RadGridView 导出excel乱码问题

private void butExp_Click(object sender, RoutedEventArgs e)
{
if (this.radGridView1.Items.ItemCount == 0)
{
return;
}
this.butAdd.IsEnabled = true;
string extension = "";
ExportFormat format = ExportFormat.ExcelML;
extension = "xls";
SaveFileDialog dialog = new SaveFileDialog()
{
DefaultExt = extension,
Filter = String.Format("{1} files (*.{0})|*.{0}|All files (*.*)|*.*", extension, "Excel"),
FilterIndex = 1
};

if (dialog.ShowDialog() == true)
{

using (Stream stream = dialog.OpenFile())
{
this.radGridView1.Export(stream,
new GridViewExportOptions()
{
Format = format,
Items = a.QueryableSourceCollection.Where(this.radGridView1.FilterDescriptors).Sort(this.radGridView1.SortDescriptors).GroupBy(this.radGridView1.GroupDescriptors).GroupBy(this.radGridView1.GroupDescriptors).GroupBy(this.radGridView1.GroupDescriptors),
ShowColumnHeaders = true,
});
}

}
}