c# - 如何避免重新启动迭代器方法？

Question

请考虑以下 C# 块：

int resultIndex = 0;

Result firstResult = results.First();
DoAVeryImportOperationWithFirstResult(firstResult);

Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
yield return firstResult;

foreach(Result result in results)
{
   Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
   yield return result;
}

如果您熟悉 Linq 和迭代器，您会注意到在foreach块的第一次迭代中，将返回 results 的第一个结果而不是第二个结果。

所以，基本上这是我的问题：我无法从迭代器方法中获取第一个值，然后在不重新启动此方法的情况下在其他地方使用它。

有人知道一些解决方法吗？

Answer 1

Others have shown the approach using a foreach loop and conditional execution. That's actually a neat approach - but here's another option in case the foreach loop is inappropriate for whatever reason.

using (var iterator = results.GetEnumerator())
{
    if (!iterator.MoveNext())
    {
        // Throw some appropriate exception here.
    }

    Result firstResult = iterator.Current;
    DoAVeryImportOperationWithFirstResult(firstResult);

    Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
    yield return firstResult;

    while (iterator.MoveNext())
    {
        Result result = iterator.Current;
        Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
        yield return result;
    }
}

Answer 2

只需明确地这样做：

bool first = true;
foreach(var item in results) {
    if(first) {
        first = false;
        // whatever
    } else {
        // whatever else
    }
}

或更复杂：

using(var iter = results.GetEnumerator()) {
    if(iter.MoveNext()) {
        // do first things with iter.Current

        while(iter.MoveNext()) {
            // do non-first things with iter.Current
        }
    }
}

Answer 3

您需要手动迭代：

var enumerator = results.GetEnumerator();
enumerator.MoveNext();
yield return enumerator.Current; //first
while(enumerator.MoveNext())
 yield return enumerator.Current; //2nd, ...

省略所有错误检查...也不要忘记处理。

Answer 4

bool isFirst = true;
foreach(Result result in results)
{
   if(isFirst)
   {
      DoAVeryImportOperationWithFirstResult(firstResult);
      isFirst = false;
   }

   Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
   yield return result;
}

Answer 5

您需要使用跳过

foreach(Result result in results.Skip(1))
{
   Console.WriteLine(String.Format("This is the {0} result.", resultIndex++));
   yield return result;
}

或手动迭代结果。

如果您考虑如何实现迭代器，您所看到的确实是预期的行为！