c# - 在 'TContext' 上找不到无参数构造函数

Question

我不太清楚为什么会发生错误。当我第一次搭建我的项目时，迁移和更新数据库命令运行良好，但在对应用程序进行一些更改后，我得到了 this error .唯一的解决方案是 this :

public class BloggingContextFactory : IDbContextFactory<BloggingContext>
{
    public BloggingContext Create()
    {
        var optionsBuilder = new DbContextOptionsBuilder<BloggingContext>();
        optionsBuilder.UseSqlite("Data Source=blog.db");

        return new BloggingContext(optionsBuilder.Options);
    }
}

我的 DbContext:

public class ApplicationDbContext : IdentityDbContext<ApplicationUser>, IApplicationDbContext
{
    public DbSet<ApplicationUserCode> ApplicationUserCodes { get; set; }

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

    protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);
        // Customize the ASP.NET Identity model and override the defaults if needed.
        // For example, you can rename the ASP.NET Identity table names and more.
        // Add your customizations after calling base.OnModelCreating(builder);
    }
}

它对我有用，但连接字符串是硬编码在类中的，这对我来说不是很好。非常感谢任何关于为什么这个错误没有一次发生但后来发生的澄清，这也是一个比在类中嵌入连接字符串更优雅的解决方案。

Answer 1

你有吗

public class BloggingContext : DbContext
{
    public BloggingContext(){          // << The reason....

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

   public DbSet<Blog> Blogs { get; set; }
}

另一个原因是在 OnModelCreating( ){} 方法本身的顶部。让我想了想为什么......然后我尝试了一些东西，低，看它是一个简单的无参数构造函数，它看起来是罪魁祸首。

 protected override void OnModelCreating(ModelBuilder builder)
    {
        base.OnModelCreating(builder);   //<< Absolutely required at the top.
        // Customize the ASP.NET Identity model and override the defaults if needed.
        // For example, you can rename the ASP.NET Identity table names and more.
        // Add your customizations after calling base.OnModelCreating(builder);

编辑

//Startup.cs ConfigureServices(IServiceCollection服务)

 services.AddDbContext<ApplicationDbContext>(options =>
      options.UseSqlite(Configuration.GetConnectionString("SqliteConnection")));

//appsettings.json

{
  "ConnectionStrings": {
   "DefaultConnection": "Data Source=(LocalDb)\\MssqlLocalDb;Initial        Catalog=PilotSystemCore;Integrated Security=True",
"SqliteConnection" :  "Data Source=SqliteDbName.db"
 },
 "Logging": {
    "IncludeScopes": false,
     "LogLevel": {
         "Default": "Debug",
         "System": "Information",
         "Microsoft": "Information"
   }
 }
}

注意:如果您之前使用 SqlServer 进行过任何迁移，则生成的项目长期有效，重做迁移。