LINQ - For each item in select populate value to one unpopulated property

Question

I'm using my Map method to create DTO object from my context class Company and Mapping looks like this:

private CompDTO Map(Company company)
{
    return new CompDTO()
    {
        Id = company.Id,
        Title = company.Title,
        ParentCompanyId = company.ParentCompanyId,
    };
} 

CompDTO looks like this:

public class CompDTO
{
    public long Id { get; set; }
    public string Title { get; set; }
    public long? ParentCompanyId { get; set; }
    public bool HasChildrens { get; set; }
}

Here's how I'm calling my Map method :

private IEnumerable<CompDTO> Map(IEnumerable<Company> companies)
{

    var result = companies.Select(c => Map(c));

     return result.Select(c => { c.HasChildrens = companies.Any(cc => cc.ParentCompanyId == c.Id)});
 }

After main mapping I'm trying to populate HasChildrens property in my return result.Select for each compDTO item.

But it doesn't work cuz it says:

But I guess there's more deeper issue because I added simply test like this:

return result.Select(c => { c.HasChildrens = true; }); 

And it said:The type arguments for method cannot be infered from usage.

Any kind of help would be awesome

Answer 1

The IEnumerable Select is supposed to create a new sequence from the input. If you want to just change a property using Select as it were a foreach then you need to return the object passed to your lambda

 return result.Select(c => 
               { 
                   c.HasChildrens = companies.Any(cc => cc.ParentCompanyId == c.Id);
                   return c;
               });

But do you really prefer this approach to a simple For Loop? I find this more clear

foreach(Company c in result)
    c.HasChildrens = companies.Any(cc => cc.ParentCompanyId == c.Id);

return result;