title: how do I add a CheckBox to a TemplateField HEADER?
body: <p>I have this:</p>
  
  <pre><code>&lt;asp:GridView ID="gvShows" runat="server" HorizontalAlign="Center" DataKeyNames="dataSource,title" Caption="Show List" AutoGenerateColumns="False" AllowSorting="True" CaptionAlign="Top" OnSorting="gvShows_Sorting"&gt;
  &lt;RowStyle BorderColor="Black" /&gt;
  &lt;Columns&gt; 
      &lt;asp:TemplateField HeaderText="Select" &gt; 
          &lt;ItemTemplate&gt; 
              &lt;asp:CheckBox ID="cbSelect" runat="server" AutoPostBack="false"/&gt; 
          &lt;/ItemTemplate&gt; 
      &lt;/asp:TemplateField&gt; 
      &lt;asp:BoundField HeaderText="Data Source" DataField="DataSource" SortExpression="DataSource"/&gt; 
      &lt;asp:BoundField HeaderText="Show Title" DataField="Title" SortExpression="Title"/&gt; 
      &lt;asp:BoundField HeaderText="Episode Title" DataField="EpisodeTitle"     SortExpression="EpisodeTitle"/&gt; 
      &lt;asp:BoundField HeaderText="Genre" DataField="Genre" SortExpression="Genre"/&gt; 
      &lt;asp:BoundField HeaderText="Show Type Description" DataField="ShowTypeDescription" SortExpression="ShowTypeDescription"/&gt; 
      &lt;asp:BoundField HeaderText="Director Name" DataField="DirectorName" SortExpression="DirectorName"/&gt; 
      &lt;asp:BoundField HeaderText="Release Year" DataField="ReleaseYear" SortExpression="ReleaseYear"/&gt; 
      &lt;asp:BoundField HeaderText="Season Episode" DataField="SeasonEpisode" SortExpression="SeasonEpisode"/&gt; 
      &lt;asp:BoundField HeaderText="Show ID" DataField="ShowId" SortExpression="ShowId"/&gt; 
      &lt;asp:BoundField HeaderText="Episode ID" DataField="EpisodeID" SortExpression="EpisodeID"/&gt; 
  &lt;/Columns&gt;  
  </code></pre>
  
  <p></p>
  
  <p>Which gives me this:</p>
  
  <p><img src="https://i.imgur.com/cPpTtk2.png"></p>
  
  <p>I want to change where the highlighted word <code>"SELECT"</code> is to an actual <code>CheckBox</code> so that when the user checks it, it checks all boxes under.</p>
  
  <p>How do I go about modifying the header text from "Select" to an actual <code>CheckBox</code>?</p>