Question

有没有人知道我如何从lambda表达式中调用lambda表达式？

如果我有：

public class CourseViewModel
{
    public int Id { get; set; }
    public string Name { get; set; }

    public static Expression<Func<Course, CourseViewModel>> AsViewModel = x =>
        new CourseViewModel
        {
            Id = x.Id,
            Name = x.Name,
        }
}

public class StudentViewModel
{
    public int Id { get; set; }
    public string Name{ get; set; }
    public string PreferredCheese { get; set; }
    public IEnumerable<CourseViewModel> Courses { get; set; }

    public static Expression<Func<Student, StudentViewModel>> AsViewModel = x =>
        new StudentViewModel
        {
            Id = x.Id,
            Name = x.Name,
            PreferredCheese = x.PreferredCheese,
            Courses = ???I'd like to call the CourseViewModel.AsViewModel here
        }
}

在上面的代码中，而不是在StudentViewModel中编写AsViewModel表达式：

Courses = new CourseViewModel
{
    Id = x.Id,
    Name = x.Name,
}

我想调用CourseViewModel.AsViewModel以允许代码重用，并保持将Course转换为CourseViewModel类中的CourseViewModel的代码。这可能吗？

Answer 1

你可以使用x.Courses.Select(c => CourseViewModel.AsViewModel(c))所以你的整个表达将是：

public static Expression<Func<Student, StudentViewModel>> AsViewModel = x =>
        new StudentViewModel
        {
            Id = x.Id,
            Name = x.Name,
            PreferredCheese = x.PreferredCheese,
            Courses = x.Courses.Select(c => CourseViewModel.AsViewModel(c))
        }

Answer 2

如果要将CourseViewModel.AsViewModel保存为嵌套表达式（如果使用LINQ查询提供程序，则可能会这样做），这会变得棘手;您实际上必须自己在AsViewModel中构建StudentViewModel表达式：

using E = System.Linq.Expressions.Expression; // for brevity

// ...

static StudentViewModel()
{
    var s = E.Parameter(typeof(Student), "s");

    //
    // Quick hack to resolve the generic `Enumerable.Select()` extension method
    // with the correct type arguments.
    //
    var selectMethod = (Expression<Func<Student, IEnumerable<CourseViewModel>>>)
                        (_ => _.Courses.Select(c => default(CourseViewModel)));

    var lambda = E.Lambda<Func<Student, StudentViewModel>>(
        E.MemberInit(
            E.New(typeof(StudentViewModel)),
            E.Bind(
                typeof(StudentViewModel).GetProperty("Id"),
                E.Property(s, "Id")),
            E.Bind(
                typeof(StudentViewModel).GetProperty("Name"),
                E.Property(s, "Name")),
            E.Bind(
                typeof(StudentViewModel).GetProperty("PreferredCheese"),
                E.Property(s, "PreferredCheese")), // LOL?
            E.Bind(
                typeof(StudentViewModel).GetProperty("Courses"),
                E.Call(
                    ((MethodCallExpression)selectMethod.Body).Method,
                    E.Property(s, "Courses"),
                    CourseViewModel.AsViewModel))
            ),
        s);

    AsViewModel = lambda;
}

生成的表达式树等同于：

s => new StudentViewModel {
    Id = s.Id,
    Name = s.Name,
    PreferredCheese = s.PreferredCheese,
    Courses = s.Courses.Select(x => new CourseViewModel { Id = x.Id, Name = x.Name })
}

嵌套的Lambda表达式

2 个答案: