Automapper says Missing map from System.String to

问题:

I've got three classes:

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; }
}

With my automapper configs set as:

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

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

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

I get this error

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

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

Just for testing I tried to make one:

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

And then it says:

Argument types do not match

Further research shows that this works:

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

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.

回答1:

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)));

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

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.

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

IEnumerable<string> => IEnumerable<char>