Question

var ID, x,y

switch(ItemTypeNo)
{

   case ItemType.A : 

    ID = from s in cntx.Tablo1
break;


   case ItemType.B : 

    ID = from s in cntx.Tablo2
break;


   case ItemType.C : 

    ID = from s in cntx.Tablo3
break;

}

Answer 1

You can only use var for inline initialization, which that isn’t. You would need to type ID appropriately, and since var has a single type that would only work if Tablo1, Tablo2 and Tablo3 are the same type (which seems unlikely).

What is it you need to do here?

There is a scenario that works here; when selecting a common type from each; let’s assume thay all have an int primary key:

IQueryable<int> ids;    
switch(ItemTypeNo)
{    
   case ItemType.A : ids= from s in cntx.Tablo1 select s.Id; break;
   case ItemType.B : ids= from s in cntx.Tablo2 select s.Id; break;
   case ItemType.C : ids= from s in cntx.Tablo3 select s.Id; break;
   default: throw new InvalidOperationException();
}

However, in the general case… not so much. You could type ID as the non-generic IQueryable, but to be honest that doesn’t let you do very many interesting things. And dynamic doesn’t play nicely with LINQ (and even if it did, it would be a hack here).