c# do while 功能等效？答案

【问题标题】：c# do while functional equivalent?c# do while 功能等效？
【发布时间】：2016-10-06 05:34:30
【问题描述】：

我正在编写一个控制台应用程序，它可以调用 web 服务并接收返回的 XML，其中 XML 将是“文件夹”或“文档”的条目。我在 C# 中编写了以下 do while 循环来循环调用 web 服务，但我想知道是否有更实用的方法来做到这一点：

 ...
        var documentUrls = new Dictionary<string, string>();

        // get the root folder's children
        var rootFeed = $"{baseUrl}/biprws/infostore/{23}/children"
            .Do(str => Console.WriteLine($"{str}:"))
            .GetResponse(headerToken)
            .GetXmlString()
            .DeserializeFromXml<Library.Children.Feed>();

        var folderUrls = rootFeed.Entries
            .Where(entry => entry.Content.ContentContainsFolderUrl(type: "folder"))
            .Select(entry => $"{entry.Link.Href}/children")
            .ToList();

        // go through each folder and pull out the documents - if a nested folder is found then check that too
        do
        {
            var feeds = folderUrls
                .Select(url => url.GetResponse(headerToken).GetXmlString().DeserializeFromXml<Library.Children.Feed>())
                .SelectMany(x => x.Entries);

            var folders = feeds
                .Where(entry => entry.Content.ContentContainsFolderUrl(type: "Folder"))
                .Select(x => $"{x.Link.Href}/children")
                .ToList();

            var documents = feeds
                .Where(entry => entry.Content.ContentContainsFolderUrl(type: "webi"))
                .ToDictionary(
                    keySelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "id").First().Text,
                    elementSelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "name").First().Text);

            // if there are no more folders or documents to check, break out of this while loop
            if (!documents.Any() && !folders.Any())
                break;

            Console.WriteLine($"Got {folders.Count} folders and {documents.Count} documents");

            folderUrls.RemoveAll(x => true); // remove all the folder urls as they have already been checked!!!!
            folderUrls.AddRange(folders);
            documents.ForEachDo(x => documentUrls.Add(x.Key, x.Value));
        } while (true);

        Console.WriteLine($"There are {documentUrls.Count()} {nameof(documentUrls)}");
...

【问题讨论】：

结束条件是什么？你的 do..while 循环目前是无止境的。在haskell中，是否将其实现为递归调用？但是通过无休止的递归调用，您可能会收到 stackoverflow 异常。
既然它只是一个无限循环，你不妨做一个普通的while循环。
一种更“实用”的方法是使用递归，但在添加错误处理时，你所拥有的很好，并且在许多方面更可取。文件夹的Queue<T> 会比您不断擦除的列表更整洁。
@juharr 中断意味着它不是无限的。
@TryToSolveItSimple 当没有找到更多文档或文件夹时，其中有一个break。

标签： c# functional-programming do-while

【解决方案1】：

做这个更实用的不是你是使用for、while还是递归。主要是为了避免副作用。

例如您如何使用folderUrls 来跟踪已检查和嵌套的文件夹不是很实用。相反，设计一个函数，它接受一个文件夹列表并返回文档。 IE。定义明确的输入和输出。从您的代码示例中，很难说出您到底想要实现什么。

这是这个函数的样子：

// I don't know what the type of "entry" is. Replace "TEntry" with the correct type.
public IEnumerable<TEntry> LoadDocuments(IEnumerable<TEntry> feedEntries)
{
    var folderUrls = feedEntries
        .Where(entry => entry.Content.ContentContainsFolderUrl(type: "folder"))
        .Select(entry => $"{entry.Link.Href}/children");

    if (!folderUrls.Any())
        return Enumerable.Empty<TEntry>();

    var feeds = folderUrls
        .Select(url => url.GetResponse(headerToken)
                          .GetXmlString()
                          .DeserializeFromXml<Library.Children.Feed>())
        .SelectMany(x => x.Entries)
        .ToList();

    // Recursive call to load nested documents.    
    var nestedDocuments = LoadDocuments(feeds);

    var documents = feeds
        // What's the type of "entry" here? Use that type for the return type of this 
        // function.
        .Where(entry => entry.Content.ContentContainsFolderUrl(type: "webi"));

    return documents.Concat(nestedDocuments);
}

调用它并将这个函数的输出转换成字典：

var rootFeed = $"{baseUrl}/biprws/infostore/{23}/children"
    .Do(str => Console.WriteLine($"{str}:"))
    .GetResponse(headerToken)
    .GetXmlString()
    .DeserializeFromXml<Library.Children.Feed>();

var documentUrls = LoadDocuments(rootFeed.Entries)
    .ToDictionary(
        x => x.Content.Attrs.Attr
              .Where(y => y.Name.ToLower() == "id")
              .First().Text,
        x => x.Content.Attrs.Attr
              .Where(y => y.Name.ToLower() == "name")
              .First().Text);

