Merge two or more lists into one in C NET

Question

Is it possible to convert two or more lists into one single list  in  NET using C    For example    public static List lt Product gt  GetAllProducts int categoryId                var productCollection1   GetAllProducts CategoryId1   var productCollection2   GetAllProducts CategoryId2   var productCollection3   GetAllProducts CategoryId3

User · Answer

Assuming you want a list containing all of the products for the specified category-Ids  you can treat your query as a projection followed by a flattening operation  There s a LINQ operator that does that  SelectMany      implicitly List lt Product gt  var products   new     CategoryId1  CategoryId2  CategoryId3                         SelectMany id   gt  GetAllProducts id                         ToList      In C  4  you can shorten the SelectMany to   SelectMany GetAllProducts   If you already have lists representing the products for each Id  then what you need is a concatenation  as others point out

User · Answer

You could use the Concat extension method   var result   productCollection1      Concat productCollection2       Concat productCollection3       ToList

User · Answer

You need to use Concat operation

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To merge or Combine to Lists into a One list    There is one thing that must be true  the type of both list will be equal  For Example  if we have list of string so we can add add another list to the existing list which have list of type string otherwise we can t    Example   class Program      static void Main string   args             List lt string gt  CustomerList One   new List lt string gt                     James             Scott             Mark             John             Sara             Mary             William             Broad             Ben             Rich             Hack             Bob                  List lt string gt  CustomerList Two   new List lt string gt                     Perter             Parker             Bond             been             Bilbo             Cooper                     Adding all contents of CustomerList Two to CustomerList One        CustomerList One AddRange CustomerList Two             Creating another Listlist and assigning all Contents of CustomerList One        List lt string gt  AllCustomers   new List lt string gt            foreach  var item in CustomerList One                   AllCustomers Add item                     Removing CustomerList One  amp  CustomerList Two        CustomerList One   null         CustomerList Two   null           CustomerList One  amp  CustomerList Two --  Garbage Collected        GC Collect           Console WriteLine  Total No  of Customers        AllCustomers Count           Console WriteLine  -------------------------------------------------          foreach  var customer in AllCustomers                   Console WriteLine  Customer       customer                 Console WriteLine  -------------------------------------------------

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you can combine them using LINQ     list   list1 Concat list2  Concat list3  ToList      the more traditional approach of using List AddRange   might be more efficient though

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When you got few list but you don t know how many exactly  use this   listsOfProducts contains few lists filled with objects   List lt Product gt  productListMerged   new List lt Product gt      listsOfProducts ForEach q   gt  q ForEach e   gt  productListMerged Add e

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I know this is an old question I thought I might just add my 2 cents   If you have a List lt Something gt    you can join them using Aggregate  public List lt TType gt  Concat lt TType gt  params List lt TType gt    lists        var result   lists Aggregate new List lt TType gt      x  y    gt  x Concat y  ToList          return result      Hope this helps

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I would make it a little bit more simple   var products   new List lt List lt product gt  gt   item1  item2  item3   SelectMany id   gt  id  ToList      This way it is a multi dimensional List and the  SelectMany   will flatten it into a IEnumerable of product then I use the  ToList   method after

User · Answer

Have a look at List AddRange to merge Lists

User · Answer

I ve already commented it but I still think is a valid option  just test if in your environment is better one solution or the other  In my particular case  using source ForEach p   gt  dest Add p   performs better than the classic AddRange but I ve not investigated why at the low level   You can see an example code here  https   gist github com mcliment 4690433  So the option would be   var allProducts   new List lt Product gt  productCollection1 Count                                       productCollection2 Count                                       productCollection3 Count    productCollection1 ForEach p   gt  allProducts Add p    productCollection2 ForEach p   gt  allProducts Add p    productCollection3 ForEach p   gt  allProducts Add p      Test it to see if it works for you   Disclaimer  I m not advocating for this solution  I find Concat the most clear one  I just stated -in my discussion with Jon- that in my machine this case performs better than AddRange  but he says  with far more knowledge than I  that this does not make sense  There s the gist if you want to compare

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In the special case   All elements of List1 goes to a new List2    e g  a string list   List lt string gt  list2   new List lt string gt  list1     In this case  list2 is generated with all elements from list1

User · Answer

list4   list1 Concat list2  Concat list3  ToList

User · Answer

You can use the LINQ Concat and ToList methods   var allProducts   productCollection1 Concat productCollection2                                       Concat productCollection3                                       ToList      Note that there are more efficient ways to do this - the above will basically loop through all the entries  creating a dynamically sized buffer  As you can predict the size to start with  you don t need this dynamic sizing    so you could use   var allProducts   new List lt Product gt  productCollection1 Count                                       productCollection2 Count                                       productCollection3 Count   allProducts AddRange productCollection1   allProducts AddRange productCollection2   allProducts AddRange productCollection3      AddRange is special-cased for ICollection lt T gt  for efficiency    I wouldn t take this approach unless you really have to though

[c#] Merge two (or more) lists into one, in C# .NET

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