exporting gridview data to excel getting columncount as zero

protected void Button1_Click(object sender, EventArgs e)

{

OracleConnection conn = new OracleConnection(oradb);

OracleCommand cmd = new OracleCommand();

conn.Open();

cmd.Connection = conn;

cmd.CommandType = CommandType.Text;

cmd.BindByName = true;

//cmd.Parameters.Add(new OracleParameter("BranchID",BranchId));

OracleParameter transdate1= new OracleParameter();

transdate1.OracleDbType = OracleDbType.Date;

transdate1.Value = TextBox1.Text;

transdate1.ParameterName = "transdate1";

cmd.Parameters.Add(transdate1);

OracleParameter transdate2 = new OracleParameter();

transdate2.OracleDbType = OracleDbType.Date;

transdate2.Value = TextBox2.Text;

transdate2.ParameterName = "transdate2";

cmd.Parameters.Add(transdate2);

cmd.CommandText = "select * from cust_details where transdate between :transdate1 and :transdate2";

OracleDataAdapter od = new OracleDataAdapter(cmd);

DataSet ds = new DataSet();

od.Fill(ds);

GridView1.DataSource = ds;

GridView1.DataBind();

//Response.Write("Connected to Oracle" + conn.ServerVersion);

// Close and Dispose OracleConnection object

lblDisplay.Text = "connected";

conn.Close();

//Response.Write("Disconnected");

}

protected void Button2_Click(object sender, EventArgs e)

{

Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();

// creating new WorkBook within Excel application

Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);

// creating new Excelsheet in workbook

Microsoft.Office.Interop.Excel._Worksheet worksheet = null;

// see the excel sheet behind the program

app.Visible = true;

// get the reference of first sheet. By default its name is Sheet1.

// store its reference to worksheet

worksheet = workbook.Sheets["Sheet1"];

worksheet = workbook.ActiveSheet;

// changing the name of active sheet

worksheet.Name = "Exported from gridview";

// storing header part in Excel

for (int i = 1; i

{

worksheet.Cells[1, i] = GridView1.Columns[i - 1].HeaderText;

}

// storing Each row and column value to excel sheet

for (int i = 0; i < GridView1.Rows.Count; i++)

{

for (int j = 0; j < GridView1.Columns.Count; j++)

{

worksheet.Cells[i + 2, j + 1] =GridView1.Rows[i].Cells[j];

}