從配置文件（appsettings.json）中讀取數據庫上下文的ASP.NET核心運行遷移

Question

我正在嘗試使用ASP.NET Core 1.1應用程序，並試圖從配置文件中讀取連接字符串。

我的數據庫上下文類看起來是這樣的：

public class ApplicationDbContext : DbContext, IDbContextFactory<ApplicationDbContext> 
{ 
    public ApplicationDbContext() 
    { 
    } 

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options) 
    { 
    } 

    protected override void OnModelCreating(ModelBuilder builder) 
    { 
     base.OnModelCreating(builder); 
    } 

    public ApplicationDbContext Create(DbContextFactoryOptions options) 
    { 
     var builder = new DbContextOptionsBuilder<ApplicationDbContext>(); 

     string connString = "some hardcoded connection string"; 
     builder.UseSqlServer(connString); 
     return new ApplicationDbContext(builder.Options); 
    } 
} 

運行的遷移工作正常（例如dotnet ef database update）。

如果我想刪除硬編碼的連接字符串，我面臨着以下問題：

• ，如果我嘗試注入IConfiguration爲ApplicationDbContext，遷移將會失敗No parameterless constructor defined for this object.
• 沒有注入的東西我無法找到一種從配置文件中讀取信息的方式

我該如何繼續？

謝謝。

[編輯]

在Startup.cs我已經配置的連接字符串，並將其正常工作時（runtime）：

public void ConfigureServices(IServiceCollection services) 
{ 
    // Add framework services. 
    //services.AddApplicationInsightsTelemetry(Configuration); 

    string connString = ConfigurationExtensions.GetConnectionString(Configuration, "DefaultConnection"); 
    services.AddDbContext<ApplicationDbContext>(options => options.UseSqlServer(connString)); 

    services.AddMvc(); 
} 

的問題是，dotnet migrations似乎需要在數據庫環境參數的構造函數並實施IDbContextFactory<>，我不知道如何注入配置。後

編輯試圖IOptions <>與ApplicationDbContext

我曾嘗試提出解決方案，但我不能讓它工作，因爲dotnet migration將調用默認的構造函數。

public class ApplicationDbContext : DbContext, IDbContextFactory<ApplicationDbContext> 
{ 
    private IOptions<DefaultConnectionValue> ConnOptions; 
    private string connectionString; 

    public ApplicationDbContext() 
    { 
     Console.WriteLine("Default constructor was called"); 
    } 

    public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options, IOptions<DefaultConnectionValue> connOptions) : base(options) 
    { 
     ConnOptions = connOptions; 
     connectionString = connOptions.Value.Value; 

     Console.WriteLine($"Injected connection string = {connectionString}"); 
    } 

    protected override void OnModelCreating(ModelBuilder builder) 
    { 
     base.OnModelCreating(builder); 
    } 

    public ApplicationDbContext Create(DbContextFactoryOptions options) 
    { 
     var builder = new DbContextOptionsBuilder<ApplicationDbContext>(); 

     //string connString = ConfigurationExtensions.GetConnectionString(null, "DefaultConnection"); 
     builder.UseSqlServer(connectionString); 
     return new ApplicationDbContext(builder.Options, ConnOptions); 
    } 

Answer 1

我面臨同樣的問題，我解決它以下列方式：

1. 與服務集合中的連接字符串註冊選項
2. 進樣選擇到的DbContext
3. 用戶連接DBContext無參數構造函數中的字符串

感謝您的支持具有遷移所需的無參數構造函數。

這裏是你如何註冊配置設置：

public class Startup 
{ 
    public Startup(IHostingEnvironment env) 
    { 
     // Set up configuration sources. 
     var builder = new ConfigurationBuilder() 
      .SetBasePath(env.ContentRootPath) 
      .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true); //let's assume that here you have your connection string 

     Configuration = builder.Build(); 
    } 

    public IConfigurationRoot Configuration { get; set; } 

    public void ConfigureServices(IServiceCollection services) 
    { 
     // Adds services required for using options. 
     services.AddOptions(); 

     // Register the IConfiguration instance which MyOptions binds against. 
     services.Configure<MyOptions>(Configuration); 

     ... 
    } 
} 

然後你就可以使用它作爲休耕（這是控制器的例子，但你可以做同樣的用的DbContext）：

public class HomeController : Controller 
{ 
    private readonly MyOptions _optionsAccessor; 

    public HomeController(IOptions<MyOptions> optionsAccessor) 
    { 
     //injection of your configuration object with connection string 
     _optionsAccessor = optionsAccessor.Value; 
    } 

    public IActionResult Index() 
    { 
     var option1 = _optionsAccessor.Option1; 
     var option2 = _optionsAccessor.Option2; 
     return Content($"option1 = {option1}, option2 = {option2}"); 
    } 
} 

有關配置設置使用的更多信息，請看這裏：Configuration。

UPDATE：

您也可以嘗試不同的方法 - 看配置的DbContext構造內：

public class ApplicationDbContext : DbContext, IDbContextFactory<ApplicationDbContext> 
{ 
    private string connectionString; 

    public ApplicationDbContext() 
    { 
     var builder = new ConfigurationBuilder().AddJsonFile("appsettings.json", optional: true, reloadOnChange: true); //let's assume that here you have your connection string 
     var configuration = builder.Build(); 

     connectionString = ""; //FILL THE CONNECTION STRING FROM THE CONFIGURATION OBJECT 
    } 

    ... 
}