【讨论】：

我不喜欢的是我的代码中的 folderUrls 变量（你提到的副作用），但除了做之外，我看不到任何其他方式来执行代码...while循环。我刚试过你放的东西，效果很好！

【解决方案2】：

没有无限循环的模式通常会在循环之前为退出条件预加载，

    var folders = feeds
            .Where(entry => entry.Content.ContentContainsFolderUrl(type: "Folder"))
            .Select(x => $"{x.Link.Href}/children")
            .ToList();

    var documents = feeds
            .Where(entry => entry.Content.ContentContainsFolderUrl(type: "webi"))
            .ToDictionary(
                keySelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "id").First().Text,
                elementSelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "name").First().Text);

然后使用while循环条件作为退出测试：

while(documents.Any() || folders.Any()){

        var feeds = folderUrls
            .Select(url => url.GetResponse(headerToken).GetXmlString().DeserializeFromXml<Library.Children.Feed>())
            .SelectMany(x => x.Entries);

        Console.WriteLine($"Got {folders.Count} folders and {documents.Count} documents");

        folderUrls.RemoveAll(x => true); // remove all the folder urls as they have already been checked!!!!
        folderUrls.AddRange(folders);
        documents.ForEachDo(x => documentUrls.Add(x.Key, x.Value));

最后在再次循环之前加载退出条件的变量：

        folders = feeds
            .Where(entry => entry.Content.ContentContainsFolderUrl(type: "Folder"))
            .Select(x => $"{x.Link.Href}/children")
            .ToList();

        documents = feeds
            .Where(entry => entry.Content.ContentContainsFolderUrl(type: "webi"))
            .ToDictionary(
                keySelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "id").First().Text,
                elementSelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "name").First().Text);

}

...

这种方法会重复代码，因此为每个查询创建方法，因此重复的代码只是对适当方法的几次调用。这将导致代码更具可读性，因为描述性方法名称在代码中比 LINQ 查询更容易遵循（更不用说它不会有无限循环）。这种模式可能需要更多的努力来处理变量范围，但在这种情况下，这不应该是一个问题（feed 应该是这种情况下方法的唯一参数）。

【讨论】：

【解决方案3】：

自己关心打字

static void Main(string[] args)
{
    var documentUrls = new Dictionary<string, string>();

    Feed rootFeed = $"{baseUrl}/biprws/infostore/{23}/children"
        .Do(str => Console.WriteLine($"{str}:"))
        .GetResponse(headerToken)
        .GetXmlString()
        .DeserializeFromXml<Library.Children.Feed>();

    IEnumerable<object> folderUrls = rootFeed.Entries
        .Where(entry => entry.Content.ContentContainsFolderUrl(type: "folder"))
        .Select(entry => $"{entry.Link.Href}/children")
        .ToList();

    object feeds;
    object folders;
    object documents;

    Init(folderUrls, out feeds, out folders, out documents);

    while (documents.Any() || folders.Any())
    {
        Console.WriteLine($"Got {folders.Count} folders and {documents.Count} documents");

        folderUrls.RemoveAll(x => true);
        folderUrls.AddRange(folders);
        documents.ForEachDo(x => documentUrls.Add(x.Key, x.Value));

        Init(folderUrls, out feeds, out folders, out documents);
    }

    Console.WriteLine($"There are {documentUrls.Count()} {nameof(documentUrls)}");

}

private static void Init(object foldersUrl, out object feeds, out object folders, out object documents)
{
    feeds = InitFeeds(foldersUrl);
    folders = InitFolders(feeds);
    documents = InitDocs(feeds);
}

private static object InitDocs(object feeds)
{
    var documents = feeds
        .Where(entry => entry.Content.ContentContainsFolderUrl(type: "webi"))
        .ToDictionary(
            keySelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "id").First().Text,
            elementSelector: x => x.Content.Attrs.Attr.Where(y => y.Name.ToLower() == "name").First().Text);
    return documents;
}

private static object InitFolders(object feeds)
{
    var folders = feeds
        .Where(entry => entry.Content.ContentContainsFolderUrl(type: "Folder"))
        .Select(x => $"{x.Link.Href}/children")
        .ToList();
    return folders;
}

private static object InitFeeds(object folderUrls)
{
    var feeds = folderUrls
        .Select(url => url.GetResponse(headerToken).GetXmlString().DeserializeFromXml<Feed>())
        .SelectMany(x => x.Entries);
    return feeds;
}

【讨论】：