The problem arises from your instance of new List() . They also need a common component:
public static List<string> items = new List<string>(); public static List<double> itemsprice = new List<double>(); public static List<double> qu = new List<double>();
That is, there is no List type, but there is a generic List<T> .
For more information and examples of creating an instance of List<T> , see the MSDN documentation.
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