnumerable lt T gt  items  Func lt T  bool gt  match                var list   items ToList            for  int idx   0  idx  lt  list Count    idx                          if  match list idx                                  list RemoveAt idx                   idx--     the list is 1 item shorter                                 return list AsEnumerable            It would look much simpler if extension methods support passing by reference   usage   var result   string    mike    john    ali   result   result Remove x   gt  x Username     mike   ToArray    Assert IsTrue result Length    2     EDIT  ensured that the list looping remains valid even when deleting items by decrementing the index  idx

User · Answer

There is another approach you can take depending on how you re using your collection  If you re downloading the assignments one time  e g   when the app runs   you could translate the collection on the fly into a hashtable where   shortname    SPRoleAssignment  If you do this  then when you want to remove an item by short name  all you need to do is remove the item from the hashtable by key   Unfortunately  if you re loading these SPRoleAssignments a lot  that obviously isn t going to be any more cost efficient in terms of time  The suggestions other people made about using Linq would be good if you re using a new version of the  NET Framework  but otherwise  you ll have to stick to the method you re using

User · Answer

Here is a pretty good way to do it  http   support microsoft com kb 555972          System Collections ArrayList arr   new System Collections ArrayList            arr Add  1            arr Add  2            arr Add  3               This throws an exception         foreach  string s in arr                        arr Remove s                                   where as this works correctly         Console WriteLine arr Count           foreach  string s in new System Collections ArrayList arr                          arr Remove s                     Console WriteLine arr Count           Console ReadKey

User · Answer

For a simple List structure the most efficient way seems to be using the Predicate RemoveAll implementation    Eg    workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName     The reasons are     The Predicate Linq RemoveAll method is implemented in List and has access to the internal array storing the actual data  It will shift the data and resize the internal array  The RemoveAt method implementation is quite slow  and will copy the entire underlying array of data into a new array  This means reverse iteration is useless for List    If you are stuck implementing this in a the pre c  3 0 era  You have 2 options     The easily maintainable option  Copy all the matching items into a new list and and swap the underlying list     Eg    List lt int gt  list2   new List lt int gt       foreach  int i in GetList          if    i   2    0                 list2 Add i           list2   list2    Or    The tricky slightly faster option  which involves shifting all the data in the list down when it does not match and then resizing the array     If you are removing stuff really frequently from a list  perhaps another structure like a HashTable   net 1 1  or a Dictionary   net 2 0  or a HashSet   net 3 5  are better suited for this purpose

User · Answer

There is another approach you can take depending on how you re using your collection  If you re downloading the assignments one time  e g   when the app runs   you could translate the collection on the fly into a hashtable where   shortname    SPRoleAssignment  If you do this  then when you want to remove an item by short name  all you need to do is remove the item from the hashtable by key   Unfortunately  if you re loading these SPRoleAssignments a lot  that obviously isn t going to be any more cost efficient in terms of time  The suggestions other people made about using Linq would be good if you re using a new version of the  NET Framework  but otherwise  you ll have to stick to the method you re using

User · Answer

For a simple List structure the most efficient way seems to be using the Predicate RemoveAll implementation    Eg    workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName     The reasons are     The Predicate Linq RemoveAll method is implemented in List and has access to the internal array storing the actual data  It will shift the data and resize the internal array  The RemoveAt method implementation is quite slow  and will copy the entire underlying array of data into a new array  This means reverse iteration is useless for List    If you are stuck implementing this in a the pre c  3 0 era  You have 2 options     The easily maintainable option  Copy all the matching items into a new list and and swap the underlying list     Eg    List lt int gt  list2   new List lt int gt       foreach  int i in GetList          if    i   2    0                 list2 Add i           list2   list2    Or    The tricky slightly faster option  which involves shifting all the data in the list down when it does not match and then resizing the array     If you are removing stuff really frequently from a list  perhaps another structure like a HashTable   net 1 1  or a Dictionary   net 2 0  or a HashSet   net 3 5  are better suited for this purpose

User · Answer

There is another approach you can take depending on how you re using your collection  If you re downloading the assignments one time  e g   when the app runs   you could translate the collection on the fly into a hashtable where   shortname    SPRoleAssignment  If you do this  then when you want to remove an item by short name  all you need to do is remove the item from the hashtable by key   Unfortunately  if you re loading these SPRoleAssignments a lot  that obviously isn t going to be any more cost efficient in terms of time  The suggestions other people made about using Linq would be good if you re using a new version of the  NET Framework  but otherwise  you ll have to stick to the method you re using

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For a simple List structure the most efficient way seems to be using the Predicate RemoveAll implementation    Eg    workSpace RoleAssignments RemoveAll x   gt x Member Name    shortName     The reasons are     The Predicate Linq RemoveAll method is implemented in List and has access to the internal array storing the actual data  It will shift the data and resize the internal array  The RemoveAt method implementation is quite slow  and will copy the entire underlying array of data into a new array  This means reverse iteration is useless for List    If you are stuck implementing this in a the pre c  3 0 era  You have 2 options     The easily maintainable option  Copy all the matching items into a new list and and swap the underlying list     Eg    List lt int gt  list2   new List lt int gt       foreach  int i in GetList          if    i   2    0                 list2 Add i           list2   list2    Or    The tricky slightly faster option  which involves shifting all the data in the list down when it does not match and then resizing the array     If you are removing stuff really frequently from a list  perhaps another structure like a HashTable   net 1 1  or a Dictionary   net 2 0  or a HashSet   net 3 5  are better suited for this purpose

[c#] Best way to remove items from a collection

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