.NET MediatR 和 EF Core DbContext 连接不支持 MultipleActiveResultSets

Question

我正在使用 MediatR 处理我的请求，我需要从 EF Core (ApplicationDbContext) 注入 DbContext。我使用了一个界面来做到这一点。

第一个请求工作正常，但紧接着，第二个请求失败并显示此消息：

The connection does not support MultipleActiveResultSets.

我不想启用 MultipleActiveResultSets，我只是想弄清楚为什么会这样。

这是我的Startup.cs:

services.AddPersistence(Configuration);
            
services.AddInfrastructure(Configuration);
            
services.AddApplication();

在 AddPersistence 中，我注册了我的 ApplicationDbContext 并将 IApplicationDbContext 接口添加为 Scoped:

services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlServer(dbConnectionString)
            );
            
services.AddScoped<IApplicationDbContext>(provider => provider.GetService<ApplicationDbContext>());

这是我的 IApplicationDbContext 接口：

public interface IApplicationDbContext
    {
        DbSet<MyEntity> MyEntities { get; set; }

        Task<int> SaveChangesAsync(CancellationToken cancellationToken);
    }

我的ApplicationDbContext:

public sealed class ApplicationDbContext : DbContext, IApplicationDbContext
    {
        public DbSet<MyEntity> MyEntities{ get; set; }

        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options)
        {
            Database.EnsureCreated();
            
            Database.Migrate();
        }
        
        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            modelBuilder.ApplyConfigurationsFromAssembly(Assembly.GetExecutingAssembly());
        }
    }

现在在 MediatR 处理程序中，我这样做：

private readonly IApplicationDbContext _applicationDbContext;

public FilterAircraftDelayCodesRequestHandler(IApplicationDbContext applicationDbContext)
{
    _applicationDbContext = applicationDbContext;
}

我只想return MediatR 处理程序中的 ApplicationDbContext DbSet 中的对象列表：

var data = await _applicationDbContext.MyEntites.ToListAsync(cancellationToken: cancellationToken);

这里是我得到的异常：

System.InvalidOperationException: The connection does not support MultipleActiveResultSets.
   at Microsoft.Data.SqlClient.SqlCommand.<>c.<ExecuteDbDataReaderAsync>b__164_0(Task`1 result)
   at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
   at System.Threading.Tasks.Task.<>c.<.cctor>b__274_0(Object obj)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.Tasks.Task.ExecuteWithThreadLocal(Task& currentTaskSlot, Thread threadPoolThread)
--- End of stack trace from previous location where exception was thrown ---
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReaderAsync(RelationalCommandParameterObject parameterObject, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.InitializeReaderAsync(DbContext _, Boolean result, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.SqlServer.Storage.Internal.SqlServerExecutionStrategy.ExecuteAsync[TState,TResult](TState state, Func`4 operation, Func`4 verifySucceeded, CancellationToken cancellationToken)
   at Microsoft.EntityFrameworkCore.Query.Internal.QueryingEnumerable`1.AsyncEnumerator.MoveNextAsync()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.ToListAsync[TSource](IQueryable`1 sou...

Answer 1

在这种情况下，问题出在 ApplicationDbContext 的构造函数中。

我使用了Database.MigrateAsync()，产生了这个错误。

我以 non-async 的方式使用 Database.Migrate() 方法解决了这个异常。