Automapper ProjectTo 将 ToList 添加到子属性中

我使用 Entity Framework Core 使用投影将实体类映射到 DTO。但是，投影将 ToList 添加到子集合属性中，这会大大减慢查询速度。

公司实体：

public class Company
{
    public Company()
    {
        Employees = new List<CompanyEmployee>();
    }

    public string Address { get; set; }
    public virtual ICollection<CompanyEmployee> Employees { get; set; }
    ...
}

公司DTO：

public class CompanyDTO
{
    public CompanyDTO()
    {
        CompanyEmployees = new List<EmployeeDTO>();
    }

    public string Address { get; set; }
    public List<EmployeeDTO> CompanyEmployees { get; set; }
    ...
}

配置：

CreateMap<Company, CompanyDTO>()
    .ForMember(c => c.CompanyEmployees, a => a.MapFrom(src => src.Employees));
CreateMap<CompanyEmployee, EmployeeDTO>();

Query:

UnitOfWork.Repository<Company>()
    .ProjectTo<CompanyDTO>(AutoMapper.Mapper.Configuration)
    .Take(10)
    .ToList();

使用以下命令检查生成的查询Expression之后的财产ProjectTo产生以下结果：

Company.AsNoTracking()
    .Select(dtoCompany => new CompanyDTO() 
    {
        Address = dtoCompany.Address, 
        ...
        CompanyEmployees = dtoCompany.Employees.Select(dtoCompanyEmployee => new EmployeeDTO() 
                            {
                                CreatedDate = dtoCompanyEmployee.CreatedDate, 
                                ...
                            }).ToList() // WHY??????
    })

That ToList调用会导致对每个实体运行选择查询，这不是我想要的，正如您所猜测的那样。我测试了没有这个查询ToList（通过手动复制表达式并运行它）并且一切都按预期进行。如何防止 AutoMapper 添加该调用？我尝试改变List输入 DTO 到IEnumerable但什么都没有改变..

让我们忽略 EF Core 的影响ToList调用并专注于 AutoMapperProjectTo.

该行为被硬编码在EnumerableExpressionBinder class:

expression = Expression.Call(typeof(Enumerable), propertyMap.DestinationPropertyType.IsArray ? "ToArray" : "ToList", new[] { destinationListType }, expression);

此类属于 AutoMapper QueryableExtensions 处理管道的一部分，负责将源可枚举类型转换为目标可枚举类型。正如我们所看到的，它总是发出ToArray or ToList.

事实上，当目标成员类型是ICollection<T> or IList<T>, the ToList需要调用，否则表达式将无法编译。但是当目标成员类型是IEnumerable<T>，这是任意的。

因此，如果您想摆脱上述场景中的这种行为，您可以注入一个自定义IExpressionBinder before the EnumerableExpressionBinder（活页夹按顺序调用，直到IsMatch回报true） 像这样 （

namespace AutoMapper
{
    using System.Collections.Generic;
    using System.Linq;
    using System.Linq.Expressions;
    using AutoMapper.Configuration.Internal;
    using AutoMapper.Mappers.Internal;
    using AutoMapper.QueryableExtensions;
    using AutoMapper.QueryableExtensions.Impl;

    public class GenericEnumerableExpressionBinder : IExpressionBinder
    {
        public bool IsMatch(PropertyMap propertyMap, TypeMap propertyTypeMap, ExpressionResolutionResult result) =>
            propertyMap.DestinationPropertyType.IsGenericType &&
            propertyMap.DestinationPropertyType.GetGenericTypeDefinition() == typeof(IEnumerable<>) &&
            PrimitiveHelper.IsEnumerableType(propertyMap.SourceType);

        public MemberAssignment Build(IConfigurationProvider configuration, PropertyMap propertyMap, TypeMap propertyTypeMap, ExpressionRequest request, ExpressionResolutionResult result, IDictionary<ExpressionRequest, int> typePairCount, LetPropertyMaps letPropertyMaps)
            => BindEnumerableExpression(configuration, propertyMap, request, result, typePairCount, letPropertyMaps);

        private static MemberAssignment BindEnumerableExpression(IConfigurationProvider configuration, PropertyMap propertyMap, ExpressionRequest request, ExpressionResolutionResult result, IDictionary<ExpressionRequest, int> typePairCount, LetPropertyMaps letPropertyMaps)
        {
            var expression = result.ResolutionExpression;

            if (propertyMap.DestinationPropertyType != expression.Type)
            {
                var destinationListType = ElementTypeHelper.GetElementType(propertyMap.DestinationPropertyType);
                var sourceListType = ElementTypeHelper.GetElementType(propertyMap.SourceType);
                var listTypePair = new ExpressionRequest(sourceListType, destinationListType, request.MembersToExpand, request);
                var transformedExpressions = configuration.ExpressionBuilder.CreateMapExpression(listTypePair, typePairCount, letPropertyMaps.New());
                if (transformedExpressions == null) return null;
                expression = transformedExpressions.Aggregate(expression, (source, lambda) => Select(source, lambda));
            }

            return Expression.Bind(propertyMap.DestinationProperty, expression);
        }

        private static Expression Select(Expression source, LambdaExpression lambda)
        {
            return Expression.Call(typeof(Enumerable), "Select", new[] { lambda.Parameters[0].Type, lambda.ReturnType }, source, lambda);
        }

        public static void InsertTo(List<IExpressionBinder> binders) =>
            binders.Insert(binders.FindIndex(b => b is EnumerableExpressionBinder), new GenericEnumerableExpressionBinder());
    }
}

它基本上是一个修改后的副本EnumerableExpressionBinder与不同的IsMatch检查并删除ToList调用发出代码。

现在，如果您将其注入到 AutoMapper 配置中：

Mapper.Initialize(cfg =>
{
    GenericEnumerableExpressionBinder.InsertTo(cfg.Advanced.QueryableBinders);
    // ...
});

并设置您的 DTO 集合类型IEnumerable<T>:

public IEnumerable<EmployeeDTO> CompanyEmployees { get; set; }

the ProjectTo将生成表达式Select但没有ToList.