saving selected item from combobox , combox is filled by database values

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.NET

.NET4

I am having problem in saving selected item from combo box. , combo box is filled by database values.
my code is given below.I have made a method "viewincombo(string str)".
private void viewincombo(string str)
{
string connectionString = (@"Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Documents and Settings\nic-15\My Documents\ALRS.mdb");
OleDbConnection con = new OleDbConnection(connectionString);
con.Open();
//string str = "select thana from thana";
OleDbDataAdapter da = new OleDbDataAdapter(str, con);
DataSet ds = new DataSet();
da.Fill(ds);
comboBoxView.Visible = true;
comboBoxView.DataSource = ds.Tables[0];

}
and i am calling this method like this
private void buttonViewCombo_Click(object sender, EventArgs e)
{
string str = "select * from Licence_Category";
viewincombo(str);
comboBoxView.DisplayMember = "Licence_Category";
}
Table name is Licence_Category and field name is licence_category.
Above code is working but

textBoxField.Text = comboBoxView.SelectedItem.ToString();

doesnt not works.
Thanks in advance.