如何使用c＃连接oracle数据库10g，连接字符串必须在app.config中？

Question

我的app.config文件就像

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <connectionStrings>
    <add name="DB" connectionString="Data Source=192.168.0.16;Persist Security Info=True;User ID=SP;Password=qfirst;Unicode=True"
        providerName="" />
    </connectionStrings>
</configuration>

我创建了一个类文件，如下所示

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Data.OracleClient;
using System.Configuration;

namespace WindowsApplication4
{
    class class1
    {
        string c = System.Configuration.ConfigurationManager.ConnectionStrings["DB"].ConnectionString;
        OracleConnection con = new OracleConnection(C);
        public void openconnection()
        {
            con.Open();
            MessageBox.Show("conn opened");
        }
    }
}

但是当我运行它时，它会显示一条错误消息。请帮助我......