实体框架核心-迁移-没有为此对象定义无参数构造函数 [英] Entity Framework Core - Migration - No Parameterless Constructor Defined for this Object
问题描述
我正在使用Visual Studio 2017中的最新.Net Core和EF Core。我创建了一个模型，并且运行良好。此后，我进行了一些修改，并在尝试添加新迁移时出现以下错误：
构建成功。
0警告
0错误
经过的时间00：00：09.08
System.MissingMethodException：没有为此对象定义无参数的构造函数。 System.RuntimeTypeHandle.CreateInstance（RuntimeType类型，布尔publicOnly，布尔noCheck，布尔& canBeCached，RuntimeMethodHandleInternal& ctor，布尔&bNeedSecurityCheck）中的
，System.RuntimeType.CreateInstanceSlow（布尔publicOnly，布尔skipCheckThis，布尔fillCache，StackCrawlMark& amp; StackMark）System.Activator.CreateInstance（类型类型，布尔非公共）处的
System.Activator.CreateInstance（Type类型）处的
Microsoft.EntityFrameworkCore.Design.Internal处的
。 DbContextOperations。<> c__DisplayClass12_0。< FindContextTypes> b__3（）
在Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext（Func`1 factory）
在Microsoft.EntityFrameworkCore.Design.Internal。 Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration（String name，String outputDir，String contextType）的DbContextOperations.CreateContext（String contextType）
Microsoft.EntityFrameworkCore的
.Design.OperationExecutor.AddMigrationImpl（字符串名称，字符串outputDir，字符串contextType）
在Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase。<> c__DisplayClass3_0`1。< Execute> b__0（）
在Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute（Action action）
没有为此对象定义无参数构造函数。
自上次签入以来，我已经比较了我的代码，并注释掉了某些代码块以查看是否错误仍然存在，无论我如何注释，它仍然会失败，并显示相同的错误。
问题：有没有办法获取更多详细信息到底什么类型没有无参数构造函数？甚至可以从VS中运行它并可能获得断点？
更新：根据一些评论，下面是一些代码。 / p>
对于 DbContext 覆盖
公共类AlmanacDb：IdentityDbContext< ApplicationUser，ApplicationRole，int> {
私人唯读ILogger记录器；
public AlmanacDb（DbContextOptions< AlmanacDb>选项，ILoggerFactory loggerFactory）：base（options）{
this.logger = loggerFactory.CreateLogger< AlmanacDb>（）;
}
受保护的覆盖无效void OnConfiguring（DbContextOptionsBuilder optionsBuilder）{
optionsBuilder.UseSqlServer（ Server = .\\SQLExpress; Database = Almanac; Trusted_Connection = True; MultipleActiveResultSets =真正;）;
}
...
}
我确实有一个 IDbContextFactory ，但是它没有在任何地方被引用，并且没有引用就可以工作。根据第二条评论中提供的链接，不确定是否存在问题。如果内存可用，只要 IDbContextFactory 在解决方案中，它应该找到它吗？
公共类AlmanacDbFactory：IDbContextFactory< AlmanacDb> {
私有IConfigurationRoot配置；
私有只读ILoggerFactory loggerFactory;
公共AlmanacDbFactory（ILoggerFactory loggerFactory）{
var builder = new ConfigurationBuilder（）
.SetBasePath（System.AppContext.BaseDirectory）
.AddJsonFile（ appsettings.json ，可选：false，reloadOnChange：true）;
this.configuration = builder.Build（）;
this.loggerFactory = loggerFactory;
}
public AlmanacDb Create（DbContextFactoryOptions options）{
var optionsBuilder = new DbContextOptionsBuilder< AlmanacDb>（）;
optionsBuilder.UseSqlServer（
configuration.GetConnectionString（ AlmanacSQL），m => {m.EnableRetryOnFailure（）;}
）;
返回新的AlmanacDb（optionsBuilder.Options，loggerFactory）;
}
}
我的 Startup.cs 类
public void ConfigureServices（IServiceCollection服务）{
试试{
服务.AddDbContext< AlmanacDb>（options =>
options.UseSqlServer（Configuration.GetConnectionString（ AlmanacSQL））
）;
services.AddScoped（typeof（IRepository<>），typeof（Repository<>）））;
services.AddIdentity< ApplicationUser，ApplicationRole>（）
.AddEntityFrameworkStores< AlmanacDb，int>（）
.AddDefaultTokenProviders（）;
services.AddScoped< SignInManager< ApplicationUser> ;, AvantiaSignInManager< ApplicationUser>>（）;
services.AddAuthorization（x => {
x.AddPolicy（ EmployeeOnly，p => p.RequireClaim（ EmployeeNumber））;
}）;
services.AddMvc（）;
} catch（Exception e）{
Console.WriteLine（e.ToString（））;
投掷；
}
}
public void Configure（IApplicationBuilder应用程序，IHostingEnvironment env，ILoggerFactory loggerFactory，AlmanacDb上下文）{
... //实际上没有触及AlmanacDb这段代码，所以我忽略了它
}
更新2：Solutiuon
看到我正在学习ASP.Net Core，我读了很多网站（包括docs.microsoft.com），当时我把它放在 IDbContextFactory 代码，如上所示。我从代码中删除了该错误，错误消失了，一切都建立并创建了迁移。
我将@alessalessio标记为我认为的答案（没有进行了测试），在 AlmanacDbFactory 构造函数中删除 ILoggerFactory loggerFactory 依赖关系也可以解决问题。
设计时工具会尝试自动查找应用程序如何创建DbContext类型的实例。如果EF无法找到合适的方式初始化DbContext，则可能会遇到此错误。
选项：
1-要么创建无参数构造函数
public AlmanacDb（）{}
受保护的重写void OnConfiguring（DbContextOptionsBuilder optionsBuilder）
{
optionsBuilder.UseSqlServer （_connString）;
}
私有只读字符串_connString =<您的conn字符串>;
2-
public AlmanacDb Create（）
{
var optionsBuilder = new DbContextOptionsBuilder< AlmanacDb>（）;
optionsBuilder.UseSqlServer（connectionString）;
返回新的AlmanacDb（optionsBuilder.Options）;
}
https://docs.microsoft.com/zh-cn/ef/core/miscellaneous/configuring-dbcontext
I am working with the latest .Net Core and EF Core in Visual Studio 2017. I have created a model and it was working great. I have since made some modifications and am getting the following error when I try to add a new migration:
Build succeeded.
