如何在 .NET Core Web App 中获取有效的连接字符串？

Question

这是我的代码，但是当我运行它时，我得到一个始终为null的连接字符串。 我还安装了 EntityFrameworkCore 和 SqlServer NuGet 包。

我得到的错误是这样的：

System.NullReferenceException：“对象引用未设置为对象的实例。” _connectionStrings 为空。

appSettings.Development.json

{
    "Logging": {
            "LogLevel": {
                "Default": "Information",
                "Microsoft": "Warning",
                "Microsoft.Hosting.Lifetime": "Information"
            }
    },

    "ConnectionStrings": {
            "MyDataContext": "[actual connection string]"
        }
}

启动.cs

public class Startup
{
    public IConfiguration Configuration { get; }

    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddRazorPages();

        services.Configure<ConnectionStrings>(Configuration.GetSection("ConnectionStrings"));
        
        services.AddDbContext<MyDataContext>(options => options.UseSqlServer(Configuration.GetConnectionString("MyDataContext")));

    }
}

连接字符串.cs

public class ConnectionStrings
{
    public string MyDataContext { get; set; }
}

我的数据上下文

public class MyDataContext : DbContext
{
    public MyDataContext(DbContextOptions<MyDataContext> options) : base(options)
    {
    }
}

索引.cs.html

public class IndexModel : PageModel
{
    private readonly ConnectionStrings _connectionStrings;


    public IndexModel(ConnectionStrings connectionStrings)
    {
        _connectionStrings = connectionStrings;
    }

    public void OnGet()
    {
        StringBuilder sb = new StringBuilder();

        string sql = "SELECT * FROM MyTable";

        string connectionString = _connectionStrings.MyDataContext;
        SqlConnection conn = new SqlConnection(connectionString);
        conn.Open();

        SqlCommand command = new SqlCommand(sql, conn);
        SqlDataReader dr = command.ExecuteReader();

        while (dr.Read())
        {
            sb.Append(dr[0]);
        }

        conn.Close();

    ...
        // Do more stuff with the sql data here
    }
}

我还获得了对对象连接字符串的空引用。 我错过了什么？

编辑

我添加了constructor ，现在得到这个错误：

InvalidOperationException：尝试激活“MyProject.Pages.IndexModel”时无法解析“MyProject.ConnectionStrings”类型的服务。

除了IndexModel页面的MyProject.Pages之外，所有类都具有MyProject的namespace 。

解决了

    private readonly ConnectionStrings _connectionStrings;
    
    public IndexModel(IOptions<ConnectionStrings> connectionStrings)
    {
        _connectionStrings = connectionStrings.Value;
    }

了解 DI 和选项模式很重要，需要进一步阅读。

Answer 1

请注意， Appsetting.Development.json是拆分文件，您必须将此文件添加到启动中的配置中，例如。

public static IWebHost BuildWebHost(string[] args)
{
    return new WebHostBuilder()
        .UseKestrel()
        .UseContentRoot(Directory.GetCurrentDirectory())
        .ConfigureAppConfiguration((hostingContext, config) =>
        {
            var env = hostingContext.HostingEnvironment;
            config.AddJsonFile("appsettings.json", optional: true)
                .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true);
            config.AddEnvironmentVariables();
        })
        .UseStartup<Startup>()
        .Build();

}

因此，您可以通过 Env Environment 将 ConnectionString 添加到开发和生产文件中

-- 阅读部分的优先级 -> 1 - AppSettings.Json 2 - AppSetting.{Env}.Json