UpdatePanel with Javascript

<script runat="server">
protected void btnDeleteAll_Click(object sender, EventArgs e)
{
if (FileHelper.DeleteAll() == true)
{
string script = @"alert('All Files Deleted Successfully!');";
ScriptManager.RegisterStartupScript(this, this.GetType(),"filesDeleted", script, true);
}
}
</script>

when i am executing FileHelper does not exist in the current context is comming. plz help me.