Question

Below my code. It returns exception “InvalidCastException”. And main question is – why?
What is wrong?

Error text:

Unable to cast object of type
‘WhereSelectListIterator`2[Monopolowy_beta.Gracz,Monopolowy_beta.Gracz]’
to type ‘Monopolowy_beta.Gracz’.

namespace Monopolowy_beta
{
    class Program
    {
        static void Main(string[] args)
        {
            List<Gracz> lista = new List<Gracz> { };
            Gracz g1 = new Gracz();
            Gracz g2 = new Gracz();
            Gracz g3 = new Gracz();
            g2.Id = 3;

            lista.Add(g1);
            lista.Add(g2);
            lista.Add(g3);

            g1 = GraczeTools.UstawAktywnegoGracza(lista, 3);


            Console.ReadKey();
        }
    }
}

Error in these lines:
var docelowy = from item in listagraczy where (item.Id==ID && item.czyAktywny == true) select listagraczy[listagraczy.IndexOf(item) + 1];
gracz = (Gracz)docelowy;

namespace Monopolowy_beta
{
    static class GraczeTools
    {

        public static Gracz UstawAktywnegoGracza(List<Gracz> listagraczy, int ID)
        { 
            Gracz gracz = new Gracz();
            if (ID == 4){
                var docelowy = from item in listagraczy where (item.czyAktywny == true && item.Id == 3) select listagraczy[1];
                gracz = (Gracz)docelowy;
            }

            if (ID != 4){
                var docelowy = from item in listagraczy where (item.Id==ID && item.czyAktywny == true) select listagraczy[listagraczy.IndexOf(item) + 1];
                gracz = (Gracz)docelowy;
            }


            return gracz;
        }

    }
}

Answer 1

var docelowy = from item in listagraczy 
               where (item.czyAktywny == true && item.Id == 3) 
               select listagraczy[1];

Let’s examine this query. It finds all items which satisfy condition (yes, it will return sequence, not single item) and for each such item, it returns.. second element of listagraczy list. Yes, you don’t have items, which matched your condition.

I think you should select item instead (this a range variable of your query), and apply FirstOrDefault to result, because by default query will return IEnumerable<Gracz> result.

var docelowy = (from item in listagraczy 
                where (item.czyAktywny == true && item.Id == 3) 
                select item).FirstOrDefault();

Which is better to write with fluent API:

var docelowy = listagraczy.FirstOrDefault(item => item.czyAktywny && item.Id == 3);

Also you can use boolean values directly in conditions (i.e. item.czyAktywny instead of item.czyAktywny == true).

---

After little refactoring your method should look like

public static Gracz UstawAktywnegoGracza(List<Gracz> listagraczy, int ID)
{ 
   return listagraczy
           .FirstOrDefault(item => item.Id == 3 && (ID != 4 || item.czyAktywny));
}

How it works:

You have two conditional blocks in your method if (ID == 4) and if (ID != 4) (which is actually if ... else. Difference is that you are filtering sequence by one more condition in first case – item.czyAktywny should be true. In second case this property does not matter. So, you can add one filtering condition instead (ID != 4 || item.czyAktywny) – czyAktywny will be verified only if ID equal to 4. Also you don’t need to create new Gracz object in your method, because you anyway return one from passed list.