使用 SQLiteDataReader 的异常

Question

我正在制作一个自定义 SQLite 包装器。这是为了允许与数据库的持久连接。但是，两次调用此函数时收到异常。

public Boolean DatabaseConnected(string databasePath)
{
    bool exists = false;
    if (ConnectionOpen())
    {
        this.Command.CommandText = string.Format(DATABASE_QUERY);
        using (reader = this.Command.ExecuteReader())
        {
            while (reader.Read())
            {
                if (string.Compare(reader[FILE_NAME_COL_HEADER].ToString(), databasePath, true) == 0)
                {
                    exists = true;
                    break;
                }
            }
            reader.Close();
        }
    }
    return exists;
}

在执行命令或尝试打开数据库之前，我使用上述函数检查数据库当前是否打开。我第一次执行该函数时，它执行没有问题。之后reader = this.Command.ExecuteReader()抛出异常

Object reference not set to an instance of an object.
StackTrace:
   at System.Data.SQLite.SQLiteStatement.Dispose()
   at System.Data.SQLite.SQLite3.Reset(SQLiteStatement stmt)
   at System.Data.SQLite.SQLite3.Step(SQLiteStatement stmt)
   at System.Data.SQLite.SQLiteDataReader.NextResult()
   at System.Data.SQLite.SQLiteDataReader..ctor(SQLiteCommand cmd, CommandBehavior behave)
   at System.Data.SQLite.SQLiteCommand.ExecuteReader(CommandBehavior behavior)
   at System.Data.SQLite.SQLiteCommand.ExecuteReader()
   at EveTraderApi.Database.SQLDatabase.DatabaseConnected(String databasePath) in C:\Documents and Settings\galford13x\My Documents\Visual Studio 2008\Projects\EveTrader\EveTraderApi\Database\Database.cs:line 579
   at EveTraderApi.Database.SQLDatabase.OpenSQLiteDB(String filename) in C:\Documents and Settings\galford13x\My Documents\Visual Studio 2008\Projects\EveTrader\EveTraderApi\Database\Database.cs:line 119
   at EveTraderApiExample.Form1.CreateTableDataTypes() in C:\Documents and Settings\galford13x\My Documents\Visual Studio 2008\Projects\EveTrader\EveTraderApiExample\Form1.cs:line 89
   at EveTraderApiExample.Form1.Button1_ExecuteCommand(Object sender, EventArgs e) in C:\Documents and Settings\galford13x\My Documents\Visual Studio 2008\Projects\EveTrader\EveTraderApiExample\Form1.cs:line 35
   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(Int32 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 EveTraderApiExample.Program.Main() in C:\Documents and Settings\galford13x\My Documents\Visual Studio 2008\Projects\EveTrader\EveTraderApiExample\Program.cs:line 18
   at System.AppDomain._nExecuteAssembly(Assembly 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.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.ThreadHelper.ThreadStart()

Answer 1

您可以使用自定义包装器来管理连接，但此包装器不应管理命令：this.Command。由于命令对象存储在包装器中，因此它将被重用。问题是，第一次调用 ExeuteReader 时会处理此命令，第二次调用它时会出现异常。尝试为每个查询创建命令：

public bool DatabaseConnected(string databasePath)
{
    if (ConnectionOpen())
    {
        using (var command = YourSQLiteConnection.CreateCommand())
        {
            command.CommandText = string.Format(DATABASE_QUERY);
            using (reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    if (string.Compare(reader[FILE_NAME_COL_HEADER].ToString(), databasePath, true) == 0)
                    {
                        return true;
                    }
                }
            }
        }
    }
    return false;
}