C# 类型为'；的未处理异常；Oracle.DataAccess.Client.OracleException'；发生在Oracle.DataAccess.dll中

我的代码有一个严重的问题，当我试图从表（对应项）中检索ID（编号）时，总是抛出一个异常

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这是一个输入错误，编辑：C#

        private void comboboxCorrespondant_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        #region connectDB

        OracleConnection con = new OracleConnection();
        con.ConnectionString = connectionString;
        con.Open();
        OracleCommand cmd = new OracleCommand();
        cmd.Connection = con;
        #endregion
        cmd.CommandText = " SELECT ID FROM CORRESPONDANT WHERE desingation = " + comboboxCorrespondant.SelectedItem.ToString();
        cmd.BindByName = true;
        object _id = cmd.ExecuteScalar();

        string returnId = _id.ToString();

        IdtextBox.Text = "";
        if (_id != null)
        {
            IdtextBox.Text = returnId;
        }

        con.Dispose();
        cmd.Dispose();

    }