Question

我已经尝试了一百万种不同的方法来实现这一目标，却发现没有任何方法可行。向下滚动到底部。（注意，这是使用SQL Server数据库的登录表单）

DatabaseConnection.cs：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace AllIn1Database
{
    class DatabaseConnection
    {
        private string sql_string;
        private string strCon;
        System.Data.SqlClient.SqlDataAdapter da_1;
        public string Sql
        {
            set
            {
                sql_string = value;
            }
        }
        public string connection_string
        {
            set { strCon = value; }
        }
        public System.Data.DataSet GetConnection
        {
            get { return MyDataSet(); }
        }
        private System.Data.DataSet MyDataSet()
        {
            System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection(strCon);
            con.Open();
            da_1 = new System.Data.SqlClient.SqlDataAdapter(sql_string, con);
            System.Data.DataSet dat_set = new System.Data.DataSet();
            da_1.Fill(dat_set, "Table_Data_1");
            con.Close();
            return dat_set;
        }
    }
}

Form1.cs中：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.Sql;
using System.Data.SqlClient;
using System.Data.SqlTypes;

namespace AllIn1Database
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        DatabaseConnection objConnect;
        string conString;
        DataSet ds;
        SqlConnection con = new SqlConnection();

        private void tnLogin_Click(object sender, EventArgs e)
        {
            try{
                System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection();
                con.Open();
                String cmp = "SELECT ISNULL(stUsername, '') AS stUsername, ISNULL(stPassword,'') AS stPassword, ISNULL(stRole,'') AS stRole FROM LoginDetails WHERE stUsername='" + txtUname.Text + "' and stPassword='" + txtPass.Text + "'";
            SqlCommand cmd = new SqlCommand(cmp, con);
           SqlDataAdapter da = new SqlDataAdapter(cmd);
                SqlDataReader dr = cmd.ExecuteReader();
            while (dr.Read())  
                {  
                string userText = txtUname.Text;
                string passText = txtPass.Text;
                        if (this.CompareStrings(dr["Name"].ToString(), userText) &&   
                       this.CompareStrings(dr["Password"].ToString(), passText)){
                           MessageBox.Show("Logged on as" + txtUname.Text);
        }
                        else
                        {
                            MessageBox.Show("Invalid Logon Details.");
                        }
                        dr.Close();
                        con.Close();
            }
            }
                catch (Exception err) {
                    MessageBox.Show(err.Message);
                }
            }

        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                objConnect = new DatabaseConnection();
                conString = Properties.Settings.Default.DatabaseConnectionString;
                objConnect.connection_string = conString;
                objConnect.Sql = Properties.Settings.Default.SQL;
                ds = objConnect.GetConnection;
            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message);
            }
        }

        private bool CompareStrings(string string1, string string2)
        {
            return String.Compare(string1, string2, true, System.Globalization.CultureInfo.InvariantCulture) == 0 ? true : false;
        }
    }
}

错误发生在

行中

System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection(strCon);

错误说

strCon在此上下文中不存在。

如何在不抛出此错误的情况下成功连接？

Answer 1

您正在实例化没有连接字符串的连接。而且我没有看到你在行之后设置它：

System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection();

假设您的连接字符串位于Properties.Settings.Default.DatabaseConnectionString中，您可以执行以下操作：

System.Data.SqlClient.SqlConnection con = new System.Data.SqlClient.SqlConnection(Properties.Settings.Default.DatabaseConnectionString);

Answer 2

你没有连接字符串/没有给strCon一个值，你需要做类似的事情：

strCon = "myConnectionString";

找出连接字符串的用途，可以使用本网站：

http://www.connectionstrings.com/

SQL Server连接字符串出错

2 个答案: