空引用的SelectedIndexChanged在關閉形式

Question

我使用組合框時，我選擇它顯示在下面的是文本框該項目的ID的項目顯示的項目，我已經寫了選定的變化指標的事件裏面的代碼，當我執行的形式和關閉形式，而不在形式做任何事情就告訴我對象引用不設置到對象的實例，在我的查詢字符串 我的代碼如下：

using System; 
    using System.Collections.Generic; 
    using System.ComponentModel; 
    using System.Data; 
    using System.Drawing; 
    using System.Linq; 
    using System.Text; 
    using System.Windows.Forms; 
    using System.Data.SqlClient; 

    namespace TinyErp 
    { 
public partial class frmaddprod : Form 
{ 
    public frmaddprod() 
    { 
     InitializeComponent(); 
    } 


    private void productsBindingNavigatorSaveItem_Click(object sender, EventArgs e) 
    { 
     this.Validate(); 
     this.productsBindingSource.EndEdit(); 
     this.tableAdapterManager.UpdateAll(this.orderMachineDataSet); 

    } 


    SqlConnection conn = new SqlConnection(Properties.Settings.Default.OrderMachineConnectionString); 
    string strgetsupp; 


    private void frmaddprod_Load(object sender, EventArgs e) 
    { 
     // TODO: This line of code loads data into the 'orderMachineDataSet.Suppliers' table. You can move, or remove it, as needed. 
     this.suppliersTableAdapter.Fill(this.orderMachineDataSet.Suppliers); 
     // TODO: This line of code loads data into the 'orderMachineDataSet.Products' table. You can move, or remove it, as needed. 
     this.productsTableAdapter.Fill(this.orderMachineDataSet.Products); 

     //string strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'"; 
     //conn.Open(); 
     //SqlCommand cmd = new SqlCommand(strgetsupp, conn); 
     //SqlDataReader read; 
     //read = cmd.ExecuteReader(); 
     //read.Read(); 
     //int sid = int.Parse(read["supplier_id"].ToString()); 
     //read.Close(); 
     //conn.Close(); 
     //supplier_idTextBox.Text = sid.ToString(); 

    } 

    private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) 
    { 
     strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'"; 
     conn.Open(); 
     SqlCommand cmd = new SqlCommand(strgetsupp, conn); 
     SqlDataReader read; 
     read = cmd.ExecuteReader(); 
     read.Read(); 
     int sid = int.Parse(read["supplier_id"].ToString()); 
     supplier_idTextBox.Text = sid.ToString(); 
     read.Close(); 
     conn.Close(); 
    } 

    private void productsDataGridView_CellContentClick(object sender, DataGridViewCellEventArgs e) 
    { 

    } 

    private void frmaddprod_FormClosing(object sender, FormClosingEventArgs e) 
    { 
     Application.Exit(); 
    } 
} 
} 

Answer 1

檢查comboBox1.SelectedItem是執行之前別的空

private void comboBox1_SelectedIndexChanged(object sender, EventArgs e) 
    { 
     if(comboBox1.SelectedItem==null) 
      return; 

     strgetsupp = "select supplier_id from Suppliers where supplier_company='" + ((DataRowView)comboBox1.SelectedItem).Row["supplier_company"].ToString() + "'"; 
     conn.Open(); 
     SqlCommand cmd = new SqlCommand(strgetsupp, conn); 
     SqlDataReader read; 
     read = cmd.ExecuteReader(); 
     read.Read(); 
     int sid = int.Parse(read["supplier_id"].ToString()); 
     supplier_idTextBox.Text = sid.ToString(); 
     read.Close(); 
     conn.Close(); 
    }