首次迁移时访问 Microsoft.Extensions.Hosting 服务时出错
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【中文标题】首次迁移时访问 Microsoft.Extensions.Hosting 服务时出错【英文标题】：An error occurred while accessing the Microsoft.Extensions.Hosting services when do first migrations 【发布时间】：2020-06-19 01:24:08 【问题描述】：我不明白哪里错了。 我试图用一个非常简单的模型在.net core mvc中制作一个简单的crud，它的字段很少。
这些是我的模型：
public class Employee
[Key] public int EmployeeId get; set;
[Required] public string FistName get; set;
[Required] public string LastName get; set;
public int PositionId get; set;
public virtual Position Position get; set;
public class Position
[Key]
public int PositionId get; set;
public string PositionName get; set;
public ICollection<Employee> Employees get; set;
然后我创建了应用上下文：
public class EmployeeContext : DbContext
public EmployeeContext(DbContextOptions<EmployeeContext> options)
: base(options)
public DbSet<Employee> Employees get; set;
public DbSet<Position> Positions get; set;
protected override void OnModelCreating(ModelBuilder modelBuilder)
modelBuilder.Entity<Employee>()
.HasOne(e => e.Position)
.WithMany()
.HasForeignKey(e => e.PositionId);
并在 Startup.cs 中注册上下文：
public void ConfigureServices(IServiceCollection services)
services.Configure<CookiePolicyOptions>(options =>
// This lambda determines whether user consent for non-essential cookies is needed for a given request.
options.CheckConsentNeeded = context => true;
options.MinimumSameSitePolicy = SameSiteMode.None;
);
services.AddDbContext<EmployeeContext>(item =>item.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
可能还需要一个.csproj和program.cs的文件代码
<Project Sdk="Microsoft.NET.Sdk.Web">
<PropertyGroup>
<TargetFramework>net461</TargetFramework>
<DebugType>full</DebugType>
</PropertyGroup>
<ItemGroup>
<PackageReference Include="Microsoft.AspNetCore" Version="2.2.0" />
<PackageReference Include="Microsoft.AspNetCore.CookiePolicy" Version="2.2.8" />
<PackageReference Include="Microsoft.AspNetCore.Mvc" Version="2.2.0" />
<PackageReference Include="Microsoft.AspNetCore.StaticFiles" Version="2.2.0" />
<PackageReference Include="Microsoft.EntityFrameworkCore" Version="3.1.2" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Design" Version="3.1.2">
<PrivateAssets>all</PrivateAssets>
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="3.1.2" />
<PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer.Design" Version="1.1.6" />
<PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="3.1.2">
<PrivateAssets>all</PrivateAssets>
<IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
</PackageReference>
<PackageReference Include="Microsoft.Extensions.Logging.Configuration" Version="3.1.2" />
<PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="3.1.1" />
</ItemGroup>
</Project>
public class Program
public static void Main(string[] args)
CreateWebHostBuilder(args).Build().Run();
public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
WebHost.CreateDefaultBuilder(args)
.UseStartup<Startup>();
然后我尝试先迁移，但我看到一个非常奇怪的错误 Add-Migration FirstInit -verbose
Using project 'Crud'.
Using startup project 'Crud'.
Build started...
Build succeeded.
C:\.nuget\packages\microsoft.entityframeworkcore.tools\3.1.2\tools\net461\win-x86\ef.exe migrations add FirstInit --json --verbose --no-color --prefix-output --assembly C:\source\repos\Crud\Crud\bin\Debug\net461\Crud.exe --startup-assembly C:\source\repos\Crud\Crud\bin\Debug\net461\Crud.exe --project-dir C:\source\repos\Crud\Crud\ --language C# --working-dir C:\source\repos\Crud --root-namespace Crud
Using assembly 'Crud'.
Using startup assembly 'Crud'.
Using application base 'C:\source\repos\Crud\Crud\bin\Debug\net461'.
Using working directory 'C:\source\repos\Crud\Crud'.
Using root namespace 'Crud'.
Using project directory 'C:\source\repos\Crud\Crud\'.
Using configuration file 'C:\source\repos\Crud\Crud\bin\Debug\net461\Crud.exe.config'.
Using assembly 'Crud'.
Using startup assembly 'Crud'.
Using application base 'C:\source\repos\Crud\Crud\bin\Debug\net461'.
Using working directory 'C:\source\repos\Crud\Crud'.
Using root namespace 'Crud'.
Using project directory 'C:\source\repos\Crud\Crud\'.
Finding DbContext classes...
Finding IDesignTimeDbContextFactory implementations...
Finding application service provider...
Finding Microsoft.Extensions.Hosting service provider...
Using environment 'Development'.
System.TypeLoadException: There is no implementation of the GetItem method in the type "Microsoft.AspNetCore.Mvc.Razor.Internal.FileProviderRazorProjectFileSystem" from assembly "Microsoft.AspNetCore.Mvc.Razor, Version=2.2.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60".
