Question

错误是：
System.NullReferenceException: Object reference not set to an instance of an object.我知道在使用某个对象但未初始化或初始化但后来被声明为null但仍被访问导致异常时会发生此错误。但我无法弄清楚违规物品的位置，所以我可以自己纠正。在我第二次从列表框控件中选择一个字符串后，将显示错误。首先它会起作用然后如果我第二次选择另一个项目，程序将突然退出，错误将显示在Linux的命令行中。

如果需要，我可以在运行单win_binary_file后显示完整错误。

经过调试，我发现了这个会引发NullReferenceException的违规代码：

public void fListItems(DataGridView datagridview1, string param_name)
    {
        NpgsqlDataAdapter dr = default(NpgsqlDataAdapter);
        DataSet ds;
        string sql;
        NpgsqlConnection dbcon;
        string connectionstr = "SERVER=" + DataBaseHost + ";DATABASE=" + DatabaseName + ";USER ID=" + DatabaseUser + ";PASSWORD=" + DatabasePassword + ";pooling=true; port=" + DatabasePort;
        dbcon = new NpgsqlConnection(connectionstr);
        try
        {
            sql = "SELECT * FROM purchase_order" +                     
           " WHERE purchase_request_num= '" + param_name + "' ORDER BY id ASC; ";                
            dbcon.Open();
            ds = new DataSet("purchase_order");
            dr = new NpgsqlDataAdapter();
            dr.SelectCommand = new NpgsqlCommand(sql, dbcon);
            dr.Fill(ds, "purchase_order");
            datagridview1.DataSource = ds.Tables["purchase_order"];
        } 
        catch (ApplicationException ex)
        {
            MessageBox.Show(ex.Message);
        }
        finally
        {
            dbcon.Close();
        }
    }

此代码将调用上述函数：

private void listboxPR_SelectedIndexChanged(object sender, EventArgs e)
    {
        Cursor = Cursors.WaitCursor;
        string prnumberSelect;
        if (varname == 180)
        {
            //Somecode here for condition varname=180
            prnumberSelect = listboxPR.Text.ToString();
            Class1.fListItems(DataGridRequests, prnumberSelect);
        }
        else
        {
            prnumberSelect = listboxPR.Text.ToString();
            Class1.fListItems(DataGridRequests, prnumberSelect);
        }
        Cursor = Cursors.Default;

    }

完整错误：

System.NullReferenceException: Object reference not set to an instance of an object
  at purchase_requisition.frmmain.DataGridRequests_SelectionChanged (System.Object sender, System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.OnSelectionChanged (System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.SetSelectedRowCore (Int32 rowIndex, Boolean selected) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.SetSelectedRowCoreInternal (Int32 rowIndex, Boolean selected) [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) System.Windows.Forms.DataGridView:SetSelectedRowCoreInternal (int,bool)
  at System.Windows.Forms.DataGridViewBand.set_Selected (Boolean value) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridViewRow.set_Selected (Boolean value) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.ClearSelection () [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.MoveCurrentCell (Int32 x, Int32 y, Boolean select, Boolean isControl, Boolean isShift, Boolean scroll) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.ClearBinding () [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.DataGridView.set_DataSource (System.Object value) [0x00000] in <filename unknown>:0 
  at (wrapper remoting-invoke-with-check) System.Windows.Forms.DataGridView:set_DataSource (object)
  at purchase_requisition.dbquery.PRMonitoringGrid (System.Windows.Forms.DataGridView DatagridRequests, System.String Purchase_Number) [0x00000] in <filename unknown>:0 
  at purchase_requisition.frmmain.listboxPR_SelectedIndexChanged (System.Object sender, System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.ListBox.OnSelectedIndexChanged (System.EventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.ListBox.OnItemClick (Int32 index) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.ListBox.OnMouseUpLB (System.Object sender, System.Windows.Forms.MouseEventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Control.OnMouseUp (System.Windows.Forms.MouseEventArgs e) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Control.WmLButtonUp (System.Windows.Forms.Message& m) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Control.WndProc (System.Windows.Forms.Message& m) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.ListBox.WndProc (System.Windows.Forms.Message& m) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Control+ControlWindowTarget.OnMessage (System.Windows.Forms.Message& m) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.Control+ControlNativeWindow.WndProc (System.Windows.Forms.Message& m) [0x00000] in <filename unknown>:0 
  at System.Windows.Forms.NativeWindow.WndProc (IntPtr hWnd, Msg msg, IntPtr wParam, IntPtr lParam) [0x00000] in <filename unknown>:0

Answer 1

我发现函数fListItems（）完全没有错。 DatagridREquests_SelectionChanged事件的代码触发了异常。这是代码实现不良的结果（我的错）。我在listboxPR_SelectedIndexChanged事件下传输了有问题的代码，因为我需要的是在数据网格由函数fListItems（）填充后从数据网格中获取数据。

private void listboxPR_SelectedIndexChanged(object sender, EventArgs e)
{
    Cursor = Cursors.WaitCursor;
    string prnumberSelect;
    if (varname == 180)
    {
        //Somecode here for condition varname=180
        prnumberSelect = listboxPR.Text.ToString();
        Class1.fListItems(DataGridRequests, prnumberSelect);
        //Transferred code from DatagridRequests_SelectionChanged event
        if (DataGridRequests.RowCount > 0 ) {
            intVar1 = Convert.ToInt32(DataGridRequests.CurrentRow.Cells["column"].Value);
        }
    }
    else
    {
        prnumberSelect = listboxPR.Text.ToString();
        Class1.fListItems(DataGridRequests, prnumberSelect);
        if (DataGridRequests.RowCount > 0 ) {
            intVar1 = Convert.ToInt32(DataGridRequests.CurrentRow.Cells["column"].Value);
        }
    }
    Cursor = Cursors.Default;

}

谢谢你们帮助我......

这个Winform C＃代码有什么问题？它在Windows中运行良好，但在单声道（Linux）中，将抛出NullReferenceException

1 个答案: