How to compare two list and add all list to new list ASP.Net MVC 4 using linq

Question

I have two list
List A

List<test> populate = new List<test>();
{
  populate.Add(new test(){ID = 1, name="AAA", nameID=1, type=1, isSelected=false});
  populate.Add(new test(){ID = 2, name="BBB", nameID=2, type=1, isSelected=false});
  populate.Add(new test(){ID = 3, name="CCC", nameID=3, type=1, isSelected=false});
}

List B

    List<build> populateBuild = new List<build>();
{
  populateBuild.Add(new test(){ID = 1, name="AAA", nameID=1, type=1, isSelected=false});
  populateBuild.Add(new test(){ID = 3, name="CCC", nameID=3, type=1, isSelected=false});
}

What I want to achieve is:
1) I want new list, (List C)

2) In List C, I want all the data in List A, but the value of isSelected in List A, will changed to TRUE, when it is compared to data in List B

3) Means, if List B is exist in list A, the value of isSelected in list A will changed to TRUE and added to list C

4) if List B is not exist in List A, it will still be added to List C, but without changing the isSelected value(remain false).

Thank you,

Answer 1

I assume you mean List<test> populateBuild = new List<test>(); (not List<build>). You can generate the 3rd list using

// Get the ID's of the 2nd list
IEnumerable<int> populateBuildIds = populateBuild.Select(x => x.ID);
// Initialize the 3rd list
List<test> listC = new List<test>();
// Copy all elements from the first list and update the isSelected property
foreach (test t in populate)
{
    listC.Add(new test()
    {
        ID = t.ID, 
        name = t.name, 
        nameID = t.nameID, 
        type = t.type, 
        isSelected = populateBuildIds.Contains(t.ID) // true if also in 2nd list
    });
}