в Microsoft.Extensions.DependencyInjection.MvcRazorMvcCoreBuilderExtensions.AddRazorViewEngineServices(IServiceCollection services)
в Microsoft.Extensions.DependencyInjection.MvcRazorMvcCoreBuilderExtensions.AddRazorViewEngine(IMvcCoreBuilder builder)
в Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddMvc(IServiceCollection services)
в Crud.Startup.ConfigureServices(IServiceCollection services) в C:\source\repos\Crud\Crud\Startup.cs:строка 38
--- End the stack trace from the previous location where the exception occurred ---
в System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
в Microsoft.AspNetCore.Hosting.ConventionBasedStartup.ConfigureServices(IServiceCollection services)
в Microsoft.AspNetCore.Hosting.Internal.WebHost.EnsureApplicationServices()
в Microsoft.AspNetCore.Hosting.Internal.WebHost.Initialize()
в Microsoft.AspNetCore.Hosting.WebHostBuilder.Build()
An error occurred while accessing the Microsoft.Extensions.Hosting services. Continuing without the application service provider. Error: There is no implementation of the GetItem method in the type "Microsoft.AspNetCore.Mvc.Razor.Internal.FileProviderRazorProjectFileSystem" from assembly "Microsoft.AspNetCore.Mvc.Razor, Version=2.2.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60".
No application service provider was found.
Finding DbContext classes in the project...
Found DbContext 'EmployeeContext'.
Microsoft.EntityFrameworkCore.Design.OperationException: Unable to create an object of type 'EmployeeContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728 ---> System.MissingMethodException:There are no parameterless constructors defined for this object..
в System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck)
в System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
в System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
в System.Activator.CreateInstance(Type type, Boolean nonPublic)
в System.Activator.CreateInstance(Type type)
в Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_3.<FindContextTypes>b__13()
--- End trace of internal exception stack ---
в Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.<>c__DisplayClass13_3.<FindContextTypes>b__13()
в Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(Func`1 factory)
в Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext(String contextType)
в Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration(String name, String outputDir, String contextType)
в Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl(String name, String outputDir, String contextType)
в Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.<>c__DisplayClass0_0.<.ctor>b__0()
в Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.<>c__DisplayClass3_0`1.<Execute>b__0()
в Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action)
Unable to create an object of type 'EmployeeContext'. For the different patterns supported at design time, see https://go.microsoft.com/fwlink/?linkid=851728
这 3 行有什么问题？
【问题讨论】：
你安装了Microsoft.EntityFrameworkCore.Design吗？ 我安装了 Microsoft.EntityFrameworkCore.Design，我看到了同样的错误 您能分享有关 Program.cs 文件的详细信息吗？ 添加问题 一切看起来都不错，你能添加-Migration FirstInit -verbose 看看你能不能得到更多细节？ 【参考方案1】：EF 调用 CreateWebHostBuilder 或 BuildWebHost 而不运行 Main。所以 Iconfiguration 为空。
创建继承自 IDesignTimeDbContextFactory 的新类。
public class YourDbContext : DbContext
//Dbcontext implementation
public class YourDbContextFactory : IDesignTimeDbContextFactory<YourDbContext>
public YourDbContext CreateDbContext(string[] args)
var optionsBuilder = new DbContextOptionsBuilder<YourDbContext>();
optionsBuilder.UseSqlServer("your connection string");
return new YourDbContext(optionsBuilder.Options);
您正在使用一个使用 IHostBuilder 的新 .net 核心 EF。（在像您这样的旧版本中，提供程序是 IWebHostBuilder）。这些工具首先尝试通过调用 Program.CreateHostBuilder()、调用 Build() 来获取服务提供者，然后访问 Services 属性。 您可以从Here 了解更多关于设计时 DbContext 创建的信息
它可能发生在您的启动文件中的某个条件或您正在注入时。例如，您有一个标志来检查 appsetting 中的某个变量是否为真以使用内存数据库实例。
EF 需要在不启动应用程序的情况下构建模型并使用 DbContext。当 EF 调用方法时，您的配置服务仍然为 null，这就是您收到错误的原因。
确保您已安装软件包
Microsoft.EntityFrameworkCore.Tools
【讨论】：
很好用！有没有办法使用 args 传递连接字符串而不是硬编码？ @Cladoo 检查此链接gist.github.com/tonysneed/… 另外值得一提的是，在某些情况下，如果在 Startup.cs 的 ConfigureServices 中有一些 DI 类没有注册，可能会出现这样的错误。这就是我的场景。【参考方案2】：正如accepted 答案所建议的那样：
EF 调用 CreateWebHostBuilder 或 BuildWebHost 而不运行 Main。所以 我的配置为空。
但您可以通过覆盖OnConfiguring来使用这个简单的解决方案，而不是使用更复杂的工厂。
public class ApplicationDbContext : DbContext
public ApplicationDbContext()
public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
if (!optionsBuilder.IsConfigured)
optionsBuilder.UseSqlServer("Data Source=(localdb)\\MSSQLLocalDB;Initial Catalog=Chinook");
*注意不要在源码（docs）中使用db连接字符串
dotnet user-secrets set ConnectionStrings:Chinook "Data Source=(localdb)\MSSQLLocalDB;Initial Catalog=Chinook"
dotnet ef dbcontext scaffold Name=ConnectionStrings:Chinook Microsoft.EntityFrameworkCore.SqlServer
还要注意在cli命令行中只使用一个\。
【讨论】：
【参考方案3】：就我而言，我删除了括号之间的参数：
之前：
services.AddDbContext<EmployeeContext>(item =>item.UseSqlServer(Configuration.GetConnectionString("DefaultConnection")));
之后：
services.AddDbContext<EmployeeContext>();
关注Docs
【讨论】：
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