Activation error occured while trying to get instance of type database, error occurring only when connection string is created dynamically

See more:

C#

.NET4.5

I have a piece of code (.NET Framework 4.5.2, Enterprise Library 5.0.505.0) where I need to connect to a SQL Server database. However, the DB name may keep changing depending on user's requirement. So, I have the following piece of code to write the connection string dynamically to the app.config file.

public void CreateNewConnectionStringInConfig(string initialCatalog)
{
      SqlConnectionStringBuilder builder = new SqlConnectionStringBuilder();
      builder.DataSource = ConfigurationManager.AppSettings["Data Source"];
      builder.PersistSecurityInfo = true;
      builder.InitialCatalog = initialCatalog; //This is the DB name
      builder.UserID = ConfigurationManager.AppSettings["User ID"];
      builder.Password = ConfigurationManager.AppSettings["Password"];

      // Get the application configuration file.
      Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

      // Create a connection string element.
      ConnectionStringSettings csSettings = new ConnectionStringSettings("UserSelectedConnectionString", builder.ConnectionString, "System.Data.SqlClient");

      // Get the connection strings section.
      ConnectionStringsSection csSection = config.ConnectionStrings;

      // Add the new element.
      csSection.ConnectionStrings.Add(csSettings);

      // Save the configuration file.
      config.Save(ConfigurationSaveMode.Modified);

     // Refresh the section so the new configuration can be re-read
      ConfigurationManager.RefreshSection("connectionStrings");

}

I checked and the connection string is getting created fine in the vshost.exe.Config file while debugging. However, when I am trying to create the Database object, I am getting an error. The code used to create the DB object is as shown below.

public class MyDac
{
    private readonly Database _db;

    public MyDac()
    {
        DatabaseProviderFactory factory = new DatabaseProviderFactory();
        _db = factory.Create("UserSelectedConnectionString");
    }
}

I am getting the following error while trying to create the _db object.

Activation error occured while trying to get instance of type Database, key "UserSelectedConnectionString"<br />
<br />
Microsoft.Practices.ServiceLocation.ActivationException: Activation error occured while trying to get instance of type Database, key "UserSelectedConnectionString" --->     Microsoft.Practices.Unity.ResolutionFailedException: Resolution of the dependency failed, type = "Microsoft.Practices.EnterpriseLibrary.Data.Database", name = "UserSelectedConnectionString".<br />
Exception occurred while: while resolving.<br />
Exception is: InvalidOperationException - The type Database does not have an accessible constructor.

Any help will be much appreciated. Thanks!

What I have tried:

1) Upgrading the Enterprise Library version to 6.0.0.0 resolves the issue but that is not an option for me. I have to keep it to version 5.0.505.0.

2) When I hard code the connection string in the App.config file from before hand (rather than writing it during run time), the app works fine. However, I can't do that in real life because the database name will keep on changing.