C# 如何使用外部迁移程序集从代码迁移EFCore数据库？

我正在使用.NET Core 3.1和Entity Framework Core创建一个包含IdentityServer 4的应用程序。它的目的是可以针对多种数据库类型

在下面的各种示例中，除了包含DBContext的IdentityServer项目本身之外，我还使用了两个独立的迁移程序集，从而使它能够同时使用SQL Server和MySQL

在Startup.cs中，我有以下内容：

var migrationsAssembly = $"IdentityServer.Migrations.{serverType}";

services.AddDbContext<Data.IdentityServerContext>(options =>
{
    switch (serverType)
    {
        case ServerType.SqlServer:
            options.UseSqlServer(connectionString, sql => sql.MigrationsAssembly(migrationsAssembly));
            break;
        case ServerType.MySql:
            options.UseMySql(connectionString, sql => sql.MigrationsAssembly(migrationsAssembly));
            break;
    }
});

我希望能够使用配置或命令行获取IdentityServer以应用迁移本身

e、 g.主要内容（）

var host=CreateHostBuilder（args.Build（）；
使用（var scope=host.Services.CreateScope（））
{
var provider=scope.ServiceProvider；
尝试
{
var configuration=provider.GetRequiredService（）；
if（configuration.GetValue（“migrateDatabases”））
{
var identityContext=provider.GetRequiredService（）；
identityContext.Database.Migrate（）；
}
}
抓住
{
}
}
host.Run（）；

但是，如果我这样做，我会得到一个例外：

System.IO.FileNotFoundException: Could not load file or assembly 'IdentityServer.Migrations.MySql, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'IdentityServer.Migrations.MySql, Culture=neutral, PublicKeyToken=null'
   at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, RuntimeAssembly assemblyContext, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, StackCrawlMark& stackMark, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef, StackCrawlMark& stackMark, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsAssembly..ctor(ICurrentDbContext currentContext, IDbContextOptions options, IMigrationsIdGenerator idGenerator, IDiagnosticsLogger`1 logger)
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetRelationalService[TService](IInfrastructure`1 databaseFacade)
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.Migrate(DatabaseFacade databaseFacade)
   at IdentityServer.Program.Main(String[] args) 

System.IO.FileNotFoundException:无法加载文件或程序集'IdentityServer.Migrations.MySql，Culture=neutral，PublicKeyToken=null'。系统找不到指定的文件。
文件名：“IdentityServer.Migrations.MySql，区域性=中性，PublicKeyToken=null”
位于System.Reflection.RuntimeAssembly.nLoad（AssemblyName文件名、字符串代码基、RuntimeAssemblyContext assemblyContext、StackScrawMark和stackMark、Boolean throwOnFileNotFound、AssemblyLoadContext AssemblyLoadContext）
位于System.Reflection.RuntimeAssembly.InternalLoadAssemblyName（AssemblyName assemblyRef、StackScrawMark和stackMark、AssemblyLoadContext AssemblyLoadContext）
在System.Reflection.Assembly.Load处（AssemblyName assemblyRef、StackScrawMark和stackMark、AssemblyLoadContext AssemblyLoadContext）
在System.Reflection.Assembly.Load处（AssemblyName assemblyRef）
位于Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsAssembly..ctor（ICurrentDbContext currentContext、IDbContextOptions选项、IMiglationSidGenerator idGenerator、IDiagnosticsLogger`1 logger）
在System.RuntimeMethodHandle.InvokeMethod（对象目标、对象[]参数、签名符号、布尔构造函数、布尔WrapeExceptions）
在System.Reflection.RuntimeConstructorInfo.Invoke（BindingFlags invokeAttr、Binder Binder、Object[]参数、CultureInfo区域性）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor（ConstructorCallSite ConstructorCallSite，RuntimeResolverContext上下文）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain（ServiceCallSite callSite，Target参数）
在Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache（ServiceCallSite callSite、RuntimeResolverContext上下文、ServiceProviderEngineScope serviceProviderEngine、RuntimeResolverLockType）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache（ServiceCallSite singletonCallSite，RuntimeResolverContext上下文）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite（ServiceCallSite callSite，Target参数）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor（ConstructorCallSite ConstructorCallSite，RuntimeResolverContext上下文）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain（ServiceCallSite callSite，Target参数）
在Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache（ServiceCallSite callSite、RuntimeResolverContext上下文、ServiceProviderEngineScope serviceProviderEngine、RuntimeResolverLockType）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache（ServiceCallSite singletonCallSite，RuntimeResolverContext上下文）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite（ServiceCallSite callSite，Target参数）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve（ServiceCallSite callSite，ServiceProviderEngineScope作用域）
在Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0（ServiceProviderEngineScope作用域）
在Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（输入serviceType，ServiceProviderEngineScope ServiceProviderEngineScope）
位于Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型serviceType）
位于Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService[T]（IServiceProvider提供程序）
位于Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetRelationalService[TService]（IInfrastructure`1 databaseFacade）
位于Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.Migrate（DatabaseFacade DatabaseFacade）
at IdentityServer.Program.Main（字符串[]参数）

