AutoMapper：将多对多关联映射到 DTO（是否支持展平集合项？）

Question

我在团队和员工之间建立了多对多关联。

public class Employee : Entity
{
    public virtual string LastName { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string EMail { get; set; }
    public virtual IList<LoanedItem> LoanedItems { get; private set; }
    public virtual ISet<Team> Teams { get; private set; }

    public Employee()
    {
        if (LoanedItems == null)
        {
            LoanedItems = new List<LoanedItem>();
        }

        if (Teams == null)
        {
            Teams = new HashedSet<Team>();
        }
    }

    public virtual Employee AddTeam(Team team)
    {
        Teams.Add(team);
        team.Employees.Add(this);

        return this;
    }

    public virtual Employee RemoveTeamFromEmployee(Team team)
    {
        Teams.Remove(team);
        team.Employees.Remove(this);

        return this;
    }
}

public class Team : Entity
{
    public virtual string Name { get; set; }
    public virtual ISet<Employee> Employees { get; private set; }

    public Team()
    {
        if (Employees == null)
        {
            Employees = new HashedSet<Employee>();
        }
    }

    public virtual Team RemoveEmployeeFromTeam (Employee employee)
    {
        Employees.Remove(employee);
        employee.Teams.Remove(this);

        return this;
    }

    public virtual Team AddEmployee(Employee employee)
    {
        Employees.Add(employee);
        employee.Teams.Add(this);

        return this;
    }
}

我想将其映射到以下 DTO。

public class EmployeeForm
{
    public string LastName { get; set; }
    public string FirstName { get; set; }
    public string EMail { get; set; }
    public string TeamName { get; set; }
    public int Id { get; set; }
}

所以 TeamName 是必须被映射的那个。

我使用以下映射代码：

Mapper.CreateMap<Employee, EmployeeForm>()
    .ForMember(dest=>dest.TeamName, opt => opt.MapFrom(s=>s.Teams.FirstOrDefault().Name));
var mappedresult = Mapper.Map<List<Employee>, List<EmployeeForm>>(result);

问题在于 Employee.Teams 成员。 顺便说一下，Teams 集合是急切的。所以它就在那里。

当 Teams 为 Null 时，我会收到 NullReference 异常。

如何将团队名称映射到 DTO？

编辑：现在我离开 Automapper 并改用这个繁琐的代码：

var result = _repository.GetAllView();

var employeeForms = new List<EmployeeForm>();
foreach (Employee employee in result)
{
    var employeeAdded = false;
    foreach (var team in employee.Teams)
    {
        employeeForms.Add(new EmployeeForm
          {
              EMail = employee.EMail,
              FirstName = employee.FirstName,
              LastName = employee.LastName,
              TeamName = team.Name
          });
        employeeAdded = true;
    }

    if (!employeeAdded)
    {
        employeeForms.Add(new EmployeeForm
        {
            EMail = employee.EMail,
            FirstName = employee.FirstName,
            LastName = employee.LastName,
            TeamName = string.Empty
        });
    }
}
return View(employeeForms);

对我来说，这闻起来很刺鼻……

Answer 1

由于 FirstOrDefault().Name，你得到了 nullreferenceexception，所以你必须在 firstordefault 之后做一个 null 检查，并告诉 automapper 你想在 null 时返回什么。其他部分应该可以正常工作。