Merge DataGrid Header

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Introduction

This article describes the technique to merge the header of a DataGrid by redirecting the Render method of the DataGrid items.

Background

I found many times the need to merge the header of a DataGrid. So, when I was having spare time, I tried to find a way to do it, and here it is. I know that if you need to merge headers, you can use the Repeater control instead. But if you are fond of DataGrid (just like me), or may be you already used DataGrid, then this article is for you.

Using the code

When rendered, a DataGrid will be converted into a HTML Table element and the header will be the first HTML TR element. So, to have a merged header, we need to have control in the rendering of the header. This can be achieved by redirecting the Render method of the DataGrid header using the SetRenderMethodDelegate of the DataGrid header on ItemCreated event, like this:

private void Datagrid1_ItemCreated(object sender, 
          System.Web.UI.WebControls.DataGridItemEventArgs e) 
{ 
    //*** Examine if the item created is the header item 
    ListItemType lit = e.Item.ItemType; 
    if(ListItemType.Header == lit) 
    { 
        //*** Redirect the default header rendering method to our own method 
        e.Item.SetRenderMethodDelegate(new RenderMethod(NewRenderMethod)); 
    } 
}

And here is our own Render method:

/// <summary> 
/// This is our custom render method for the grid header item 
/// </summary> 
/// <param name="writer"></param> 
/// <param name="ctl"></param> 
private void NewRenderMethod(HtmlTextWriter writer, Control ctl) 
{ 
    //***  We don't need to write the <TR> tag
    //     because it's already written by the writer 
    //     so now we write the Name column 
    writer.Write("<TD colspan=\"3\" align=\"center\">Name</TD>\n"); 

    //***  The Age column must have the rowspan attribute
    //     and must be rendered inside the 
    //     first row so it will centered vertically 
    TableCell cell = (TableCell)ctl.Controls[ctl.Controls.Count-1]; 
    cell.Attributes.Add("rowspan","2"); 
    cell.RenderControl(writer); 

    //***     Now we close the first row, so we can insert the second one 
    writer.Write("</TR>\n"); 

    //***  Add the style attributes that was defined in design time 
    //     to our second row so they both will have the same appearance 
    DataGrid1.HeaderStyle.AddAttributesToRender(writer); 

    //***     Insert the second row 
    writer.RenderBeginTag("TR"); 

    //***  Render all the cells that was defined
    //     in design time, except the last one 
    //     because we already rendered it above 
    for(int i=0; i<= ctl.Controls.Count-2; i++) 
    { 
        ctl.Controls[i].RenderControl(writer); 
    } 

    //***  We don't need to write the </TR> close tag
    //     because the writer will do that for us 
    //     and so we're done :) 
}

I have created a decorator class to decorate a DataGrid (ASPNetDatagridDecorator class) to have a merge header, and all you need to do is define the header cell like this (you can use the auto format feature, but doesn't work for all):

private void Page_Load(object sender, System.EventArgs e)
{
    // Put user code to initialize the page here
    if(!this.IsPostBack)
    {
        TableCell cell = null;
        DataGrid1.DataSource = GetData();
        DataGrid1.DataBind(); 

        m_add.DatagridToDecorate = Datagrid2;
        ArrayList header = new ArrayList();

        // cell = new TableCell();
        // cell.Text = "Code";
        // cell.RowSpan = 2;
        // cell.HorizontalAlign = HorizontalAlign.Center;
        // header.Add(cell);

        cell = new TableCell();
        cell.Text = "Name";
        cell.RowSpan = 2;
        cell.HorizontalAlign = HorizontalAlign.Center;
        header.Add(cell);

        cell = new TableCell();
        cell.Text = "Name";
        cell.ColumnSpan = 3;
        cell.HorizontalAlign = HorizontalAlign.Center;
        header.Add(cell);

        cell = new TableCell();
        cell.Text = "Age";
        cell.RowSpan = 2;
        cell.HorizontalAlign = HorizontalAlign.Center;
        header.Add(cell);

        cell = new TableCell();
        cell.Text = "School";
        cell.ColumnSpan = 3;
        cell.HorizontalAlign = HorizontalAlign.Center;
        header.Add(cell);

        cell = new TableCell();
        cell.Text = "Religion";
        cell.RowSpan = 2;
        cell.HorizontalAlign = HorizontalAlign.Center;
        header.Add(cell);

        m_add.AddMergeHeader(header);

        Datagrid2.DataSource = GetData();
        Datagrid2.DataBind();

    }

}