迁移程序集与项目DLL和EXE位于同一目录中

我无法理解的是，为什么它无法加载迁移程序集

作为一个实验，我连接了

AssemblyLoadContext.Default.Resolving

回调，并使用

Assembly.LoadFrom（）

手动加载程序集。这可以正常工作并正确应用迁移，但这是一个非常混乱的解决方法

有人能解释一下为什么从

context.Database.Migrate（）

调用程序集时加载失败，但在使用ef工具或在

解析

回调中手动加载时加载失败

---

编辑

我想我已经找到了一个解决方案，它与我们在同一解决方案的另一个项目中所做的工作相匹配

我们是

var host = CreateHostBuilder(args).Build();

using (var scope = host.Services.CreateScope())
{
    var provider = scope.ServiceProvider;
    try
    {
        var configuration = provider.GetRequiredService<IConfiguration>();
        if (configuration.GetValue<bool>("migrateDatabases"))
        {
            var identityContext = provider.GetRequiredService<IdentityServerContext>();
            identityContext.Database.Migrate();
        }
    }
    catch
    {
    }
}

host.Run();

System.IO.FileNotFoundException: Could not load file or assembly 'IdentityServer.Migrations.MySql, Culture=neutral, PublicKeyToken=null'. The system cannot find the file specified.
File name: 'IdentityServer.Migrations.MySql, Culture=neutral, PublicKeyToken=null'
   at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, RuntimeAssembly assemblyContext, StackCrawlMark& stackMark, Boolean throwOnFileNotFound, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName(AssemblyName assemblyRef, StackCrawlMark& stackMark, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef, StackCrawlMark& stackMark, AssemblyLoadContext assemblyLoadContext)
   at System.Reflection.Assembly.Load(AssemblyName assemblyRef)
   at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationsAssembly..ctor(ICurrentDbContext currentContext, IDbContextOptions options, IMigrationsIdGenerator idGenerator, IDiagnosticsLogger`1 logger)
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeConstructorInfo.Invoke(BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitConstructor(ConstructorCallSite constructorCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSiteMain(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitCache(ServiceCallSite callSite, RuntimeResolverContext context, ServiceProviderEngineScope serviceProviderEngine, RuntimeResolverLock lockType)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScopeCache(ServiceCallSite singletonCallSite, RuntimeResolverContext context)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor`2.VisitCallSite(ServiceCallSite callSite, TArgument argument)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.Resolve(ServiceCallSite callSite, ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.<>c__DisplayClass1_0.<RealizeService>b__0(ServiceProviderEngineScope scope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService(Type serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
   at Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService(Type serviceType)
   at Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetService[T](IServiceProvider provider)
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.GetRelationalService[TService](IInfrastructure`1 databaseFacade)
   at Microsoft.EntityFrameworkCore.RelationalDatabaseFacadeExtensions.Migrate(DatabaseFacade databaseFacade)
   at IdentityServer.Program.Main(String[] args)