c# - 调用使用 yield 返回的方法时出现错误 'Iterator cannot contain return statement '

Question

我希望有更好的方法来编写此方法和重载，同时减少代码重复。我想返回列表中项目之间的一系列增量。这个方法:-

    public static IEnumerable<decimal> CalculateDeltas(this IEnumerable<decimal> sequence)
    {
        decimal prev = default(decimal);
        foreach (var item in sequence)
        {
            var current = item;
            decimal diff = current - prev;
            prev = item;
            yield return diff;
        }
    }

工作得很好。

然后我想到了一个允许绝对增量的重载，但如果不需要绝对值，则会调用原始方法:-

    public static IEnumerable<decimal> CalculateDeltas(this IEnumerable<decimal> sequence,bool absolute)
    {
        if (absolute)
        {
            decimal prev = default(decimal);
            foreach (var item in sequence)
            {
                var current = item;
                decimal diff = Math.Abs(current - prev);
                prev = item;
                yield return diff;
            }
        }
        else
        {
            return CalculateDeltas(sequence);
        }
    }

但是由于错误，这不能编译

"Cannot return a value from an iterator. Use the yield return statement to return a value, or yield break to end the iteration."

我看过this post似乎除了重复原始方法中的代码之外我将无法做任何事情:-

    public static IEnumerable<decimal> CalculateDeltas(this IEnumerable<decimal> sequence,bool absolute)
    {
        if (absolute)
        {
            decimal prev = default(decimal);
            foreach (var item in sequence)
            {
                var current = item;
                decimal diff = Math.Abs(current - prev);
                prev = item;
                yield return diff;
            }
        }
        else
        {
            decimal prev = default(decimal);
            foreach (var item in sequence)
            {
                var current = item;
                decimal diff = current - prev;
                prev = item;
                yield return diff;
            }
        }
    }

谁能提出更好的方法？

Answer 1

单个方法不能同时yield return 和return。您必须选择其中之一。

您可以执行 foreach 以 yield return 列表:

else
{
    foreach (var item in CalculateDeltas(sequence))
        yield return item;
}

或者将您的代码分成两个方法:

if (absolute)
    return CalculateAbsoluteDeltas(sequence);
else
    return CalculateDeltas(sequence);