.NET Core 3.1升级后EF Core LINQ查询报错NavigationExpandingExpressionVisitor失败

• 同步升级EF Core版本：.NET Core 3.1必须搭配EF Core 3.1.x版本使用，若仅升级框架未同步更新EF Core包，会因表达式解析逻辑不兼容触发报错。
• 重构LINQ左连接写法：EF Core 3.0+对LINQ表达式解析的严格度提升，旧版本允许的松散写法不再兼容。需调整为规范的左连接写法：

  var query = from offer in dbContext.Offers
              join membership in dbContext.VendorOfferMembershipSchemes
                  on offer.Id equals membership.OfferId into membershipGroup
              from membership in membershipGroup.DefaultIfEmpty()
              select new { offer, membership };
  

• 避免在LINQ查询中直接调用自定义方法：EF Core 3.0+默认禁用客户端评估，无法将prepareOfferData这类自定义方法转换为SQL语句。需先查询原始数据到内存，再调用方法处理：

  // 先拉取原始数据至内存
  var rawData = (from offer in dbContext.Offers
                join membership in dbContext.VendorOfferMembershipSchemes
                    on offer.Id equals membership.OfferId into membershipGroup
                from membership in membershipGroup.DefaultIfEmpty()
                select new { Offer = offer, Membership = membership })
                .ToList();
  
  // 内存中调用自定义方法
  var result = rawData.Select(item => prepareOfferData(item.Offer, item.Membership));
  

• 检查实体关系配置：确认Offer与VendorOfferMembershipScheme的外键、导航属性配置正确，EF Core 3.1对关系映射的校验更严格，错误配置会导致表达式解析失败。
• 启用日志排查细节：配置EF Core日志输出客户端评估警告，定位具体报错的表达式片段：

  services.AddDbContext<YourDbContext>(options =>
      options.UseSqlServer("your_connection_string")
             .ConfigureWarnings(warnings => warnings.Log(CoreEventId.ClientEvaluationWarning)));
  

内容的提问来源于stack exchange，提问作者Dilakshan AT