如何在一个方法中创建datatable，并在另一个方法中引用它？

Question

c#/oop noob这里。我正在从sql server数据库的架构创建一个datatable。我想要有一个按钮，让用户从这个模式中选择一些表并执行一些操作(例如插入行)。但是，该方法无法引用我的第一个方法的datatable -如何从另一个方法引用在第一个方法中创建的datatable？

下面是我使用的代码：

   public Form1()
    {
        /****************************************************************/
        string sqlSchema = "  SELECT TABLE_CATALOG AS tblname ";
        sqlSchema += " , TABLE_SCHEMA AS tblschema  ";
        sqlSchema += " , TABLE_NAME AS tblname  ";
        sqlSchema += " , COLUMN_NAME AS colname  ";
        sqlSchema += " , DATA_TYPE AS DATA_TYPE  ";
        sqlSchema += " , CHARACTER_MAXIMUM_LENGTH AS colsize  ";
        sqlSchema += " FROM INFORMATION_SCHEMA.COLUMNS  ";


        InitializeComponent();
        OleDbConnection cn = new OleDbConnection(@"Provider=SQLNCLI;Server=THINK\SQLExpress;Database=Northwind;Trusted_Connection=yes;");
         OleDbDataAdapter  da = new OleDbDataAdapter(sqlSchema, cn);
        DataTable dt = new DataTable();

        da.TableMappings.Add("Table","Schema");
        da.Fill(dt);
        dataGridView1.DataSource = dt;
    }
    public void sql_PopulateTable(string tablenm, int rows)
    {
        // error on this line:
        DataTable table1 = da.Tables["Schema"];


    }

我得到的错误是'da‘在当前上下文中无法识别。我是否应该将sqlSchema变量声明为静态变量？我应该如何从语法上着手呢？

谢谢--

Answer 1

您的变量是在Form1构造函数的作用域内声明的-因此，一旦构造函数退出，变量就会被销毁。

您需要创建一个类级属性或变量来包含您的信息，以便所有方法都可以访问它。

 public Form1()
    {
        /****************************************************************/
        string sqlSchema = "  SELECT TABLE_CATALOG AS tblname ";
        sqlSchema += " , TABLE_SCHEMA AS tblschema  ";
        sqlSchema += " , TABLE_NAME AS tblname  ";
        sqlSchema += " , COLUMN_NAME AS colname  ";
        sqlSchema += " , DATA_TYPE AS DATA_TYPE  ";
        sqlSchema += " , CHARACTER_MAXIMUM_LENGTH AS colsize  ";
        sqlSchema += " FROM INFORMATION_SCHEMA.COLUMNS  ";


        InitializeComponent();
        OleDbConnection cn = new OleDbConnection(@"Provider=SQLNCLI;Server=THINK\SQLExpress;Database=Northwind;Trusted_Connection=yes;");
         Form1DataAdapter = new OleDbDataAdapter(sqlSchema, cn);
        DataTable dt = new DataTable();

        da.TableMappings.Add("Table","Schema");
        da.Fill(dt);
        dataGridView1.DataSource = dt;
    }
    public void sql_PopulateTable(string tablenm, int rows)
    {
        // error on this line:
        DataTable table1 = Form1DataAdapter.Tables["Schema"];


    }

    public OleDbDataAdapter Form1DataAdapter { get; set; }

Answer 2

为什么不使用GetSchema呢？

 SqlConnection connection =
    new SqlConnection(connectionString);
connection.Open();
DataTable dt = connection.GetSchema();
connection.Close();