This article will discuss about multiple header in GridView like below image.

Step 1: Create a gridview with two columns, id and name and a datakey category.

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" DataKeyNames="Category" OnRowDataBound="GridView1_OnRowDataBound">
      <Columns>
            <asp:BoundField DataField="id" HeaderText="ID" />
            <asp:BoundField DataField="Name" HeaderText="Name" />
      </Columns>
      <HeaderStyle BackColor="Brown" ForeColor="White" />
</asp:GridView>

Step 2: Create a datatable which will contain name of fruits, flowers and plants.

private DataTable GetData()
{
      DataTable dt = new DataTable();
      DataColumn dc;
      dc = new DataColumn("id");
      dt.Columns.Add(dc);
      dc = new DataColumn("Category");
      dt.Columns.Add(dc);
      dc = new DataColumn("Name");
      dt.Columns.Add(dc);    

      DataRow dr;
      dr = dt.NewRow();
      dr["id"] = "1";
      dr["Category"] = "Fruits";
      dr["Name"] = "Apple";
      dt.Rows.Add(dr);

      dr = dt.NewRow();
      dr["id"] = "2";
      dr["Category"] = "Fruits";
      dr["Name"] = "Mango";
      dt.Rows.Add(dr);

      dr = dt.NewRow();
      dr["id"] = "1";
      dr["Category"] = "Flowers";
      dr["Name"] = "Rose";
      dt.Rows.Add(dr);

      dr = dt.NewRow();
      dr["id"] = "2";
      dr["Category"] = "Flowers";
      dr["Name"] = "Lily";
      dt.Rows.Add(dr);

      dr = dt.NewRow();
      dr["id"] = "1";
      dr["Category"] = "Plants";
      dr["Name"] = "Alpine Poppy";
      dt.Rows.Add(dr);

      dr = dt.NewRow();
      dr["id"] = "2";
      dr["Category"] = "Plants";
      dr["Name"] = "Applecactus ";
      dt.Rows.Add(dr);

      return dt;
}

Step 3: Bind the above created datatable to GridView on Page_Load



protected void Page_Load(object sender, EventArgs e)
{
      GridView1.DataSource = GetData();
      GridView1.DataBind();
}



Step 4: On GridView1_OnRowDataBound, custom CreateHeader method will be called when category will change while binding the data.

protected void GridView1_OnRowDataBound(object sender, GridViewRowEventArgs e)
{
      DataKey dk, dk1;
      string strCategory = string.Empty;
      if (e.Row.RowType == DataControlRowType.DataRow)
      {
            dk = GridView1.DataKeys[e.Row.DataItemIndex];
            if (e.Row.DataItemIndex == 0)
            {
                  dk1 = GridView1.DataKeys[e.Row.DataItemIndex];
            }
            else
            {
                  dk1 = GridView1.DataKeys[e.Row.DataItemIndex - 1];
                  strCategory = dk1.Values["Category"].ToString();
            }
            string sCategory = dk.Values["Category"].ToString();
            switch (sCategory)
            {
                  case "Fruits":
                        if (!dk.Values["Category"].ToString().Equals(strCategory))
                        {
                              CreateHeader("Fruits", e.Row.Cells.Count, e.Row.DataItemIndex + 1);
                        }
                        break;

                  case "Flowers":
                        if (!(dk.Values["Category"].ToString() == dk1.Values["Category"].ToString()))
                        {
                              CreateHeader("Flowers", e.Row.Cells.Count, e.Row.DataItemIndex + 2);
                        }
                        break;

                  case "Plants":
                        if (!(dk.Values["Category"].ToString() == dk1.Values["Category"].ToString()))
                        {
                              CreateHeader("Plants", e.Row.Cells.Count, e.Row.DataItemIndex + 3);
                        }
                        break;
               }
      }
}



Step 5: CreateHeader method will create GridViewRow, TableHeaderCell will create header which will be added in the GridViewRow.

public void CreateHeader(string strCategory, int rowCount, int rowDataIndex)
{
      GridViewRow rowHeader = new GridViewRow(0, 0, DataControlRowType.Header, DataControlRowState.Normal);
      Literal newCells = new Literal();
      newCells.Text = strCategory;
      TableHeaderCell headerCell = new TableHeaderCell();
      headerCell.BackColor = System.Drawing.Color.Black;
      headerCell.Controls.Add(newCells);
      headerCell.ColumnSpan = rowCount;
      rowHeader.Cells.Add(headerCell);
      rowHeader.Visible = true;
      GridView1.Controls[0].Controls.AddAt(rowDataIndex, rowHeader);
}