Merging columns in GridView/DataGrid header

Background
As necessity to show header columns in a few rows occurs fairly often it would be good to have such functionality in the GridView/DataGrid control as an in-built feature. But meanwhile everyone solves this problem in his own way.

The described below variant of the merging implementation is based on irwansyah's idea to use the SetRenderMethodDelegate method for custom rendering of grid columns header. I guess this approach can be simplified in order to get more compact and handy code for reuse.

The code overview

As it may be required to merge a few groups of columns - for example, 1,2 and 4,5,6 - we need a class to store common information about all united columns.
[Serializable]
private class MergedColumnsInfo
{
// indexes of merged columns
public List MergedColumns = new List();
// key-value pairs: key = the first column index, value = number of the merged columns
public Hashtable StartColumns = new Hashtable();
// key-value pairs: key = the first column index, value = common title of the merged columns
public Hashtable Titles = new Hashtable();

//parameters: the merged columns indexes, common title of the merged columns
public void AddMergedColumns(int[] columnsIndexes, string title)
{
MergedColumns.AddRange(columnsIndexes);
StartColumns.Add(columnsIndexes[0], columnsIndexes.Length);
Titles.Add(columnsIndexes[0], title);
}
}

Attribute Serializable is added in order to have a possibility to store information about merged columns in ViewState - it is required if paging or sorting is used.
That is the only additional action. Now the code usage.

.ascx file:

//for GridView

//for DataGrid


Columns can be defined in design time or can be auto generated - it does not matter and doesn't influence the further code. Merging also does not harm sorting and paging if they are used in the GridView/DataGrid.

.cs file:

//property for storing of information about merged columns
private MergedColumnsInfo info
{
get
{
if (ViewState["info"] == null)
ViewState["info"] = new MergedColumnsInfo();
return (MergedColumnsInfo)ViewState["info"];
}
}
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
//merge the second, third and fourth columns with common title "Subjects"
info.AddMergedColumns(new int[] { 1, 2, 3 }, "Subjects");
grid.DataSource = ... //some data source
grid.DataBind();
}
}


Particular code for GridView: protected void GridView_RowCreated(object sender, GridViewRowEventArgs e)
{
//call the method for custom rendering the columns headers
if (e.Row.RowType == DataControlRowType.Header)
e.Row.SetRenderMethodDelegate(RenderHeader);
}


and for DataGrid: protected void DataGrid_ItemCreated(object sender, DataGridItemEventArgs e)
{
//call the method for custom rendering the columns headers
if (e.Item.ItemType == ListItemType.Header)
e.Item.SetRenderMethodDelegate(RenderHeader);
}

Next code is common for both GridView and DataGrid:

//method for rendering the columns headers

private void RenderHeader(HtmlTextWriter output, Control container)
{
for (int i = 0; i < container.Controls.Count; i++)
{
TableCell cell = (TableCell)container.Controls[i];
//stretch non merged columns for two rows
if (!info.MergedColumns.Contains(i))
{
cell.Attributes["rowspan"] = "2";
cell.RenderControl(output);
}
else //render merged columns common title
if (info.StartColumns.Contains(i))
{
output.Write(string.Format("{1}",
info.StartColumns[i], info.Titles[i]));
}
}

//close the first row
output.RenderEndTag();
//set attributes for the second row
grid.HeaderStyle.AddAttributesToRender(output);
//start the second row
output.RenderBeginTag("tr");

//render the second row (only the merged columns)
for (int i = 0; i < info.MergedColumns.Count; i++)
{
TableCell cell = (TableCell)container.Controls[info.MergedColumns[i]];
cell.RenderControl(output);
}
}