ADO.NET实体框架 无法删除此对象，因为未在 ObjectStateManager 中找到它。

使用  ADO.NET实体框架 删除时异常  “无法删除此对象，因为未在 ObjectStateManager 中找到它。”求解
 在 System.Data.Objects.ObjectContext.DeleteObject(Object entity)
   在 WMSDAL.VenderDAL.Del(Vender vender)
   在 WMSBLL.VenderBLL.Del(Vender vender) 位置 D:\My Documents\VS_2008_PRO\Pro\WMS\WMSBLL\VenderBLL.cs:行号 43
   在 WinForm.Model.VenderManagementControl.tbbtnDel_ItemClick(Object sender, ItemClickEventArgs e) 位置 D:\My Documents\VS_2008_PRO\Pro\WMS\WinForm\Model\VenderManagementControl.cs:行号 100
   在 DevExpress.XtraBars.BarItem.OnClick(BarItemLink link)
   在 DevExpress.XtraBars.BarBaseButtonItem.OnClick(BarItemLink link)
   在 DevExpress.XtraBars.BarItemLink.OnLinkClick()
   在 DevExpress.XtraBars.BarItemLink.OnLinkAction(BarLinkAction action, Object actionArgs)
   在 DevExpress.XtraBars.BarButtonItemLink.OnLinkAction(BarLinkAction action, Object actionArgs)
   在 DevExpress.XtraBars.BarItemLink.OnLinkActionCore(BarLinkAction action, Object actionArgs)
   在 DevExpress.XtraBars.ViewInfo.BarSelectionInfo.ClickLink(BarItemLink link)
   在 DevExpress.XtraBars.ViewInfo.BarSelectionInfo.UnPressLink(BarItemLink link)
   在 DevExpress.XtraBars.Controls.CustomLinksControl.OnMouseUp(MouseEventArgs e)
   在 System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   在 System.Windows.Forms.Control.WndProc(Message& m)
   在 DevExpress.XtraBars.Controls.DockedBarControl.WndProc(Message& msg)
   在 System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   在 System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   在 System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   在 System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   在 System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
   在 System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   在 System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   在 System.Windows.Forms.Application.Run(Form mainForm)
   在 WinForm.Program.Main() 位置 D:\My Documents\VS_2008_PRO\Pro\WMS\WinForm\Program.cs:行号 18
   在 System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
   在 System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
   在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
   在 System.Threading.ThreadHelper.ThreadStart_Context(Object state)
   在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   在 System.Threading.ThreadHelper.ThreadStart()