// 傳統(tǒng)方式：返回完整集合
public static List<int> GetNumbers()
{
    List<int> numbers = new List<int>();
    numbers.Add(1);
    numbers.Add(2);
    numbers.Add(3);
    return numbers;
}

// 使用yield：延遲生成值
public static IEnumerable<int> GetNumbers()
{
    yield return 1;
    yield return 2;
    yield return 3;
}

public static IEnumerable<int> GetNumbers()
{
    Console.WriteLine("開始執(zhí)行");
    yield return 1;
    Console.WriteLine("返回了第一個值");
    yield return 2;
    Console.WriteLine("返回了第二個值");
    yield return 3;
    Console.WriteLine("返回了第三個值");
}

// 調(diào)用代碼
var numbers = GetNumbers();
Console.WriteLine("迭代器已經(jīng)創(chuàng)建");

foreach (int number in numbers)
{
    Console.WriteLine($"獲取到值: {number}");
    Console.WriteLine("--------");
}

private sealed class IteratorStateMachine : IEnumerator<int>, IEnumerable<int>
{
    // 狀態(tài)標(biāo)識
    private int state;
    // 當(dāng)前返回值
    private int current;
    
    // 每次調(diào)用GetEnumerator()時創(chuàng)建新實例
    public IteratorStateMachine(int state) => this.state = state;
    
    // IEnumerable接口實現(xiàn)
    public IEnumerator<int> GetEnumerator() => this;
    object IEnumerator.Current => current;
    public int Current => current;
    
    // 核心方法：控制狀態(tài)轉(zhuǎn)換
    public bool MoveNext()
    {
        switch (state)
        {
            case 0: // 初始狀態(tài)
                state = -1; // 默認(rèn)標(biāo)記為完成
                current = 1; // 設(shè)置第一個返回值
                state = 1;   // 跳轉(zhuǎn)到第一個yield后的狀態(tài)
                return true;
                
            case 1: // 第一個yield后
                state = -1;
                current = 2;
                state = 2;
                return true;
                
            case 2: // 第二個yield后
                state = -1;
                current = 3;
                state = 3;
                return true;
                
            case 3: // 第三個yield后
                return false; // 迭代結(jié)束
                
            default:
                return false;
        }
    }
    
    // 其他接口方法
    public void Dispose() => state = -1;
    public void Reset() => throw new NotSupportedException();
}

public static IEnumerable<DataItem> LoadLargeData()
{
    using (var connection = new SqlConnection(connectionString))
    {
        connection.Open();
        using (var command = new SqlCommand(query, connection))
        {
            using (var reader = command.ExecuteReader())
            {
                while (reader.Read())
                {
                    yield return new DataItem
                    {
                        Id = reader.GetInt32(0),
                        Name = reader.GetString(1)
                    };
                }
            }
        }
    }
}

public class MyCollection
{
    private readonly int[] _items;

    public MyCollection(int[] items)
    {
        _items = items;
    }

    public IEnumerable<int> EvenNumbers()
    {
        foreach (var item in _items)
        {
            if (item % 2 == 0)
            {
                yield return item;
            }
        }
    }
}

public static IEnumerable<int> Fibonacci()
{
    int a = 0, b = 1;
    while (true)
    {
        yield return a;
        int temp = a;
        a = b;
        b = temp + b;
    }
}