Update a variable in list from another list using c# linq

Question

I want to copy data from one list to another list. While copying to the new list, i want to assign new values also. Can someone please assist quickly. I want to achieve something like below,

public HashSet<string> MobileNumber { get; set; }        
var contactViewData = MobileNumber.Select(p => p).SelectMany(o => o.SplitFromCsv()).Select(x => new ContactViewMap()
{
     ContactId = Guid.NewGuid().ToString(),
     NewMobileNumber = x,
     OldMobileNumber = p
}).ToList();

public class ContactViewMap
{
    public string ContactId { get; set; }
    public string NewMobileNumber { get; set; }
    public string OldMobileNumber { get; set; }
}


public static IEnumerable<string> SplitFromCsv(this string csv)
            => csv.Split(',', StringSplitOptions.None);

Input:
{
  "MobileNumber": [
    "+9112352",
    "+9112353"
  ]
}

Answer 1

Select(p => p) Just returns the same item again.

I think instead of this: Select(p => p).SelectMany(o => o.SplitFromCsv())
You need this: Select(p => (p, csv: o.SplitFromCsv()) )

Now you can use that tuple in the final Select:

.Select(x => new ContactViewMap()
{
     ContactId = Guid.NewGuid().ToString(),
     NewMobileNumber = x.csv,
     OldMobileNumber = x.p
})