Question

我正在创建一个应该与MySql数据库一起使用的应用程序。我收到了这个错误，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();
            }
        }

如果我的一个表中有“priemer”列，这应该是chceck。如果是，它应该结束/中断，如果不是，我的表应该用新列改名，名为“priemer”。

感谢您的建议。

编辑：堆叠 - 追踪：

  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

因此，如果我理解正确，如果第一个字段未命名为priemer，则会创建一个新列以尝试关闭阅读器。但是你不能停止你的循环，因此你会在阅读器关闭的情况下执行第二个循环。

有一种更好的方法可以确定您的表是否包含所需的列

bool found = false;
using(MySqlConnection cnn = new MySqlConnection("....................."))
{
    cnn.Open();
    DataTable dt = cnn.GetSchema("COLUMNS", new string[] {null,null,"feture", null});
    foreach(DataRow r in dt.Rows)
    {
        if(r["COLUMN_NAME"].ToString() == "priemer")
        {
            found = true;
            break;
        }
    }
    if(!found)
    {
        .. create your column here
    }
}

Answer 2

根据以下讨论，您可以运行此查询：

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

如果你得到CNT = 0（在结果集的第一个单元格中），那么你必须运行你的ALTER TABLE查询，否则它已经存在。仍然不需要运行for循环。

对象引用未设置为对象的实例 - MySqlCommand

2 个答案: