Filtering lists using LINQ

Question

I ve got a list of People that are returned from an external app and I m creating an exclusion list in my local app to give me the option of manually removing people from the list     I have a composite key which I have created that is common to both and I want to find an efficient way of removing people from my List using my List   e g   class Person       prop string compositeKey   get  set       class Exclusions       prop string compositeKey   get  set       List lt Person gt  people   GetFromDB   List lt Exclusions gt  exclusions   GetFromOtherDB   List lt Person gt  filteredResults   People - exclustions using the composite key as a comparer   I thought LINQ was the ideal way of doing this but after trying joins  extension methods  using yields  etc  I m still having trouble    If this were SQL I would use a not in         query

User · Answer

This LINQ below will generate the SQL for a left outer join and then take all of the results that don't find a match in your exclusion list.

List<Person> filteredResults =from p in people
        join e in exclusions on p.compositeKey equals e.compositeKey into temp
        from t in temp.DefaultIfEmpty()
        where t.compositeKey == null
        select p

let me know if it works!

User · Answer

Many thanks for this guys    I mangaged to get this down to one line     var results   from p in People                  where   from e in exclusions                          select e CompositeKey  Contains p CompositeKey                   select p    Thanks again everyone

User · Answer

I would just use the FindAll method on the List class  i e    List lt Person gt  filteredResults        people FindAll p   gt  return  exclusions Contains p      Not sure if the syntax will exactly match your objects  but I think you can see where I m going with this

User · Answer

var thisList   new List lt string gt    a    b    c     var otherList   new List lt string gt    a    b     var theOnesThatDontMatch   thisList          Where item  gt  otherList All otherItem  gt  item    otherItem            ToList     var theOnesThatDoMatch   thisList          Where item  gt  otherList Any otherItem  gt  item    otherItem            ToList     Console WriteLine  don t match   0    string Join      theOnesThatDontMatch    Console WriteLine  do match   0    string Join      theOnesThatDoMatch       Output    don t match  c   do match  a b   Adapt the list types and lambdas accordingly  and you can filter out anything   https   dotnetfiddle net 6bMCvN

User · Answer

Have a look at the Except method  which you use like this   var resultingList        listOfOriginalItems Except listOfItemsToLeaveOut  equalityComparer    You ll want to use the overload I ve linked to  which lets you specify a custom IEqualityComparer   That way you can specify how items match based on your composite key    If you ve already overridden Equals  though  you shouldn t need the IEqualityComparer    Edit   Since it appears you re using two different types of classes  here s another way that might be simpler   Assuming a List lt Person gt  called persons and a List lt Exclusion gt  called exclusions   var exclusionKeys            exclusions Select x   gt  x compositeKey   var resultingPersons            persons Where x   gt   exclusionKeys Contains x compositeKey      In other words   Select from exclusions just the keys  then pick from persons all the Person objects that don t have any of those keys

User · Answer

I couldn t figure out how to do this in pure MS LINQ  so I wrote my own extension method to do it   public static bool In lt T gt  this T objToCheck  params T   values        if  values    null    values Length    0                 return false    early out           else               foreach  T t in values                        if  t Equals objToCheck                   return true      RETURN found                     return false    nothing found

User · Answer

You can use the  Except  extension method  see http   msdn microsoft com en-us library bb337804 aspx   In your code  var difference   people Except exclusions

User · Answer

This LINQ below will generate the SQL for a left outer join and then take all of the results that don't find a match in your exclusion list.

List<Person> filteredResults =from p in people
        join e in exclusions on p.compositeKey equals e.compositeKey into temp
        from t in temp.DefaultIfEmpty()
        where t.compositeKey == null
        select p

let me know if it works!

User · Answer

This LINQ below will generate the SQL for a left outer join and then take all of the results that don't find a match in your exclusion list.

List<Person> filteredResults =from p in people
        join e in exclusions on p.compositeKey equals e.compositeKey into temp
        from t in temp.DefaultIfEmpty()
        where t.compositeKey == null
        select p

let me know if it works!