0 Warning(s)
0 Error(s)
Time Elapsed 00:00:09.08
System.MissingMethodException: No parameterless constructor defined for this object.
at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck)
at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
at System.Activator.CreateInstance(Type type, Boolean nonPublic)
at System.Activator.CreateInstance(Type type)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass12_0.<FindContextTypes>b__3()
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
at Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
No parameterless constructor defined for this object.
I have compared my code since the last check in and have commented out certain chunks of code to see if the error persists and whatever I comment out, it still fails with the same error.
Question: Is there a way to get more detailed information on exactly WHAT type does not have a parameterless constructor? Or even run this from within VS and perhaps get a breakpoint?
Update: Based on some of the comments, here is some code.
For the DbContext override
public class AlmanacDb : IdentityDbContext<ApplicationUser, ApplicationRole, int> {
private readonly ILogger logger;
public AlmanacDb(DbContextOptions<AlmanacDb> options, ILoggerFactory loggerFactory) : base(options) {
this.logger = loggerFactory.CreateLogger<AlmanacDb>();
}
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) {
optionsBuilder.UseSqlServer("Server=.\\SQLExpress;Database=Almanac;Trusted_Connection=True;MultipleActiveResultSets=True;");
}
...
}
I do have a IDbContextFactory but it is not referenced anywhere and worked without a reference. Not sure if this is a problem or not based on a link provided in the 2nd comment. If memory serves, as long as the IDbContextFactory is within the solution, it should find it?
public class AlmanacDbFactory : IDbContextFactory<AlmanacDb> {
private IConfigurationRoot configuration;
private readonly ILoggerFactory loggerFactory;
public AlmanacDbFactory(ILoggerFactory loggerFactory) {
var builder = new ConfigurationBuilder()
.SetBasePath(System.AppContext.BaseDirectory)
.AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);
this.configuration = builder.Build();
this.loggerFactory = loggerFactory;
}
public AlmanacDb Create(DbContextFactoryOptions options) {
var optionsBuilder = new DbContextOptionsBuilder<AlmanacDb>();
optionsBuilder.UseSqlServer(
configuration.GetConnectionString("AlmanacSQL"), m => { m.EnableRetryOnFailure(); }
);
return new AlmanacDb(optionsBuilder.Options, loggerFactory);
}
}
My Startup.cs class
public void ConfigureServices(IServiceCollection services) {
try {
services.AddDbContext<AlmanacDb>(options =>
options.UseSqlServer(Configuration.GetConnectionString("AlmanacSQL"))
);
services.AddScoped(typeof(IRepository<>), typeof(Repository<>));
services.AddIdentity<ApplicationUser, ApplicationRole>()
.AddEntityFrameworkStores<AlmanacDb, int>()
.AddDefaultTokenProviders();
services.AddScoped<SignInManager<ApplicationUser>, AvantiaSignInManager<ApplicationUser>>();
services.AddAuthorization(x => {
x.AddPolicy("EmployeeOnly", p => p.RequireClaim("EmployeeNumber"));
});
services.AddMvc();
} catch (Exception e) {
Console.WriteLine(e.ToString());
throw;
}
}
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory, AlmanacDb context) {
... // Nothing actually touches the AlmanacDb within this code so I left it out
}
Update 2: Solutiuon
Seeing that I am learning ASP.Net Core I am reading a ton of sites (including docs.microsoft.com) at one point I put in the IDbContextFactory code as you can see above. I removed that from my code and the error went away and everything built and created the migration.
I am going to mark @alessalessio as the answer as I assume (have not tested it yet) that taking out the ILoggerFactory loggerFactory dependency within the AlmanacDbFactory constructor will do the trick as well.
Design-time tools attempt to automatically find how your application creates instances of your DbContext type. If EF cannot find a suitable way to initialize your DbContext, you may encounter this error.
Options: 1- Either create a parameterless constructor
public AlmanacDb() { }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer(_connString);
}
private readonly string _connString = "<your conn string>";
2-
public AlmanacDb Create()
{
var optionsBuilder = new DbContextOptionsBuilder<AlmanacDb>();
optionsBuilder.UseSqlServer(connectionString);
return new AlmanacDb(optionsBuilder.Options);
}
https://docs.microsoft.com/en-us/ef/core/miscellaneous/configuring-dbcontext
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