Asp.Net核心-如何设定数据种子-未设置为对象实例的对象引用
Asp.Net核心-如何设定数据种子-未设置为对象实例的对象引用
提问于 2019-06-20 07:28:14
在转换所有表以开始使用标识列中的Guid类型后，我未能设定数据种子，因此我简化了大量代码以定位错误，并以一个种子类结束，如下所示：
public class SeedTest
{
private readonly MyDbContext _context;
public SeedTest(MyDbContext context)
{
_context = context;
}
public async Task SeedTest()
{
Values value1 = new Values
{
Id = Guid.Parse("29c48913-1b5c-47b8-g144-08d6d2273deb"),
ValueName = "value 1",
Created = DateTime.Now
};
_context.Values.Add(value1);
await _context.SaveChangesAsync();
}
public SeedTest()
{
}
}这个类是从另一个调用的：
public interface IDatabaseInitializer
{
Task SeedAsync();
}
public class DatabaseInitializer : IDatabaseInitializer
{
public async Task SeedAsync()
{
SeedTest _seedTest = new SeedTest();
await _seedTest.SeedTest();
}
}从startup.cs调用的
public class Startup
{
public IConfiguration Configuration { get; }
private readonly IHostingEnvironment _hostingEnvironment;
public Startup(IConfiguration configuration, IHostingEnvironment env)
{
Configuration = configuration;
_hostingEnvironment = env;
}
public void ConfigureServices(IServiceCollection services)
{
...
services.AddMyDbContext<MyDbContext>(options =>
options.UseSqlServer("ConnectionStrings:MyCn"));
...
// DB Seeding
services.AddTransient<IDatabaseInitializer, DatabaseInitializer>();
...
...
}下面是如何从program.cs触发的
public class Program
{
public static void Main(string[] args)
{
var host = BuildWebHost(args);
using (var scope = host.Services.CreateScope())
{
var services = scope.ServiceProvider;
try
{
var databaseInitializer = services.GetRequiredService<IDatabaseInitializer>();
databaseInitializer.SeedAsync().Wait();
}
catch (Exception ex)
{
var logger = services.GetRequiredService<ILogger<Program>>();
logger.LogCritical(LoggingEvents.INIT_DATABASE, ex, LoggingEvents.INIT_DATABASE.Name);
}
}
host.Run();
}
public static IWebHost BuildWebHost(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>()
.Build();
}不幸的是，这个实现没有在数据库中植入任何数据，我能找到的唯一错误是在日志文件中，它说：
System.NullReferenceException:对象引用未设置为对象的实例。它指向SeedTest类的最后一行。
那么我到底做错了什么呢？
回答 3
Stack Overflow用户
回答已采纳
发布于 2019-06-20 08:07:38
new SeedTest()不初始化其_context字段。您可以在DatabaseInitializer上使用DI来实例化具有MyDbContext的SeedTest。
public class DatabaseInitializer : IDatabaseInitializer
{
private readonly MyDbContext _context;
public DatabaseInitializer(MyDbContext context)
{
_context = context;
}
public async Task SeedAsync()
{
SeedTest _seedTest = new SeedTest(_context);
await _seedTest.SeedTest();
}
}Stack Overflow用户
发布于 2019-06-20 08:09:59
您在DatabaseInitialize中显式地创建了一个SeedTest实例，而依赖项注入服务正在创建DatabaseInitialize的实例。在具有正确作用域的服务中注册SeedTest类，并让依赖项注入完成它的工作。
在ConfigureServices中添加如下内容
services.AddTransient<SeedTest>();修改DatabaseInitializer
public class DatabaseInitializer : IDatabaseInitializer{
private readonly SeedTest _seedTest;
public DatabaseInitializer(SeedTest seedTest)
{
_seedTest = seedTest;
}
public async Task SeedAsync()
{
await _seedTest.SeedTest();
}
}删除无参数的SeedTest构造函数，并确保注册的MyDbContext类型是传递给另一个构造函数的类型，因为您同时拥有MyDbContext和DbContext。
Stack Overflow用户
发布于 2019-06-20 09:40:09
你可以试试这个，我在这个示例中使用了.net Core2.2-
MyDbContext.cs
public class MyDbContext : DbContext
{
public MyDbContext(DbContextOptions<MyDbContext> options) : base(options)
{
Database.EnsureCreated();
}
public DbSet<Values> Values { get; set; }
}SeedTest.cs
public class SeedTest
{
private readonly MyDbContext _context;
public SeedTest(MyDbContext context)
{
_context = context;
}
public async Task SeedTest1()
{
Values value1 = new Values
{
Id = Guid.Parse("AFE1052A-A694-48AF-AA77-56D2D945DE31"),
ValueName = "value 1",
Created = DateTime.Now
};
_context.Values.Add(value1);
var value = await _context.SaveChangesAsync();
}
public SeedTest()
{
}
}服务
public interface IDatabaseInitializer
{
Task SeedAsync();
}
public class DatabaseInitializer : IDatabaseInitializer
{
private readonly MyDbContext _cotext;
// Inject DbContext
public DatabaseInitializer(MyDbContext dbContext)
{
_cotext = dbContext;
}
public async Task SeedAsync()
{
// Object with contructor which having DbContext parameter
SeedTest _seedTest = new SeedTest(_cotext);
await _seedTest.SeedTest1();
}
}startup.cs
services.AddTransient<IDatabaseInitializer, DatabaseInitializer>();
services.AddDbContext<MyDbContext>(option=> option.UseSqlServer("Data Source=localhost;Initial Catalog=StackOverFlow1;Integrated Security=True"));program.cs
public class Program
{
public static void Main(string[] args)
{
var host = CreateWebHostBuilder(args).Build();
using (var scope = host.Services.CreateScope())
{
var services = scope.ServiceProvider;
try
{
var databaseInitializer = services.GetRequiredService<IDatabaseInitializer>();
databaseInitializer.SeedAsync().Wait();
}
catch (Exception ex)
{
}
}
host.Run();
}
public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>();
}你可以阅读更多关于seed data的内容。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/56677148
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