绑定IEnumerable到Silverlight datagrid

[OperationContract]        
public IEnumerable GetCategoryList()
{
    return _service.GetCategoryList().ToList();            
}

CategoryService.Category1Client client = new CategoryService.Category1Client  ();            
client.GetCategoryListCompleted += (s, ea) =>
{
    dataGrid1.AutoGenerateColumns = true;
    dataGrid1.ItemsSource = ea.Result.ToList();
};
client.GetCategoryListAsync();

[OperationContract]        
public Category[] GetCategoryList()
{
    return _service.GetCategoryList().ToArray();            
}