void Item_Command(Object sender, DataListCommandEventArgs e)
{
if(e.commnandname=="Reply")
{
Label lblUserName= e.Item.FindControl("lblUserName");
lblUserName.Text= Convert.ToString(e.CommandArgument);
}
}
<asp:datalist id="ItemsList" runat="server" onitemcommand="Item_Command" xmlns:asp="#unknown">
<itemtemplate>
<asp:linkbutton id="SelectButton" text="Reply">
CommandName="Reply" runat="Reply" CommandArgument='<%# Eval("Username") %>' />
</asp:linkbutton></itemtemplate>
</asp:datalist>