在c#中将对象的平面列表转换为嵌套的对象列表

using System;
using System.Collections.Generic;
using System.Linq;

public class Category
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int? ParentId { get; set; }
    public List<Category> Children { get; set; }
}

public class Program
{
    public static void Main()
    {
        List<Category> categories = new List<Category>
        {
            new Category { Id = 1, Name = "Category 1", ParentId = null },
            new Category { Id = 2, Name = "Category 2", ParentId = null },
            new Category { Id = 3, Name = "Category 1.1", ParentId = 1 },
            new Category { Id = 4, Name = "Category 1.2", ParentId = 1 },
            new Category { Id = 5, Name = "Category 2.1", ParentId = 2 },
            new Category { Id = 6, Name = "Category 2.2", ParentId = 2 },
            new Category { Id = 7, Name = "Category 1.1.1", ParentId = 3 },
            new Category { Id = 8, Name = "Category 1.1.2", ParentId = 3 }
        };

        List<Category> nestedCategories = GetNestedCategories(categories, null);

        foreach (var category in nestedCategories)
        {
            PrintCategory(category);
        }
    }

    public static List<Category> GetNestedCategories(List<Category> categories, int? parentId)
    {
        return categories
            .Where(c => c.ParentId == parentId)
            .Select(c => new Category
            {
                Id = c.Id,
                Name = c.Name,
                ParentId = c.ParentId,
                Children = GetNestedCategories(categories, c.Id)
            })
            .ToList();
    }

    public static void PrintCategory(Category category, string indent = "")
    {
        Console.WriteLine(indent + category.Name);

        if (category.Children != null)
        {
            foreach (var child in category.Children)
            {
                PrintCategory(child, indent + "  ");
            }
        }
    }
}