User · Answer

Have a look at the Except method  which you use like this   var resultingList        listOfOriginalItems Except listOfItemsToLeaveOut  equalityComparer    You ll want to use the overload I ve linked to  which lets you specify a custom IEqualityComparer   That way you can specify how items match based on your composite key    If you ve already overridden Equals  though  you shouldn t need the IEqualityComparer    Edit   Since it appears you re using two different types of classes  here s another way that might be simpler   Assuming a List lt Person gt  called persons and a List lt Exclusion gt  called exclusions   var exclusionKeys            exclusions Select x   gt  x compositeKey   var resultingPersons            persons Where x   gt   exclusionKeys Contains x compositeKey      In other words   Select from exclusions just the keys  then pick from persons all the Person objects that don t have any of those keys

User · Answer

I couldn t figure out how to do this in pure MS LINQ  so I wrote my own extension method to do it   public static bool In lt T gt  this T objToCheck  params T   values        if  values    null    values Length    0                 return false    early out           else               foreach  T t in values                        if  t Equals objToCheck                   return true      RETURN found                     return false    nothing found

User · Answer

You can use the  Except  extension method  see http   msdn microsoft com en-us library bb337804 aspx   In your code  var difference   people Except exclusions

User · Answer

I couldn t figure out how to do this in pure MS LINQ  so I wrote my own extension method to do it   public static bool In lt T gt  this T objToCheck  params T   values        if  values    null    values Length    0                 return false    early out           else               foreach  T t in values                        if  t Equals objToCheck                   return true      RETURN found                     return false    nothing found

User · Answer

You can use the  Except  extension method  see http   msdn microsoft com en-us library bb337804 aspx   In your code  var difference   people Except exclusions

User · Answer

I would just use the FindAll method on the List class  i e    List lt Person gt  filteredResults        people FindAll p   gt  return  exclusions Contains p      Not sure if the syntax will exactly match your objects  but I think you can see where I m going with this

User · Answer

var result   Data Where x   gt                            bool condition   true              double accord    double x Table Columns IndexOf FiltercomboBox Text                return condition  amp  amp  accord  gt   double Parse FilterLowertextBox Text   amp  amp  accord  lt   double Parse FilterUppertextBox Text

User · Answer

I would do something like this but i bet there is a simpler way  i think the sql from linqtosql would use a select from person Where NOT EXIST select from your exclusion list   static class Program       public class Person               public string Key   get  set            public Person string key                       Key   key                      public class NotPerson               public string Key   get  set            public NotPerson string key                       Key   key                      static void Main                 List lt Person gt  persons   new List lt Person gt                         new Person   1               new Person   2               new Person   3               new Person   4                     List lt NotPerson gt  notpersons   new List lt NotPerson gt                         new NotPerson   3               new NotPerson   4                     var filteredResults   from n in persons                              where  notpersons Any y   gt  n Key    y Key                               select n          foreach  var item in filteredResults                     Console WriteLine item Key

User · Answer

I would just use the FindAll method on the List class  i e    List lt Person gt  filteredResults        people FindAll p   gt  return  exclusions Contains p      Not sure if the syntax will exactly match your objects  but I think you can see where I m going with this

User · Answer

I would do something like this but i bet there is a simpler way  i think the sql from linqtosql would use a select from person Where NOT EXIST select from your exclusion list   static class Program       public class Person               public string Key   get  set            public Person string key                       Key   key                      public class NotPerson               public string Key   get  set            public NotPerson string key                       Key   key                      static void Main                 List lt Person gt  persons   new List lt Person gt                         new Person   1               new Person   2               new Person   3               new Person   4                     List lt NotPerson gt  notpersons   new List lt NotPerson gt                         new NotPerson   3               new NotPerson   4                     var filteredResults   from n in persons                              where  notpersons Any y   gt  n Key    y Key                               select n          foreach  var item in filteredResults                     Console WriteLine item Key

User · Answer

This LINQ below will generate the SQL for a left outer join and then take all of the results that don't find a match in your exclusion list.

List<Person> filteredResults =from p in people
        join e in exclusions on p.compositeKey equals e.compositeKey into temp
        from t in temp.DefaultIfEmpty()
        where t.compositeKey == null
        select p

let me know if it works!

User · Answer

You can use the  Except  extension method  see http   msdn microsoft com en-us library bb337804 aspx   In your code  var difference   people Except exclusions

User · Answer

Have a look at the Except method  which you use like this   var resultingList        listOfOriginalItems Except listOfItemsToLeaveOut  equalityComparer    You ll want to use the overload I ve linked to  which lets you specify a custom IEqualityComparer   That way you can specify how items match based on your composite key    If you ve already overridden Equals  though  you shouldn t need the IEqualityComparer    Edit   Since it appears you re using two different types of classes  here s another way that might be simpler   Assuming a List lt Person gt  called persons and a List lt Exclusion gt  called exclusions   var exclusionKeys            exclusions Select x   gt  x compositeKey   var resultingPersons            persons Where x   gt   exclusionKeys Contains x compositeKey      In other words   Select from exclusions just the keys  then pick from persons all the Person objects that don t have any of those keys

