how to Unable to cast COM object of type 'System.__ComObject

Directory.CreateDirectory(textBox2.Text + ":\\" + textBox3.Text);

int i = 0;

foreach (string txt in richTextBox1.Lines)

if ((richTextBox1.Lines[i].Length) != 0)

Directory.CreateDirectory(textBox2.Text + ":\\" + textBox3.Text + "\\" + s);

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

Microsoft.Office.Interop.Excel.Workbook book = app.Workbooks.Add(XlSheetType.xlWorksheet);

Microsoft.Office.Interop.Excel.Worksheet sheet = book.Worksheets[1] as Microsoft.Office.Interop.Excel.Worksheet;

app.Visible = false;

Range rng1 = sheet.get_Range("A1", "A1");

rng1.Value2 = "?????";

rng1.Font.Bold = true;

Range rng2 = sheet.get_Range("B1", "B1");

rng2.Value2 = "??? ???";

rng2.Font.Bold = true;

Range rng3 = sheet.get_Range("C1", "C1");

rng3.Value2 = "?? ????";

rng3.Font.Bold = true;

Class1.com3.Connection = Class1.con;

Class1.con.Close();

Class1.con.Open();

Class1.com3.CommandText = "SELECT * FROM [mobile_list] WHERE (city = '" + s + "') and (codeposti='" + richTextBox1.Lines[i] + "') ";

Class1.datareader1 = Class1.com3.ExecuteReader();

int j = 1;

while (Class1.datareader1.Read())

Range r1 = sheet.get_Range("A" + j.ToString(), "A" + j.ToString());

r1.Value2 = Class1.datareader1.GetValue(0).ToString();

Range r2 = sheet.get_Range("B" + j.ToString(), "B" + j.ToString());

r2.Value2 = Class1.datareader1.GetValue(1).ToString();

Range r3 = sheet.get_Range("C" + j.ToString(), "C" + j.ToString());

r3.Value2 = Class1.datareader1.GetValue(2).ToString();

richTextBox2.Text = "???? ??? ???? ??? ?? ??? ????? ???? ?? ????";

richTextBox2.Text = "???? ???? ?? ????" + richTextBox1.Lines[i] + "????? ?? ?? ????" + textBox2.Text + ":\\" + textBox3.Text + "\\" + s + "\\" + richTextBox1.Lines[i] + ".xls";

book.SaveAs(textBox2.Text + ":\\" + textBox3.Text + "\\" + s + "\\" + richTextBox1.Lines[i] + ".xls", Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);

else