.Net Core Migration System.ArgumentNullException：值不能为空

Question

这是我的 DbContext

public class DocumentStudioDbContext : DbContext
{
    public DocumentStudioDbContext(DbContextOptions<DocumentStudioDbContext> options) : base(options) { }
    public DbSet<Documents> Documents { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.HasDefaultSchema(schema: Environment.GetEnvironmentVariable(DSCASGlobals.DS_DatabaseSchema));
        base.OnModelCreating(modelBuilder);
    }

    public override int SaveChanges()
    {
        Audit();
        return base.SaveChanges();
    }

    public async Task<int> SaveChangesAsync()
    {
        Audit();
        return await base.SaveChangesAsync();
    }

    private void Audit()
    {
        var entries = ChangeTracker.Entries().Where(x => x.Entity is Documents && (x.State == EntityState.Added || x.State == EntityState.Modified));
        foreach (var entry in entries)
        {
            if (entry.State == EntityState.Added)
            {
                ((Documents)entry.Entity).CreatedOn = DateTime.UtcNow;
            }
        ((Documents)entry.Entity).UpdatedOn = DateTime.UtcNow;
        }
    }
}

当我尝试运行迁移时，我收到此错误：

System.ArgumentNullException：值不能为空。参数名称： 连接字符串在 Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty（字符串值， 字符串参数名称）在 Microsoft.EntityFrameworkCore.SqlServerDbContextOptionsExtensions.UseSqlServer(DbContextOptionsBuilder optionsBuilder，字符串 connectionString，Action1 sqlServerOptionsAction) at DocumentStudio.Startup.<>c.<ConfigureServices>b__1_0(DbContextOptionsBuilder options) in C:\Users\amosa\source\repos\DotNetCore\DocumentStudio\DocumentStudio\Startup.cs:line 48 at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass1_02.b__0(IServiceProvider p, DbContextOptionsBuilder b) 在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.DbContextOptionsFactory[TContext](IServiceProvider applicationServiceProvider, Action2 optionsAction) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass10_01.b__0(IServiceProvider p) 在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory（FactoryCallSite factoryCallSite、ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0(ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（类型 serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型 服务类型）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider 提供者，类型 serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider 提供者）在 Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.c__101.<AddCoreServices>b__10_1(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped（ScopedCallSite scopedCallSite、ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor2.VisitCallSite(IServiceCallSite callSite, TArgument 参数）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine.c__DisplayClass1_0.b__0(ServiceProviderEngineScope 范围）在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（类型 serviceType, ServiceProviderEngineScope serviceProviderEngineScope)
在 Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（类型 服务类型）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService(IServiceProvider 提供者，类型 serviceType) 在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService[T](IServiceProvider 提供者）在 Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetServices[T](IServiceProvider 提供者）在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes() 在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType（字符串 名称）在 Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext（字符串 上下文类型）在 Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration（字符串 名称，字符串 outputDir，字符串 contextType）在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl（字符串 名称，字符串 outputDir，字符串 contextType）在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigration.c__DisplayClass0_1.<.ctor>b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.c__DisplayClass3_0`1.b__0() 在 Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute（操作 动作）值不能为空。参数名称：连接字符串

我的 ConnectionString 来自 Visual Studio 中设置的环境变量。我不确定如何执行迁移。

我还需要做什么？

编辑：

Startup.cs 中的 DbContext

services.AddDbContext<DocumentStudioDbContext>(options => options.UseSqlServer(Environment.GetEnvironmentVariable(DSCASGlobals.DS_ConnectionString)));

编辑2：

设法从环境变量中取出变量

Answer 1

  <configuration>

          <connectionStrings>
              <add name="DocumentStudioDb"
                connectionString="Server=(localdb)\mssqllocaldb;Database=DocumentStudioDb;Trusted_Connection=True;" />
                   </connectionStrings>
   </configuration>

/////在你的上下文中编写这段代码

 protected override void OnConfiguring(DbContextOptionsBuilder   optionsBuilder)
               {
       optionsBuilder.UseSqlServer(ConfigurationManager.ConnectionStrings["DocumentStudioDb"]  .ConnectionString);
                }