Invalid Cast Exception

1st i am retriving file from opendialouge box and saving in into mysql database.....using this code.....

private string curFileName = null;
OpenFileDialog openDlg = new OpenFileDialog();

openDlg.Filter = "JPeg Image|*.jpg|Bitmap Image|*.bmp|Gif Image|*.gif";

string filter = openDlg.Filter;

openDlg.Title = "Open only Image Files";

if (openDlg.ShowDialog() == DialogResult.OK)
{

curFileName = openDlg.FileName;

textBox2.Text = curFileName;
}
FileStream fs = new FileStream(curFileName,

FileMode.OpenOrCreate, FileAccess.Read);
int file = (int)fs.Length;
byte[] rawData = new byte[file];

fs.Read(rawData, 0, (int)file);

fs.Close();
string str = "select * from Doc";
if (con.State != ConnectionState.Open)
con.Open();
MySqlDataAdapter da = new MySqlDataAdapter(str,con);
MySqlCommandBuilder mcb = new MySqlCommandBuilder(da);
DataSet ds = new DataSet("Doc");
da.MissingSchemaAction = MissingSchemaAction.AddWithKey;
da.Fill(ds,"Doc");
DataRow row=ds.Tables["Doc"].NewRow();
row["pfno"] = label7.Text;
row["depname"] = label8.Text;
row["docname"] = comboBox1.Text;
row["docdisc"] = textBox1.Text;
row["addedby"] = sd.uname;
row["depdoc"] =rawData;
ds.Tables["Doc"].Rows.Add(row);
da.Update(ds,"Doc");
if(con!=null)
{
if(con.State==ConnectionState.Open)
{
con.Close();
con.Dispose();
}
MessageBox.Show("Document Saved Successfully");
}
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it is running succesfully for saving but now the main fault when i am retriving that ID specific image on winform using this code....

try
{
string str1 = "select * from doc where pfno='" + label7.Text + "' and depname='"+label8.Text+"'";
con.Open();
MySqlCommand cmd = new MySqlCommand(str1, con);
MySqlDataReader data = cmd.ExecuteReader();
if (data.Read())
{
string name = data.GetString(data.GetOrdinal("depname"));
" error Ocurring on this line below and error is Invalid Cast Exception(Object must implement IConvertible.)"
int filesize = data.GetInt32(data.GetOrdinal("depdoc"));
byte[] rawData = new byte[filesize];
data.GetBytes(data.GetOrdinal("depdoc"), 0, rawData, 0, filesize);
FileStream fs = new FileStream(name, FileMode.Create, FileAccess.Write);
fs.Write(rawData, 0, filesize);
fs.Close();
pictureBox1.Image = new Bitmap(name);
}

}
catch(Exception ex)
{
MessageBox.Show("Document Test " + ex);
}
finally
{
if (con.State == ConnectionState.Open)
{
con.Close();
con.Dispose();

}
so sir now any help???