C# Return Calculated Field in Model

I want ExtendedPrice to be calculated from values of two fields below.  I am getting below errors:

"A field initializer cannot reference the non-static field, method, or property 'MyApp.Models.ItemsModel.UnitPrice.get"
"A field initializer cannot reference the non-static field, method, or property 'MyApp.Models.ItemsModel.Total.get"

 public class ItemsModel
    {
        public string CustomerName { get; set; }
        public string PO { get; set; }
        public string DateOrder { get; set; }
        public string Total { get; set; }
        public string UnitPrice { get; set; }
        public string extendedPrice = Convert.ToDouble(UnitPrice) * Convert.ToDouble(Total);
    }

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How do I return a calculated field in the model.