为什么asp:ImageButton不触发OnClick事件?

我不知道为什么OnClick事件不工作:

<div  class="col-sm-1 pull-left" style="margin-top:20px">
    <asp:ImageButton ID="delete_file" 
                     OnClientClick="return confirm('Are you Sure ?');"  
                     OnClick="delete_file_Click"  
                     CausesValidation="false" 
                     Visible="false" 
                     class="btn btn-lg btn-default pull-left" 
                     runat="server" 
                     ImageUrl="~/img/del.png" />
</div>

c#代码:

 protected void delete_file_Click(object sender, ImageClickEventArgs e)
    {
        int index = GridView2.SelectedIndex;
        GridView2.DeleteRow(index);
    }