AccessViolationException [ArcEngine]

Question

I am currently developing using ArcObects 10.2.1 and I've encountered this tricky exception:

AccessViolationException was encountered

Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

Source=System.Windows.Forms

StackTrace:

   System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
   System.Windows.Forms.NativeWindow.DefWndProc(Message& m)
   System.Windows.Forms.Control.DefWndProc(Message& m)
   System.Windows.Forms.Control.WndProc(Message& m)
   System.Windows.Forms.AxHost.WndProc(Message& m)
   System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

InnerException:

I tried to annotate the code and find out where it was wrong，I found it's in the last line.but I still don't know how to solve it.

    public void SetSimpleLabel(IFeatureLayer featLayer, string field, string where = "")
    {
        IGeoFeatureLayer geoFeatureLayer = featLayer as IGeoFeatureLayer;
        IAnnotateLayerPropertiesCollection annoProperties = geoFeatureLayer.AnnotationProperties;
        annoProperties.Clear();

        ILineLabelPosition position = new LineLabelPosition();
        position.Parallel = true;
        position.Perpendicular = false;

        ILineLabelPlacementPriorities placement = new LineLabelPlacementPriorities();
        IBasicOverposterLayerProperties basic = new BasicOverposterLayerProperties();
        basic.FeatureType = esriBasicOverposterFeatureType.esriOverposterPoint;
        basic.LineLabelPlacementPriorities = placement;
        basic.LineLabelPosition = position;

        ILabelEngineLayerProperties labelProperties = new LabelEngineLayerProperties() as ILabelEngineLayerProperties;
        labelProperties.Symbol = sf.CreateTextSymbol(null, 12, null, null, esriTextHorizontalAlignment.esriTHACenter);
        labelProperties.BasicOverposterLayerProperties = basic;
        labelProperties.Expression = "[" + field + "]";
        IAnnotateLayerProperties annoLayerProperties = labelProperties as IAnnotateLayerProperties;
        if (where != "")
            annoLayerProperties.WhereClause = where;
        annoProperties.Add(annoLayerProperties);

        geoFeatureLayer.DisplayAnnotation = true; //it cause
    }

why does it cause the exception?Does anybody have any ideas?