C#集合之不变集合的用法

//使用静态的Create方法创建该数组，Create方法被重载，可以传递任意数量的元素
          ImmutableArray<string> a1 = ImmutableArray.Create<string>();
          //Add 方法不会改变不变集合本身，而是返回一个新的不变集合
          ImmutableArray<string> a2 = a1.Add("Williams");
          //可以一次调用多个Add方法
          ImmutableArray<string> a3 = a2.Add("Ferrari").Add("Mercedes").Add("Red Bull Racing");
          foreach (var item in a3)
          {
            Console.WriteLine(item);
          }

List<Account> accounts = new List<Account>() {
        new Account {
          Name = "Scrooge McDuck", 
          Amount = 667377678765m
        },
        new Account {
          Name = "Donald Duck",
          Amount = -200m
        },
       new Account {
         Name = "Ludwig von Drake",
         Amount = 20000m
        }};
      ImmutableList<Account> immutableAccounts = accounts.ToImmutableList();

      ImmutableList<Account>.Builder builder = immutableAccounts.ToBuilder();
      for (int i = 0; i < builder.Count; i++)
      {
        Account a = builder[i];
        if (a.Amount > 0)
        {
          builder.Remove(a);
        }
      }

      ImmutableList<Account> overdrawnAccounts = builder.ToImmutable();


      foreach (var item in overdrawnAccounts)
      {
        Console.WriteLine("{0} {1}", item.Name, item.Amount);
      }
      
      public class Account
      {
        public string Name { get; set; }
        public decimal Amount { get; set; }
      }