如何将 DataTable 从类传递到页面

Question

我正在学习 C# 中的类，并尝试构建一个“DataAccessClass”。我有一个正在工作的“OpenSqlConnection”空白，并使用它作为构建“OpenSqlDatareader”空白的指南。我正在尝试合并此类中的所有数据库调用。我困惑的是如何将 DataTable 从类传递到 aspx 页面以将其与 ListView 绑定。我已经阅读并尝试了一天来完成这项工作，但无法弄清楚我做错了什么...

这是 Page_Load：

// --------------------------------------------------------------------------------
// Populate the Customers page.
// --------------------------------------------------------------------------------


// Define the query
string sqlQuery = " SELECT CustomerID, LastName + ', ' + FirstName AS CustomerName, Email, City, State, Phone"
                            + " FROM Customer"
                            + " ORDER BY LastName, FirstName";

string strErrorMessage = "";

if (DataAccessClass.OpenSqlConnection(out strErrorMessage) == false)
{
    string strErrorType = "Database Connection Error:";
    SendErrorMessageToClient(strErrorType, strErrorMessage);
}

else if (DataAccessClass.OpenSqlDatareader(sqlQuery, out dt, out strErrorMessage) == false)
{
    string strErrorType = "Datareader Error:";
    SendErrorMessageToClient(strErrorType, strErrorMessage);
}

else

{
    // Bind the Listview
    lvCustomers.DataSource = dt;
    lvCustomers.DataBind();
    dt.Dispose();
}

这是类：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Configuration;
using System.Data;

/// <summary>
/// Summary description for DataAccessClass
/// </summary>
/// 
public class DataAccessClass
{
    public DataAccessClass()
{
    //
    // TODO: Add constructor logic here
    //
}



// -----------------------------------------------------------------------------------------
// Name: OpenSqlConnection
// Abstract: Open a connection to a SQL Server
// -----------------------------------------------------------------------------------------
public static bool OpenSqlConnection(out string strErrorMessage)
{

    SqlConnection theConnection = new SqlConnection();
    bool blnResult = false;
    string strConnectionString = ConfigurationManager.ConnectionStrings["MyConn"].ConnectionString;
    strErrorMessage = "";

    if (theConnection == null)
        theConnection = new SqlConnection();

    try
    {
        switch (theConnection.State)
        {
            case ConnectionState.Broken:
                theConnection.Close();
                theConnection.ConnectionString = strConnectionString;
                theConnection.Open();
                blnResult = true;
                break;

            case ConnectionState.Closed:
                theConnection.ConnectionString = strConnectionString;
                theConnection.Open();
                blnResult = true;
                break;

            case ConnectionState.Open:
                blnResult = true;
                break;

            default:
                strErrorMessage = "Connection state is " + theConnection.State.ToString();
                break;
        }
    }
    catch (Exception excError)
    {
        strErrorMessage = excError.Message;
    }

    return blnResult;
}

// -----------------------------------------------------------------------------------------
// Name: OpenSqlDataReader
// Abstract: Open a SQL DataReader
// -----------------------------------------------------------------------------------------
public static bool OpenSqlDatareader(string sqlQuery, out dt, out string strErrorMessage)
{

    SqlConnection theConnection = new SqlConnection();

    bool blnResult = false;
    string strConnectionString = ConfigurationManager.ConnectionStrings["MyConn"].ConnectionString;
    strErrorMessage = "";

    if (theConnection == null)
        theConnection = new SqlConnection();

    try
    {
        // Declare a SQL Adapter
        SqlDataAdapter da = new SqlDataAdapter(sqlQuery, theConnection);

        // Declare a DataTable
        DataTable dt = new DataTable();

        // Populate the DataTable
        da.Fill(dt);

        // Clean up.
        dt.Dispose();
        da.Dispose();
        theConnection.Close();
    }
    catch (Exception excError)
    {
        strErrorMessage = excError.Message;
    }

    return blnResult;
  }
}

错误消息是：“类型或命名空间名称 'dt' 无法被发现（您是否缺少 using 指令或程序集引用？）” 有什么建议吗？

Answer 1

要添加到此代码的问题列表（您尚未发现，因为您还没有做到这一点）：

在您的 OpenSqlDataReader 方法中，您需要实例化您的 SqlConnection< /code> 使用您定义的连接字符串的对象。现在您只是使用空构造函数创建它，这意味着它不会充当实际的连接对象。

此外，您还需要在连接对象上显式调用 .Open()。否则，它将不会打开，并且当您尝试使用它时会抛出异常。

Answer 2

在调用 OpenSqlDatareader 之前，您需要声明 dt

DataTable dt;// no need to assign 

....
else if (DataAccessClass.OpenSqlDatareader(sqlQuery, out dt, out strErrorMessage) == false)
{
    string strErrorType = "Datareader Error:";
    SendErrorMessageToClient(strErrorType, strErrorMessage);
}
...

使用 out DataTable dt 更改 OpenSqlDatareader 的签名

public static bool OpenSqlDatareader(string sqlQuery, out DataTable dt, out string strErrorMessage)

在方法内

// create new  DataTable
dt = new DataTable();

Answer 3

在 Page_Load 中，您还需要声明 dt。