c# – ‘yield return’语句不能出现在try / catch块约束中

发布时间：2020-12-16 00:01:00 所属栏目：百科来源：网络整理

导读：参见英文答案 Why can’t yield return appear inside a try block with a catch?????????????????????????????????????5个这两种方法几乎相同,但第一种方法无法编译.我无法弄清楚这种约束存在的原因 /// summary /// Dynamically loads all document extra

参见英文答案 > Why can’t yield return appear inside a try block with a catch?????????????????????????????????????5个
这两种方法几乎相同,但第一种方法无法编译.我无法弄清楚这种约束存在的原因

/// <summary>
    /// Dynamically loads all document extractors from implementation assemblies into an enumeration
    /// </summary>
    private static IEnumerable<IDocumentExtractor> EnumerateInstances()
    {
        IEnumerable<Type> types = EnumerateTypes();

        foreach(Type type in types)
        {
            try
            {
                IDocumentExtractor extractor = Activator.CreateInstance(type) as IDocumentExtractor;
                yield return extractor;
            }
            catch
            {
                _log.WarnFormat("Type {0} couldn't be instanced.",type.Name);
            }
        }
    }

并且实际编译的版本没有问题：

/// <summary>
    /// Dynamically loads all document extractors from implementation assemblies into an enumeration
    /// </summary>
    private static IEnumerable<IDocumentExtractor> EnumerateInstances()
    {
        IEnumerable<Type> types = EnumerateTypes();

        foreach (Type type in types)
        {
            IDocumentExtractor extractor = null;
            try
            {
                extractor = Activator.CreateInstance(type) as IDocumentExtractor;
            }
            catch
            {
                _log.WarnFormat("Type {0} couldn't be instanced.",type.Name);
            }

            if (extractor != null)
                yield return extractor;
        }
    }

解决方法

您可以在Eric Lippert的Iterator Blocks系列中找到这个问题的答案.具体来看看标题为 Iterator Blocks,Part Five: Push vs Pull的帖子.七部分系列从 Iterator Blocks,Part One开始.

（编辑：李大同）

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