So I have a Query that searches for various items. But I just want their Id so I used a projection to return me only the Ids and not the other elements of the item. but converting from ObjectId .ToString() and then .ToList() returns me a List<char> instead List<string>

var items = await this.ItemAppService.GetAllAsync(expression,
               x => new
               {
                  Ids = x.Id.ToString().ToList(),
               });
           var Ids = items.SelectMany(x => x.Ids.Select(x => x)).ToList();

I would like to understand why i'm returning a List<Char> and how I convert it to List<String>

CodePudding user response：

.ToString().ToList() returns a New List<char>

Yes, because a string is an IEnumerable<char> (a bunch of characters that can be enumerated over). ToList is an extension method on all IEnumerable<T>, that returns a List<T>.

So the Ids property in the anonymous object is already a List<char>:

x => new
{
   // this "Ids" is already a List<char>
   Ids = x.Id.ToString().ToList(),
});

You then do some more shuffling on it, but not meaningfully changing anything. x.Ids.Select(x => x) returns an Innumerable<char> with the same contents as x.Ids. And SelectMany adds up all those IEnumerable<char> in each of the anonymous objects into one big IEnumerable<char>, which you then convert to a list.

I'm not sure why you have used an anonymous object here. If you just want a List<string> with all the IDs, just do:

var ids = await this.ItemAppService.GetAllAsync(
              expression,
              // assuming x.Id is not already a string
              x => x.Id.ToString()
          ).ToList();

CodePudding user response：

The first ToList is unnecessary, you want strings, and with that ToList() invocation you are converting your strings to char arrays. So the code should be rewritten as:

var items = await this.ItemAppService.GetAllAsync(expression,
    x => new
    {
        Id = x.Id.ToString(),
    });
var ids = items.Select(x => x.Id).ToList();