水晶报表Access to report file denied. Another program may be using it(重谢)

我做水晶报表得导出时老报Access to report file denied. Another program may be using it这个异常，我也去网上查了，但不理想，希望哪位大侠帮帮我解决这个问题。以下是我的代码。
 PieReport ReportDoc =new PieReport();//报表类
private void Page_Load(object sender, System.EventArgs e)
{
    string connectionString="server=E5JNPC182;database=pubs;uid=sa;pwd=wxy";
string select="select * from MonthSale";
SqlConnection con=new SqlConnection(connectionString);
con.Open();
SqlDataAdapter adapter=new SqlDataAdapter(select,con);
adapter.Fill(monthSale1,"MonthSale");
con.Close();
ReportDoc.SetDataSource(monthSale1);
CrystalReportViewer1.ReportSource=ReportDoc;
CrystalReportViewer1.DataBind();
}
private void ReportExport_Click(object sender, System.EventArgs e)
{
CrystalDecisions.Shared.DiskFileDestinationOptions DiskOpts = new  CrystalDecisions.Shared.DiskFileDestinationOptions();
ReportDoc.ExportOptions.ExportDestinationType = CrystalDecisions.Shared.ExportDestinationType.DiskFile;

switch (DropDownList1.SelectedItem.Text)
{
case "RTF":
　      ReportDoc.ExportOptions.ExportFormatType =   CrystalDecisions.Shared.ExportFormatType.RichText;
　      DiskOpts.DiskFileName = "d:\\demo.rtf";
break;
case "PDF":
　      ReportDoc.ExportOptions.ExportFormatType =   CrystalDecisions.Shared.ExportFormatType.PortableDocFormat;
　      DiskOpts.DiskFileName = "d:\\demo.pdf";
break;
case "DOC":
　      ReportDoc.ExportOptions.ExportFormatType =   CrystalDecisions.Shared.ExportFormatType.WordForWindows;
　      DiskOpts.DiskFileName = "d:\\demo.doc";
break;
case "XLS":
　      ReportDoc.ExportOptions.ExportFormatType =   CrystalDecisions.Shared.ExportFormatType.Excel;
　      DiskOpts.DiskFileName = "d:\\demo.xls";
break;
default:
break;
}
ReportDoc.ExportOptions.DestinationOptions = DiskOpts;
　          ReportDoc.Export(); }