.Net Core迁移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，出现此错误：

  

System.ArgumentNullException：值不能为null。参数名称：   connectionString在   Microsoft.EntityFrameworkCore.Utilities.Check.NotEmpty（字符串值，   字符串parameterName）在   Microsoft.EntityFrameworkCore.SqlServerDbContextOptionsExtensions.UseSqlServer（DbContextOptionsBuilder   optionsBuilder，字符串连接字符串，操作1 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_0 2.b__0（IServiceProvider   p，DbContextOptionsBuilder b）在   Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.DbContextOptionsFactory [TContext]（IServiceProvider   applicationServiceProvider，操作2 optionsAction) at Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions.<>c__DisplayClass10_0 1.b__0（IServiceProvider   p）在   Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory（FactoryCallSite   factoryCallSite，ServiceProviderEngineScope范围）   Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped(ScopedCallSite scopedCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite（IServiceCallSite   callSite，TArgument参数）在   Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine。<> c__DisplayClass1_0.b__0（ServiceProviderEngineScope   范围）   Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（Type   serviceType，ServiceProviderEngineScope serviceProviderEngineScope）
  在   Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（Type   serviceType）在   Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService（IServiceProvider   提供者，输入serviceType）   Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService [T]（IServiceProvider   提供者）   Microsoft.Extensions.DependencyInjection.EntityFrameworkServiceCollectionExtensions。<> c__10 1.<AddCoreServices>b__10_1(IServiceProvider p) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitFactory(FactoryCallSite factoryCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite（IServiceCallSite   callSite，TArgument参数）在   Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitScoped（ScopedCallSite   scopedCallSite，ServiceProviderEngineScope范围）   Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite(IServiceCallSite callSite, TArgument argument) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteRuntimeResolver.VisitIEnumerable(IEnumerableCallSite enumerableCallSite, ServiceProviderEngineScope scope) at Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteVisitor 2.VisitCallSite（IServiceCallSite   callSite，TArgument参数）在   Microsoft.Extensions.DependencyInjection.ServiceLookup.DynamicServiceProviderEngine。<> c__DisplayClass1_0.b__0（ServiceProviderEngineScope   范围）   Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngine.GetService（Type   serviceType，ServiceProviderEngineScope serviceProviderEngineScope）
  在   Microsoft.Extensions.DependencyInjection.ServiceLookup.ServiceProviderEngineScope.GetService（Type   serviceType）在   Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService（IServiceProvider   提供者，输入serviceType）   Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetRequiredService [T]（IServiceProvider   提供者）   Microsoft.Extensions.DependencyInjection.ServiceProviderServiceExtensions.GetServices [T]（IServiceProvider   提供者）   Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextTypes（）   在   Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.FindContextType（String   名称）   Microsoft.EntityFrameworkCore.Design.Internal.DbContextOperations.CreateContext（String   contextType）   Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.AddMigration（String   名称，字符串outputDir，字符串contextType）位于   Microsoft.EntityFrameworkCore.Design.OperationExecutor.AddMigrationImpl（String   名称，字符串outputDir，字符串contextType）位于   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null。参数名称：connectionString

我的ConnectionString来自在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)));

EDIT2：

设法从环境变量中取出变量

Answer 1

  <button mat-button color="primary">Primary</button>

//////在您的上下文中编写此代码

Intent intent = new Intent();
intent.setAction(Intent.ACTION_OPEN_DOCUMENT);
intent.addCategory(Intent.CATEGORY_OPENABLE);
String[] mimeTypes = {"image/png", "image/jpg", "image/jpeg"};
intent.setType("*/*");
intent.putExtra(Intent.EXTRA_MIME_TYPES, mimeTypes);
startActivityForResult(intent, REQUEST_GALLERY);