specific gridview data to be moved into excel file - asp.net

Question

I am trying to move the specific gridview data into the excel file but I am getting an error as object reference not set to the instance of an object
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns = "false" Font-Names = "Arial"
Font-Size = "11pt" AlternatingRowStyle-BackColor = "#C2D69B"
HeaderStyle-BackColor = "green">
<Columns>
<asp:TemplateField>
<HeaderTemplate>
<asp:CheckBox ID="Chckcol0" runat="server" Checked="true"/>
<asp:Label ID="lbl_catid" runat="server" Text=" CategoryID"></asp:Label>
</HeaderTemplate>
<ItemTemplate>
<asp:Label ID="lbl_catid" runat="server" Text='<%# Eval("cat_id")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<HeaderTemplate>
<asp:CheckBox ID="Chckcol1" runat="server" Checked="true"/>
<asp:Label ID="lbl_catname" runat="server" Text="category Name"></asp:Label>
</HeaderTemplate>
<ItemTemplate>
<asp:Label ID="lbl_categoryname" runat="server" Text='<%# Eval("categoryname")%>'> </asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<HeaderTemplate>
<asp:CheckBox ID="Chckcol2" runat="server" Checked="true"/>
<asp:Label ID="description" runat="server" Text="description"></asp:Label>
</HeaderTemplate>
<ItemTemplate>
<asp:Label ID="lbl_description" runat="server" Text='<%# Eval("description")%>'> </asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
And my code behind:
public partial class _Default : System.Web.UI.Page
{
MySqlConnection con = new MySqlConnection("Data Source=localhost;Database=mylibrary_db;User ID=root;Password=MyLuck2010");
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
fillgrid();
}
}
protected void fillgrid()
{
MySqlDataAdapter da = new MySqlDataAdapter("select cat_id,categoryname,description from library_category", con);
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
}
protected void getcheckedgridview()
{
CheckBox chkCol0 = (CheckBox)GridView1.HeaderRow.Cells[0].FindControl("chkCol0");
CheckBox chkCol1 = (CheckBox)GridView1.HeaderRow.Cells[0].FindControl("chkCol1");
CheckBox chkCol2 = (CheckBox)GridView1.HeaderRow.Cells[0].FindControl("chkCol2");
ArrayList arr;
if (ViewState["States"] == null)
{
arr = new ArrayList();
}
else
{
arr = (ArrayList)ViewState["States"];
}
arr.Add(chkCol0.Checked);
arr.Add(chkCol1.Checked);
arr.Add(chkCol2.Checked);
ViewState["States"] = arr;
}
protected void btnExportExcel_Click(object sender, EventArgs e)
{
Response.Clear();
Response.Buffer = true;
Response.AddHeader("content-disposition","attachment;filename=GridViewExport.xls");
Response.Charset = "";
Response.ContentType = "application/vnd.ms-excel";
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
GridView1.AllowPaging = false;
GridView1.DataBind();
GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");
GridView1.HeaderRow.Cells[0].Style.Add("background-color", "green");
GridView1.HeaderRow.Cells[1].Style.Add("background-color", "green");
GridView1.HeaderRow.Cells[2].Style.Add("background-color", "green");
ArrayList arr = (ArrayList)ViewState["States"];
GridView1.HeaderRow.Cells[0].Visible = Convert.ToBoolean(arr[0]);
GridView1.HeaderRow.Cells[1].Visible = Convert.ToBoolean(arr[1]);
GridView1.HeaderRow.Cells[2].Visible = Convert.ToBoolean(arr[2]);
GridView1.HeaderRow.Cells[0].FindControl("chkCol0").Visible = false;
GridView1.HeaderRow.Cells[1].FindControl("chkCol1").Visible = false;
GridView1.HeaderRow.Cells[2].FindControl("chkCol2").Visible = false;
for (int i = 0; i < GridView1.Rows.Count; i++)
{
GridViewRow row = GridView1.Rows[i];
row.Cells[0].Visible = Convert.ToBoolean(arr[0]);
row.Cells[1].Visible = Convert.ToBoolean(arr[1]);
row.Cells[2].Visible = Convert.ToBoolean(arr[2]);
row.BackColor = System.Drawing.Color.White;
row.Attributes.Add("class", "textmode");
if (i % 2 != 0)
{
row.Cells[0].Style.Add("background-color", "#C2D69B");
row.Cells[1].Style.Add("background-color", "#C2D69B");
row.Cells[2].Style.Add("background-color", "#C2D69B");
}
}
GridView1.RenderControl(hw);
string style = #"<style> .textmode { mso-number-format:\#; } </style>";
Response.Write(style);
Response.Output.Write(sw.ToString());
Response.End();
}
}

Answer 1

The fault is most likely to be with the Convert.ToBoolean(arr[0]); (see the line GridView1.HeaderRow.Cells[0].Visible = Convert.ToBoolean(arr[0]);)
I would put a break tag on the faulting line and see what the value of arr[0] is. I would guess arr is empty or null and your ViewState is not returning the data you are expecting. This maybe because your method getcheckedgridview() is not called by your code (or at least, not by the code you've displayed)!
You could try (where the only change is the first line of the method)
protected void btnExportExcel_Click(object sender, EventArgs e)
{
getcheckedgridview();//Only change to your code
Response.Clear();
Response.Buffer = true;
Response.AddHeader("content-disposition","attachment;filename=GridViewExport.xls");
Response.Charset = "";
Response.ContentType = "application/vnd.ms-excel";
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
GridView1.AllowPaging = false;
GridView1.DataBind();
GridView1.HeaderRow.Style.Add("background-color", "#FFFFFF");
GridView1.HeaderRow.Cells[0].Style.Add("background-color", "green");
GridView1.HeaderRow.Cells[1].Style.Add("background-color", "green");
GridView1.HeaderRow.Cells[2].Style.Add("background-color", "green");
ArrayList arr = (ArrayList)ViewState["States"];
GridView1.HeaderRow.Cells[0].Visible = Convert.ToBoolean(arr[0]);
GridView1.HeaderRow.Cells[1].Visible = Convert.ToBoolean(arr[1]);
GridView1.HeaderRow.Cells[2].Visible = Convert.ToBoolean(arr[2]);
GridView1.HeaderRow.Cells[0].FindControl("chkCol0").Visible = false;
GridView1.HeaderRow.Cells[1].FindControl("chkCol1").Visible = false;
GridView1.HeaderRow.Cells[2].FindControl("chkCol2").Visible = false;
for (int i = 0; i < GridView1.Rows.Count; i++)
{
GridViewRow row = GridView1.Rows[i];
row.Cells[0].Visible = Convert.ToBoolean(arr[0]);
row.Cells[1].Visible = Convert.ToBoolean(arr[1]);
row.Cells[2].Visible = Convert.ToBoolean(arr[2]);
row.BackColor = System.Drawing.Color.White;
row.Attributes.Add("class", "textmode");
if (i % 2 != 0)
{
row.Cells[0].Style.Add("background-color", "#C2D69B");
row.Cells[1].Style.Add("background-color", "#C2D69B");
row.Cells[2].Style.Add("background-color", "#C2D69B");
}
}
GridView1.RenderControl(hw);
string style = #"<style> .textmode { mso-number-format:\#; } </style>";
Response.Write(style);
Response.Output.Write(sw.ToString());
Response.End();
}
Edit
Based upon your comments, I think the error message is now due to your code not finding the control in question. So
CheckBox chkCol0 = (CheckBox)GridView1.HeaderRow.Cells[0].FindControl("chkCol0");
does not find the control called "chkCol0" and so therefore chkCol0 is null.