c# - 未将对象引用设置为对象实例 - MySqlCommand

Question

我正在制作一个应与 MySql 数据库一起使用的应用程序。我收到此错误，但我不知道为什么:

Object reference not set to an instance of an object

这是我的代码:

public void rtMean()
    {
        MySqlConnection conect = con.ConnectToMySQL();
        MySqlDataReader reader;
        MySqlDataReader readerMean;

        // OVERENIE, CI UZ STLPEC PRIEMER EXISTUJE

        string query1 = "SELECT * FROM feture";
        MySqlCommand cmd1 = new MySqlCommand(query1, conect);
        reader = cmd1.ExecuteReader();

        for (int j=0; j < reader.FieldCount; j++)
        {
            if (reader.GetName(j) == "priemer")
            {
                break;
            }
            else
            {
                reader.Close();
                cmd1.Cancel();
                string queryAlterTable = "ALTER TABLE feture ADD COLUMN priemer FLOAT NOT NULL";
                MySqlCommand cmd = new MySqlCommand(queryAlterTable, conect); //here I am getting the error
                cmd.ExecuteScalar();
            }
        }

这应该检查我的一个表中是否有“primer”列。如果是，它应该结束/中断，如果不是，我的表应该用名为“priemer”的新列进行更改。

感谢您的建议。

编辑:堆栈 - 跟踪:

  at MySql.Data.MySqlClient.MySqlCommand.ExecuteScalar()
  at WindowsFormsApplication1.ClusteringReady.rtMean() in    c:\Users\Martin\Desktop\CLUSTER\WindowsFormsApplication1\ClusteringReady.cs:line 43
  at WindowsFormsApplication1.Form1.clusterize_Click(Object sender, EventArgs e) in c:\Users\Martin\Desktop\CLUSTER\WindowsFormsApplication1\Form1.cs:line 130
  at System.Windows.Forms.Control.OnClick(EventArgs e)
  at System.Windows.Forms.Button.OnClick(EventArgs e)
  at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
  at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
  at System.Windows.Forms.Control.WndProc(Message& m)
  at System.Windows.Forms.ButtonBase.WndProc(Message& m)
  at System.Windows.Forms.Button.WndProc(Message& m)
  at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
  at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
  at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
  at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
  at    System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
  at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
  at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
  at System.Windows.Forms.Application.Run(Form mainForm)
  at WindowsFormsApplication1.Program.Main() in c:\Users\Martin\Desktop\CLUSTER\WindowsFormsApplication1\Program.cs:line 19
  at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
  at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
  at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
  at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
  at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
  at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
  at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
  at System.Threading.ThreadHelper.ThreadStart()

Answer 1

好的，根据下面的讨论，您可以运行此查询:

SELECT COUNT(*) as cnt FROM INFORMATION_SCHEMA.COLUMNS WHERE table_name = 'feture'
AND COLUMN_NAME = 'premier'

如果您得到CNT = 0(在结果集的第一个单元格中)，那么您必须运行您的 ALTER TABLE 查询，否则它已经存在。仍然不需要运行 for 循环。