如何從應用程序配置獲取連接字符串在C＃

Question

我的app.config文件：

<configuration> 
    <connectionStrings> 
     <add name="MegaPixelBizConn" 
      connectionString="Data Source=PSHERATH-PC;Initial Catalog=MegaPixelBiz;Integrated Security=True" 
      providerName="System.Data.SqlClient" /> 
    </connectionStrings> 
</configuration> 

然後創建DBConnection的文件：

class DBConnection 
    { 
     #region Database connection method 
     public SqlConnection Connect() 
     { 
      SqlConnection conn = new SqlConnection(); 
      conn.ConnectionString = ConfigurationManager.ConnectionStrings["MegaPixelBizConn"].ToString(); 

      try 
      { 
       conn.Open(); 
      } 
      catch 
      { 
      } 
      return conn; 
     } 
     #endregion 
    } 

這是我的登錄表單：

SqlCommand cmd; 
     DataSet ds = new DataSet(); 
     DBConnection db = new DBConnection(); 

     private void btnLogin_Click(object sender, EventArgs e) 
     { 
      try 
      { 
       if (txtUserName.Text == "" || txtPassword.Text == "") 
       { 
        lblError.Visible = true; 
        lblError.Text = "*Enter UserName and Password"; 
        //MessageBox.Show(" Enter UserName and Password ."); 
        return; 
       } 
       else 
       { 
        string sql = "SELECT * FROM login_info WHERE userName = '" + txtUserName.Text + "' and password = '" + txtPassword.Text + "'"; 
        cmd = new SqlCommand(sql, db.Connect()); 
        SqlDataAdapter da = new SqlDataAdapter(cmd); 
        da.Fill(ds); 
        int i = ds.Tables[0].Rows.Count; 
        if (i == 1) 
        { 
         this.Hide(); 
         Home hm = new Home(); 
         hm.Show(); 
         ds.Clear(); 

        } 
        else 
        { 
         lblError.Text = "*Not Registered User or Invalid Name/Password"; 
         //MessageBox.Show("Not Registered User or Invalid Name/Password"); 
         txtPassword.Text = ""; 
        } 
       } 


      } 
      catch (Exception ex) 
      { 
       MessageBox.Show(ex.Message); 
      } 


     } 

但是當我的項目運行時，出現這個錯誤： 「對象引用未設置爲對象的實例。」 請給我一個解決方案。我利用一切適當的參考資料

Answer 1

嘗試

SqlConnection con = new SqlConnection(System.Configuration.ConfigurationManager.ConnectionStrings["ConnectionStringNameFromWebConfig"].ConnectionString); 

它應該工作

Answer 2

立足我的答案上this solution，嘗試：

conn.ConnectionString = ConfigurationManager.ConnectionStrings["MegaPixelBizConn"].ConnectionString; 

確保您有System.Configuration參考爲好。

還記得關閉你的connection。

編輯

根據您的新的編輯，嘗試一下本作你的代碼（注意：我很遺憾不能測試，如果這個工程，由於我的電腦被打破）。

private void btnLogin_Click(object sender, EventArgs e) 
    { 
     try 
     { 
      if (txtUserName.Text == "" || txtPassword.Text == "") 
      { 
       lblError.Visible = true; 
       lblError.Text = "*Enter UserName and Password"; 
       //MessageBox.Show(" Enter UserName and Password ."); 
       return; 
      } 
      else 
      { 

       using (SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["MegaPixelBizConn"].ConnectionString)) 
       { 

        con.Open(); 

        using (SqlCommand cmd = new SqlCommand("SELECT * FROM login_info WHERE userName = @Username AND password = @Password", con)) 
        { 
         // If these two lines don't work, replace "Username" and "Password" with "@Username"/"@Password" 
         cmd.Parameters.Add(new SqlParameter("Username", txtUserName.Text); 
         cmd.Parameters.Add(new SqlParameter("Password", txtPassword.Text); 
         SqlDataReader r = cmd.ExecuteReader(); 
         if(r.HasRows()) 
         { 
          // this assumes that there is only one user/password and no two users with same UID and PWORD 
          this.Hide(); 
          Home hm = new Home(); 
          hm.Show(); 
         } 

         else 
         { 
          lblError.Text = "*Not Registered User or Invalid Name/Password"; 
          //MessageBox.Show("Not Registered User or Invalid Name/Password"); 
          txtPassword.Text = ""; 
         } 
        } 
       } 
      } 
     } 
     catch (Exception ex) 
     { 
      MessageBox.Show(ex.Message); 
     } 


    } 

這也有助於防止出現SqlInjections。