如何动态的选择ConnectionString的值

using System;
using System.Configuration;
namespace Maticsoft.DBUtility
{
    
    public class PubConstant
    {        
        /// <summary>
        /// 获取连接字符串
        /// </summary>
        public static string ConnectionString
        {           
            get 
            {
                string _connectionString = ConfigurationManager.AppSettings["ConnectionString"];       
                string ConStringEncrypt = ConfigurationManager.AppSettings["ConStringEncrypt"];
                if (ConStringEncrypt == "true")
                {
                    _connectionString = DESEncrypt.Decrypt(_connectionString);
                }
                return _connectionString; 
            }
        }        /// <summary>
        /// 得到web.config里配置项的数据库连接字符串。
        /// </summary>
        /// <param name="configName"></param>
        /// <returns></returns>
        public static string GetConnectionString(string configName)
        {
            string connectionString = ConfigurationManager.AppSettings[configName];
            string ConStringEncrypt = ConfigurationManager.AppSettings["ConStringEncrypt"];
            if (ConStringEncrypt == "true")
            {
                connectionString = DESEncrypt.Decrypt(connectionString);
            }
            return connectionString;
        }
    }
}
如上是我获取connectionString的代码, 现在问题是我们有三家工厂,每个工厂数据库不同,但同在一个SQL SERVER 里面, 用户登录后,我会出现三家工厂(A,B,C)的按钮, 当用户点A的时候,connectionString自动获得配置为A的连接字符串,点B的时候,自动获得配置为B的连接字符串呢? 我感觉好难呀