如果应用程序的 App_Data 目录已存在，则 Web 服务器帐户只要求对应用程序的 App_Data 目录具有读写访问权限。这是必要的权限，因为 Web 服务器帐户将尝试验证应用程序的 App_Data 目录中是否已存在 SQL Server Express 数据库。如果撤消 Web 服务器帐户对 App_Data 目录的读访问权限，提供程序便无法正确地确定 SQL Server Express 数据库是否已存在。如果提供程序尝试创建已存在的数据库的副本，则会出错。写访问权限也是必需的，因为创建新数据库时需要使用 Web 服务器帐户凭据。

If the application's App_Data directory already exists, the web server account only requires read and write access to the application's App_Data directory. This is necessary because the web server account will attempt to verify that the Sql Server Express database already exists within the application's App_Data directory. Revoking read access on the App_Data directory from the web server account will prevent the provider from correctly determining if the Sql Server Express database already exists. This will cause an error when the provider attempts to create a duplicate of an already existing database. Write access is required because the web server account's credentials are used when creating the new database.

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