使用LINQ方法语法返回父实体及其子实体

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

public class Child
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int ParentId { get; set; }
}

List<Parent> parents = new List<Parent>
{
    new Parent { Id = 1, Name = "Parent 1" },
    new Parent { Id = 2, Name = "Parent 2" },
    new Parent { Id = 3, Name = "Parent 3" }
};

List<Child> children = new List<Child>
{
    new Child { Id = 1, Name = "Child 1", ParentId = 1 },
    new Child { Id = 2, Name = "Child 2", ParentId = 1 },
    new Child { Id = 3, Name = "Child 3", ParentId = 2 },
    new Child { Id = 4, Name = "Child 4", ParentId = 3 },
    new Child { Id = 5, Name = "Child 5", ParentId = 3 }
};

var result = parents.GroupJoin(children, parent => parent.Id, child => child.ParentId,
    (parent, childGroup) => new
    {
        Parent = parent,
        Children = childGroup.ToList()
    });

foreach (var item in result)
{
    Console.WriteLine($"Parent: {item.Parent.Name}");
    foreach (var child in item.Children)
    {
        Console.WriteLine($"Child: {child.Name}");
    }
    Console.WriteLine();
}