c# - C#、SQL 2005、VS 2008 中的 "Object reference not set to an instance of an object."

Question

我正在尝试从 SQL Server 2005 的“t_Food”表中选择“food_ItemName”和“food_UnitPrice”。

我有以下代码:

private void GetDatabaseConnection()
{
    string connectionString = @"Server = RZS-F839AD139AA\SQLEXPRESS; Integrated Security = SSPI; Database = HotelCustomerManagementDatabase";
    connection = new SqlConnection(connectionString);
    connection.Open();
}

还有......

public Food PopulateFoodItemListview()
{
    GetDatabaseConnection();
    string selectFoodItemQuery = @"SELECT food_ItemName, food_UnitPrice FROM t_Food";
    SqlCommand command = new SqlCommand(selectFoodItemQuery, connection);
    SqlDataReader reader = command.ExecuteReader();

    while (reader.Read())
    {
        food.ItemName.Add(reader.GetString(0));  // Exception is generated in this line
        food.UnitPrice.Add(reader.GetDouble(1));
    }
    connection.Close();
    return food;
}

在食品类中，我有以下代码:

public class Food
{
    private List<string> itemName;
    private List<double> unitPrice;
    private double itemUnit;
    private Customer foodCustomer = new Customer();

    public Food ()
    {
    }

    public Food(List<string> itemName, List<double> unitPrice) : this()
    {
        this.itemName = itemName;
        this.unitPrice = unitPrice;
    }

    public List<string> ItemName
    {
        get { return itemName; }
        set { itemName = value ; }
    }

    public List<double> UnitPrice
    {
        get { return unitPrice; }
        set { unitPrice = value; }
    }

    public double ItemUnit
    {
        get { return itemUnit; }
        set { itemUnit = value; }
    }
}

但它生成以下异常。为什么？

“未将对象引用设置为对象的实例。”

Answer 1

您需要先创建“itemName”和“unitPrice”变量的实例，然后才能添加到它们中的任何一个。您可以在声明它们时继续执行此操作。

private List<string> itemName = new List<string>();
private List<double> unitPrice = new List<string>();