c# - 尝试在数据集中的表之间创建关系时出现异常 : System. NullReferenceException

Question

使用 C#、.net 框架 4.5、VS 2012

尝试在数据集中的表之间创建简单关系，但出现 System.NullReferenceException，正如我在 MSDN 上看到的，这意味着 当您尝试在代码中引用不存在的对象时发生。但是，认为我创建了所有必需的对象。

我的代码如下:

    //create place for storing all tables from data base
    private DataSet myDS = new DataSet("AutoLot");

    //command builders for easy way access to tables
    private SqlCommandBuilder sqlCInventory;
    private SqlCommandBuilder sqlCOrders;
    private SqlCommandBuilder sqlCCustomers;

    //adapters for each table
    private SqlDataAdapter sqlAInventory;
    private SqlDataAdapter sqlAOrders;
    private SqlDataAdapter sqlACustomers;

    //connection string
    private string cnStr = string.Empty;

    public MainForm()
    {
        InitializeComponent();
        //get connection string from .config file
        cnStr = 
            ConfigurationManager.ConnectionStrings["AutoLotSqlProvider"].ConnectionString;
        //create adapters
        sqlACustomers = new SqlDataAdapter("Select * From Customers", cnStr);
        sqlAInventory = new SqlDataAdapter("Select * From Inventory", cnStr);
        sqlAOrders = new SqlDataAdapter("Select * From Orders", cnStr);

        //automatic generate commands 
        sqlCCustomers = new SqlCommandBuilder(sqlACustomers);
        sqlCInventory = new SqlCommandBuilder(sqlAInventory);
        sqlCOrders = new SqlCommandBuilder(sqlAOrders);

        //add table to data Set
        sqlAInventory.Fill(myDS);
        sqlAOrders.Fill(myDS);
        sqlACustomers.Fill(myDS);

        //create relationship between tables
        BuildTableRelationShip();

        //create DataSourse for datGrids on UI
        dataGridViewCustomer.DataSource = myDS.Tables["Inventory"];
        dataGridViewOrders.DataSource = myDS.Tables["Orders"];
        dataGridViewCustomer.DataSource = myDS.Tables["Customers"];
    }

这里我遇到了异常

    private void BuildTableRelationShip()
    {
        //create object of relationShips
        DataRelation dr = new DataRelation("CustomersOrders", //name of relation
            myDS.Tables["Customers"].Columns["CustID"], //main columns
            myDS.Tables["Orders"].Columns["OrderID"]); //related columns
        myDS.Relations.Add(dr);

        //second relation
        dr = new DataRelation("InventoryOrder",
            myDS.Tables["Inventory"].Columns["CarID"],
            myDS.Tables["Orders"].Columns["OrderID"]);
        //add relations to dataset
        myDS.Relations.Add(dr);
    }

为什么我得到这个空引用异常？我想念什么？

编辑

Answer 1

您应该对单个 DataTable 而不是整个 DataSet 调用 fill。

sqlAInventory.Fill(myDS);
sqlAOrders.Fill(myDS);
sqlACustomers.Fill(myDS);

会变成

sqlAInventory.Fill(myDS, "Inventory");
sqlAOrders.Fill(myDS, "Orders");
sqlACustomers.Fill(myDS, "Customers");

此方法将自动向您的 DataSet 添加一个表(如果它不存在)，如果存在则用数据填充它。 MSDN有关于这种填充方法的更多信息。