How do you concatenate Lists in C

Question

If I have   List lt string gt  myList1  List lt string gt  myList2   myList1   getMeAList       Checked myList1  it contains 4 strings  myList2   getMeAnotherList       Checked myList2  it contains 6 strings  myList1 Concat myList2      Checked mylist1  it contains 4 strings    why    I ran code similar to this in Visual Studio 2008 and set break points after each execution  After myList1   getMeAList     myList1 contains four strings  and I pressed the plus button to make sure they weren t all nulls   After  myList2   getMeAnotherList     myList2 contains six strings  and I checked to make sure they weren t null    After myList1 Concat myList2   myList1 contained only four strings  Why is that

User · Answer

It also worth noting that Concat works in constant time and in constant memory. For example, the following code

        long boundary = 60000000;
        for (long i = 0; i < boundary; i++)
        {
            list1.Add(i);
            list2.Add(i);
        }
        var listConcat = list1.Concat(list2);
        var list = listConcat.ToList();
        list1.AddRange(list2);

gives the following timing/memory metrics:

After lists filled mem used: 1048730 KB
concat two enumerables: 00:00:00.0023309 mem used: 1048730 KB
convert concat to list: 00:00:03.7430633 mem used: 2097307 KB
list1.AddRange(list2) : 00:00:00.8439870 mem used: 2621595 KB

User · Answer

targetList   list1 Concat list2  ToList      It s working fine I think so  As previously said  Concat returns a new sequence and while converting the result to List  it does the job perfectly

User · Answer

Try this   myList1   myList1 Concat myList2  ToList      Concat returns an IEnumerable lt T  that is the two lists put together  it doesn t modify either existing list   Also  since it returns an IEnumerable  if you want to assign it to a variable that is List lt T   you ll have to call ToList   on the IEnumerable lt T  that is returned

User · Answer

Take a look at my implementation  It s safe from null lists    IList lt string gt  all  new List lt string gt       if  letterForm SecretaryPhone  null    first list may be null      all all Concat letterForm SecretaryPhone  ToList      if  letterForm EmployeePhone    null    second list may be null      all  all Concat letterForm EmployeePhone  ToList       if  letterForm DepartmentManagerName    null     this is not list  its just string variable  so wrap it inside list then concat it       all   all Concat new    letterForm DepartmentManagerPhone   ToList

User · Answer

Concat returns a new sequence without modifying the original list  Try myList1 AddRange myList2

User · Answer

I know this is old but I came upon this post quickly thinking Concat would be my answer  Union worked great for me  Note  it returns only unique values but knowing that I was getting unique values anyway this solution worked for me    namespace TestProject       public partial class Form1  Form               public Form1                         InitializeComponent                 List lt string gt  FirstList   new List lt string gt                 FirstList Add  1234                FirstList Add  4567                    In my code  I know I would not have this here but I put it in as a demonstration that it will not be in the secondList twice             FirstList Add  Three                   List lt string gt  secondList   GetList FirstList                           foreach  string item in secondList                  Console WriteLine item                      private List lt String gt  GetList List lt string gt  SortBy                        List lt string gt  list   new List lt string gt                 list Add  One                list Add  Two                list Add  Three                 list   list Union SortBy  ToList                 return list                      The output is   One Two Three 1234 4567

[c#] How do you concatenate Lists in C#?

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