User · Answer

var result   Data Where x   gt                            bool condition   true              double accord    double x Table Columns IndexOf FiltercomboBox Text                return condition  amp  amp  accord  gt   double Parse FilterLowertextBox Text   amp  amp  accord  lt   double Parse FilterUppertextBox Text

User · Answer

I would do something like this but i bet there is a simpler way  i think the sql from linqtosql would use a select from person Where NOT EXIST select from your exclusion list   static class Program       public class Person               public string Key   get  set            public Person string key                       Key   key                      public class NotPerson               public string Key   get  set            public NotPerson string key                       Key   key                      static void Main                 List lt Person gt  persons   new List lt Person gt                         new Person   1               new Person   2               new Person   3               new Person   4                     List lt NotPerson gt  notpersons   new List lt NotPerson gt                         new NotPerson   3               new NotPerson   4                     var filteredResults   from n in persons                              where  notpersons Any y   gt  n Key    y Key                               select n          foreach  var item in filteredResults                     Console WriteLine item Key

User · Answer

Many thanks for this guys    I mangaged to get this down to one line     var results   from p in People                  where   from e in exclusions                          select e CompositeKey  Contains p CompositeKey                   select p    Thanks again everyone

User · Answer

Have a look at the Except method  which you use like this   var resultingList        listOfOriginalItems Except listOfItemsToLeaveOut  equalityComparer    You ll want to use the overload I ve linked to  which lets you specify a custom IEqualityComparer   That way you can specify how items match based on your composite key    If you ve already overridden Equals  though  you shouldn t need the IEqualityComparer    Edit   Since it appears you re using two different types of classes  here s another way that might be simpler   Assuming a List lt Person gt  called persons and a List lt Exclusion gt  called exclusions   var exclusionKeys            exclusions Select x   gt  x compositeKey   var resultingPersons            persons Where x   gt   exclusionKeys Contains x compositeKey      In other words   Select from exclusions just the keys  then pick from persons all the Person objects that don t have any of those keys

User · Answer

Many thanks for this guys    I mangaged to get this down to one line     var results   from p in People                  where   from e in exclusions                          select e CompositeKey  Contains p CompositeKey                   select p    Thanks again everyone

User · Answer

I would just use the FindAll method on the List class  i e    List lt Person gt  filteredResults        people FindAll p   gt  return  exclusions Contains p      Not sure if the syntax will exactly match your objects  but I think you can see where I m going with this

User · Answer

Many thanks for this guys    I mangaged to get this down to one line     var results   from p in People                  where   from e in exclusions                          select e CompositeKey  Contains p CompositeKey                   select p    Thanks again everyone

User · Answer

I couldn t figure out how to do this in pure MS LINQ  so I wrote my own extension method to do it   public static bool In lt T gt  this T objToCheck  params T   values        if  values    null    values Length    0                 return false    early out           else               foreach  T t in values                        if  t Equals objToCheck                   return true      RETURN found                     return false    nothing found

User · Answer

I would do something like this but i bet there is a simpler way  i think the sql from linqtosql would use a select from person Where NOT EXIST select from your exclusion list   static class Program       public class Person               public string Key   get  set            public Person string key                       Key   key                      public class NotPerson               public string Key   get  set            public NotPerson string key                       Key   key                      static void Main                 List lt Person gt  persons   new List lt Person gt                         new Person   1               new Person   2               new Person   3               new Person   4                     List lt NotPerson gt  notpersons   new List lt NotPerson gt                         new NotPerson   3               new NotPerson   4                     var filteredResults   from n in persons                              where  notpersons Any y   gt  n Key    y Key                               select n          foreach  var item in filteredResults                     Console WriteLine item Key

User · Answer

var thisList   new List lt string gt    a    b    c     var otherList   new List lt string gt    a    b     var theOnesThatDontMatch   thisList          Where item  gt  otherList All otherItem  gt  item    otherItem            ToList     var theOnesThatDoMatch   thisList          Where item  gt  otherList Any otherItem  gt  item    otherItem            ToList     Console WriteLine  don t match   0    string Join      theOnesThatDontMatch    Console WriteLine  do match   0    string Join      theOnesThatDoMatch       Output    don t match  c   do match  a b   Adapt the list types and lambdas accordingly  and you can filter out anything   https   dotnetfiddle net 6bMCvN

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