尝试获取数据库类型的实例时发生激活错误，仅在动态创建连接字符串时发生错误

Question

我有一段代码（.NET Framework 4.5.2，Enterprise Library 5.0.505.0）需要连接到 SQL 服务器数据库。但是，数据库名称可能会根据用户的要求不断变化。因此，我有以下代码将连接字符串动态写入 app.config 文件。

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");

}

我检查过，调试时在 vshost.exe.Config 文件中创建的连接字符串正常。但是，当我尝试创建数据库对象时，出现错误。用于创建DB对象的代码如下所示。

public class MyDac
{
    private readonly Database _db;

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

我在尝试创建 _db 对象时遇到以下错误。

Activation error occured while trying to get instance of type Database, key "UserSelectedConnectionString"

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".
Exception occurred while: while resolving.
Exception is: InvalidOperationException - The type Database does not have an accessible constructor.

我尝试过的东西：

1) 将企业库版本升级到 6.0.0.0 可以解决问题，但这对我来说不是一个选项。我必须保持它到版本 5.0.505.0.

2) 当我事先在 App.config 文件中硬编码连接字符串（而不是在运行期间写入）时，应用程序运行良好。但是，我不能在现实生活中这样做，因为数据库名称会不断变化。

任何帮助将不胜感激。谢谢！

Answer 1

我如下所示更改了我的代码并开始工作。但是，我对此没有任何解释：

public class MyDac
{
    private readonly Database _db;

    public MyDac()
    {
        _db = new SqlDatabase("UserSelectedConnectionString");
    }
}

尝试获取数据库类型的实例时发生激活错误，仅在动态创建连接字符串时发生错误

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

c#

database-connection

connection-string

enterprise-library-5