如何使LINQ to SQL使用运行时正在修改的连接字符串?

我正在尝试使用一些困难 Connection String Builders (ADO.NET) 在LINQ到SQL中。让我告诉你们我在做什么:

这个 app.config 文件:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <configSections>
    </configSections>
    <connectionStrings>
        <add name="LoremIpsum"
             connectionString="Data Source=SomeServer;Initial Catalog=SomeDB;User ID=joe;"
             providerName="System.Data.SqlClient" />
    </connectionStrings>
</configuration>

ConnectionStringSettings settings = 
    ConfigurationManager.ConnectionStrings["LoremIpsum"];
if (null != settings)
{
    string connection = settings.ConnectionString;
    SqlConnectionStringBuilder builder = 
         new SqlConnectionStringBuilder(connection);

    // passwordTextBox being the control where joe the user actually 
    // enters his credentials           
    builder.Password = passwordTextBox.Text;
}

LINQTOSQLDataClassDataContext db = new LINQTOSQLDataClassDataContext();

// finally some rather anecdotic LINQ sentence here:
var foo = db.Table.Single(bar => bar.Table == whatever);

另一方面,检查 :

?builder.ConnectionString
"Data Source=SomeServer;Initial Catalog=SomeDB;User ID=joe;Password=swordfish"

我总是遇到一个异常:用户“joe”登录失败。有什么想法吗?非常感谢。