LINQ Select an object and change some properties without creating a new object

Question

I want to change some properties of a LINQ query result object without creating a new object and manually setting every property   Is this possible   Example   var list   from something in someList            select x    but change one property

User · Answer

I prefer this one  It can be combined with other linq commands   from item in list let xyz   item PropertyToChange   calcValue   select item

User · Answer

In 2020 I use the MoreLinq Pipe method   https   morelinq github io 2 3 ref api html M MoreLinq MoreEnumerable Pipe  1 htm

User · Answer

var item    from something in someList        select x  firstordefault      Would get the item  and then you could do item prop1 5  to change the specific property   Or are you wanting to get a list of items from the db and have it change the property prop1 on each item in that returned list to a specified value  If so you could do this  I m doing it in VB because I know it better    dim list   from something in someList select x for each item in list     item prop1 5 next    list will contain all the items returned with your changes

User · Answer

It is not possible with the standard query operators - it is Language Integrated Query  not Language Integrated Update  But you could hide your update in extension methods   public static class UpdateExtension       public static IEnumerable lt Car gt  ChangeColorTo         this IEnumerable lt Car gt  cars  Color color               foreach  Car car in cars                     car Color   color            yield return car                     Now you can use it as follows   cars Where car   gt  car Color    Color Blue  ChangeColorTo Color Red

User · Answer

If you just want to update the property on all elements then   someList All x   gt    x SomeProp    foo   return true

User · Answer

There shouldn t be any LINQ magic keeping you from doing this  Don t use projection though that ll return an anonymous type   User u   UserCollection FirstOrDefault u   gt  u Id    1   u FirstName    Bob    That will modify the real object  as well as   foreach  User u in UserCollection Where u   gt  u Id  gt  10        u Property   SomeValue

User · Answer

We often run into this where we want to include a the index value and first and last indicators in a list without creating a new object   This allows you to know the position of the item in your list  enumeration  etc  without having to modify the existing class and then knowing whether you re at the first item in the list or the last   foreach  Item item in this Items      Select  x  i    gt        x ListIndex   i      x IsFirst   i    0      x IsLast   i    this Items Count-1      return x        You can simply extend any class by using   public abstract class IteratorExtender       public int ListIndex   get  set        public bool IsFirst   get  set         public bool IsLast   get  set        public class Item   IteratorExtender

User · Answer

If you want to update items with a Where clause  using a  Where      will truncate your results if you do   mylist   mylist Where n   gt  n Id    ID  Select n   gt    n Property       return n     ToList      You can do updates to specific item s  in the list like so   mylist   mylist Select n   gt    if  n Id    ID    n Property         return n     ToList      Always return item even if you don t make any changes  This way it will be kept in the list

User · Answer

User u   UserCollection Single u   gt  u Id    1   u FirstName    Bob

User · Answer

I m not sure what the query syntax is   But here is the expanded LINQ expression example   var query   someList Select x   gt    x SomeProp    foo   return x       What this does is use an anonymous method vs and expression   This allows you to use several statements in one lambda   So you can combine the two operations of setting the property and returning the object into this somewhat succinct method

User · Answer

Since I didn  t find the answer here which I consider the best solution  here my way   Using  Select  to modify data is possible  but just with a trick  Anyway   Select  is not made for that  It just executes the modification when used with  ToList   because Linq doesn  t execute before the data is being needed  Anyway  the best solution is using  foreach   In the following code  you can see       class Person               public int Age             class Program               private static void Main string   args                        var persons   new List lt Person gt  new    new Person  Age   20   new Person  Age   22                 PrintPersons persons                  this doesn t work              persons Select p   gt                                p Age                    return p                              PrintPersons persons                  with  ToList  it works             persons Select p   gt                                p Age                    return p                 ToList                PrintPersons persons                  This is the best solution             persons ForEach p   gt                                p Age                                PrintPersons persons               Console ReadLine                       private static void PrintPersons List lt Person gt  persons                        Console WriteLine                                  foreach  var person in persons                                Console WriteLine  Age   0    person Age                                                 Before  foreach   you can also make a linq selection

[c#] LINQ: Select an object and change some properties without creating a new object

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