c#.net gridview how do I add a clickable buttonfield to the header?

I am making a gridview programmatically since I am using it in a webpart.  This means that I CAN NOT make it in the designer, so please don't suggest that.

I want to be able to click on a header text so I can call a function.

How do I do this?

this is my code to generate the gridview:

            gv = new GridView();
            ods1 = new ObjectDataSource();
            gv.DataBound += new EventHandler(gv_DataBound);
            SetDataSource("Tagged Documents", 4, 1);
           
            gv.ID = "GridView1";
            gv.AutoGenerateColumns = false;
            gv.DataSourceID = "ObjectDataSource1";

            ButtonField bfld = new ButtonField();
            bfld.CommandName = "OpenDocument";
            bfld.DataTextField = "DocName";
            bfld.HeaderText = "Document";
            bfld.Text = "Button";

            BoundField boundFiled1 = new BoundField();
            boundFiled1.DataField = "DocModified";
            boundFiled1.HeaderText = "Modified";

            BoundField boundFiled2 = new BoundField();
            boundFiled2.DataField = "DocModifiedBy";
            boundFiled2.HeaderText = "Modified By";

            BoundField boundFiled3 = new BoundField();
            boundFiled3.DataField = "DocType";
            boundFiled3.HeaderText = "Type";
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