EF Core 7搭配Pomelo连接MySQL启用延迟加载代理报错

问题概述 #

使用MySQL + EF Core + Pomelo.EntityFrameworkCore.MySql开发项目，基础查询正常，但遇到两个核心问题：

1. 启用UseLazyLoadingProxies()后抛出System.InvalidOperationException: This MySqlConnection is already in use错误
2. 使用Include(obj => obj.Connections)预加载关联数据无效，EF仅生成People表的查询，无法获取Connections数据

错误堆栈信息 #

System.InvalidOperationException: This MySqlConnection is already in use

System.InvalidOperationException: This MySqlConnection is already in use. See https://fl.vu/mysql-conn-reuse
   at MySqlConnector.Core.ServerSession.StartQuerying(ICancellableCommand command) in /_/src/MySqlConnector/Core/ServerSession.cs:line 283
   at MySqlConnector.Core.CommandExecutor.ExecuteReaderAsync(IReadOnlyList`1 commands, ICommandPayloadCreator payloadCreator, CommandBehavior behavior, Activity activity, IOBehavior ioBehavior, CancellationToken cancellationToken) in /_/src/MySqlConnector/Core/CommandExecutor.cs:line 51
   at MySqlConnector.MySqlCommand.ExecuteReaderAsync(CommandBehavior behavior, IOBehavior ioBehavior, CancellationToken cancellationToken) in /_/src/MySqlConnector/MySqlCommand.cs:line 344
   at MySqlConnector.MySqlCommand.ExecuteDbDataReader(CommandBehavior behavior) in /_/src/MySqlConnector/MySqlCommand.cs:line 278
   at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteReader(RelationalCommandParameterObject parameterObject)
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.InitializeReader(Enumerator enumerator)
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.<>c.<MoveNext>b__21_0(DbContext _, Enumerator enumerator)
   at Pomelo.EntityFrameworkCore.MySql.Storage.Internal.MySqlExecutionStrategy.Execute[TState,TResult](TState state, Func`3 operation, Func`3 verifySucceeded)
   at Microsoft.EntityFrameworkCore.Query.Internal.SingleQueryingEnumerable`1.Enumerator.MoveNext()
   at Microsoft.EntityFrameworkCore.EntityFrameworkQueryableExtensions.Load[TSource](IQueryable`1 source)
   at Microsoft.EntityFrameworkCore.Internal.EntityFinder`1.Load(INavigation navigation, InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.Internal.EntityFinderCollectionLoaderAdapter.Load(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.CollectionEntry.Load()
   at Microsoft.EntityFrameworkCore.Infrastructure.Internal.LazyLoader.Load(Object entity, String navigationName)
   at Microsoft.EntityFrameworkCore.Proxies.Internal.LazyLoadingInterceptor.Intercept(IInvocation invocation)
   at Castle.DynamicProxy.AbstractInvocation.Proceed()
   at Castle.Proxies.PersonProxy.get_Connections()
   at System.Text.Json.Serialization.Metadata.JsonPropertyInfo`1.GetMemberAndWriteJson(Object obj, WriteStack& state, Utf8JsonWriter writer)
   at System.Text.Json.Serialization.Converters.ObjectDefaultConverter`1.OnTryWrite(Utf8JsonWriter writer, T value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.Converters.IAsyncEnumerableOfTConverter`2.OnWriteResume(Utf8JsonWriter writer, TAsyncEnumerable value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonCollectionConverter`2.OnTryWrite(Utf8JsonWriter writer, TCollection value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.TryWrite(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCore(Utf8JsonWriter writer, T& value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.Serialization.JsonConverter`1.WriteCoreAsObject(Utf8JsonWriter writer, Object value, JsonSerializerOptions options, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteCore[TValue](Utf8JsonWriter writer, TValue& value, JsonTypeInfo jsonTypeInfo, WriteStack& state)
   at System.Text.Json.JsonSerializer.WriteStreamAsync[TValue](Stream utf8Json, TValue value, JsonTypeInfo jsonTypeInfo, CancellationToken cancellationToken)
   at System.Text.Json.JsonSerializer.WriteStreamAsync[TValue](Stream utf8Json, TValue value, JsonTypeInfo jsonTypeInfo, CancellationToken cancellationToken)
   at System.Text.Json.JsonSerializer.WriteStreamAsync[TValue](Stream utf8Json, TValue value, JsonTypeInfo jsonTypeInfo, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Mvc.Formatters.SystemTextJsonOutputFormatter.WriteResponseBodyAsync(OutputFormatterWriteContext context, Encoding selectedEncoding)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeNextResultFilterAsync>g__Awaited|30_0[TFilter,TFilterAsync](ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.Rethrow(ResultExecutedContextSealed context)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeResultFilters>g__Awaited|28_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeFilterPipelineAsync>g__Awaited|20_0(ResourceInvoker invoker, Task lastTask, State next, Scope scope, Object state, Boolean isCompleted)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Mvc.Infrastructure.ResourceInvoker.<InvokeAsync>g__Awaited|17_0(ResourceInvoker invoker, Task task, IDisposable scope)
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|6_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Swashbuckle.AspNetCore.SwaggerUI.SwaggerUIMiddleware.Invoke(HttpContext httpContext)
   at Swashbuckle.AspNetCore.Swagger.SwaggerMiddleware.Invoke(HttpContext httpContext, ISwaggerProvider swaggerProvider)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

解决方案 #

1. 解决懒加载的连接占用错误 #

核心原因：接口返回IEnumerable<Person>时，EF采用延迟执行模式，直到JSON序列化阶段才会真正执行数据库查询。此时DbContext可能已被回收（Scoped生命周期下，Action执行完毕上下文即释放），或序列化过程中懒加载触发新查询导致连接被重复占用。

修复步骤：
将返回类型改为立即执行的集合（如List<Person>），确保在DbContext作用域内完成数据加载：

[HttpGet(Name = "GetPeople")]
public List<Person> Get()
{
    return _context.People.ToList();
}

2. 解决预加载Include无效问题 #

核心原因：原代码未正确调用Include，或实体类导航属性配置不完整。

修复步骤：

• 明确添加Include方法加载关联数据，并立即执行查询：

[HttpGet(Name = "GetPeople")]
public List<Person> Get()
{
    return _context.People
        .Include(p => p.Connections) // 加载关联的Connections数据
        .ToList();
}

• 检查实体类配置：确保Person类的Connections导航属性为virtual（懒加载代理需要重写该属性），且已通过DataAnnotations或Fluent API配置正确的外键关系：

public class Person
{
    public int ID { get; set; }
    // 其他属性...
    public virtual ICollection<Connection> Connections { get; set; } // 需标记为virtual
}

3. 确保懒加载配置生效 #

检查Program.cs中的DbContext配置，确保已正确启用懒加载代理：

builder.Services.AddDbContext<YourDbContext>(options =>
    options.UseMySql(
        builder.Configuration.GetConnectionString("DefaultConnection"),
        ServerVersion.AutoDetect(builder.Configuration.GetConnectionString("DefaultConnection"))
    )
    .UseLazyLoadingProxies()); // 启用懒加载代理

内容的提问来源于stack exchange，提问作者Kristian B. Kuhrt