public void NewProdct_Click(object sender, EventArgs e)
{
    string productname;
    string warenumber;
    string Price_;

    productname = ProdNamn.Text;
    warenumber = Varu_Nr.Text;
    Price_ = pris_.Text;

    List<produkt> List = new List<produkt>();

    if(List.Any( produkt => produkt.Name == productname || produkt.Serialnumber == warenumber))
    {
        MessageBox.Show("Product with that name or warenumber");
        return;
    }

    var newProdukt = new produkt()
    {
        Name = productname,
        Serialnumber = warenumber,
        price = Price_
    };
    string random;
    random = newProdukt.Name + newProdukt.price + newProdukt.Serialnumber ;

    List.Add(newProdukt);
    Console.WriteLine(newProdukt);
}