自动映射器说缺少从System.String到System.Char的映射?但我看不到Char属性 [英] Automapper says Missing map from System.String to System.Char? But I see no Char property

问题描述

我有三节课:

public class UserReport : Entity
{
    public string Name { get; set; }
    public string Email { get; set; }
    public List<string> Departments { get; set; }
    public List<string> Titles { get; set; }
}

public abstract class Entity
{
    public Guid Id { get; set; }
    public DateTime DateCreated { get; set; }
    public DateTime DateLastModified { get; set; }
    public string CreatedBy { get; set; }
    public string LastModifiedBy { get; set; }
    public bool Active { get; set; }

    protected Entity()
    {
        DateCreated = DateTime.Now;
        DateLastModified = DateTime.Now;
        Id = Guid.NewGuid();
    }
}

public class UserModel
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public string Departments { get; set; }
    public string Titles { get; set; }
}

将我的自动映射器配置设置为:

With my automapper configs set as:

CreateMap<List<string>, string>().ConvertUsing(strings => {
            if (strings != null)
                return string.Join("\n", strings);
            return "";
        });
CreateMap<UserReport, UserModel>();

当尝试使用Automapper Ef Extensions从通用方法进行调用时:

When trying to call from a generic method using the Automapper Ef Extensions:

IQueryable<TModel> modelQueryable = _reportService.GetReportBaseQuery().ProjectToQueryable<TModel>();

我收到此错误

缺少从System.String到System.Char的映射.使用Mapper.CreateMap创建.

Missing map from System.String to System.Char. Create using Mapper.CreateMap.

GetReportBaseQuery()返回IQueryable<TReport>，因此在这种情况下为UserReport.我没有看到任何char属性，为什么会出现这种情况?

GetReportBaseQuery() returns an IQueryable<TReport>, so the UserReport in this instance. I don't see any char properties, why is this coming up?

仅出于测试目的，我尝试制作一个:

Just for testing I tried to make one:

CreateMap<String, Char>().ConvertUsing(x => x.FirstOrDefault());

然后它说:

参数类型不匹配

Argument types do not match

进一步的研究表明，这种方法有效:

Further research shows that this works:

Mapper.Map<List<TReport>, List<TModel>>(_reportService.GetReportBaseQuery().ToList());

但是我不能使用它，因为我需要它成为可查询的返回值.因此，当我尝试进行EF投影时，情况有所不同，但不确定是什么.将选择语句从一个语句编写到另一个语句很容易，但这不是通用的并且不可重用.

But I can't use that since I need it to be a queryable returned. So something is different when I try to do an EF projection, not sure what that is though. Writing a select statement from one to the other is easy enough, but that's not generic and re-usable.

推荐答案

解决方案是显式设置Departments和Titles字段的映射:

The solution is to explicitly set mapping for the Departments and Titles fields:

CreateMap<UserReport, UserModel>()
    .ForMember(x => x.Departments, o => o.MapFrom(s => string.Join("\n", s.Departments)))
    .ForMember(x => x.Titles, o => o.MapFrom(s => string.Join("\n", s.Titles)));

但是，这并不能解释为什么会发生这种情况. 正如DOTang指出的那样:

But, this does not explain, why this situation occurs. As DOTang pointed out:

LINQ to Entities无法识别方法"System.String Join(System.String，System.Collections.Generic.IEnumerable1 [System.String])"方法，并且该方法无法转换为商店表达式.

LINQ to Entities does not recognize the method 'System.String Join(System.String, System.Collections.Generic.IEnumerable1[System.String])' method, and this method cannot be translated into a store expression.

AutoMapper扩展似乎试图映射以下内容:

The AutoMapper extension seems to try to map the following thing:

IEnumerable<string> => IEnumerable<char>

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