var query=db.PC.GroupBy(pc=>new{pc.Product.PID,pc.Product.PName}).ToList()
.Select(g=>new {PID=g.Key.PID,PName=g.Key.PName,CName=string.Join(",",g.Select(x=>x.Categroy.CName).ToArray())});
var query=db.PC.GroupBy(pc=>pc.Product.PID)
.Select(g=>new {PID=g.Key,PName=g.First().Product.PName,CName=string.Join(",",g.Select(x=>x.Categroy.CName).ToArray())});