ConcurrentDictionary missing values?

I have the following code

ConcurrentDictionary<string, XElement> DocumentsElement = new ConcurrentDictionary<string, XElement>();

        public void AddDocument(string docId)
        {
            bool added;
            try
            {
                var documentElement = new XElement("Document", new XAttribute("DocID", docId));

                lock (lockObject)
                {
                    added = DocumentsElement.TryAdd(docId, documentElement);

                    string path = $"{_batchNumber}";
                    if(!Directory.Exists(path)) { Directory.CreateDirectory(path); }
                    File.AppendAllText($"{path}/{docId}.txt", documentElement.ToString());

                }
            }
            catch (Exception ex)
            {
                added = false;
            }

            if (added)
            {
                Debug.WriteLine("Document added successfully.");
            }
            else
            {
                Debug.WriteLine("Failed to add document.");
            }

            lock (lockObject)
            {
                Debug.WriteLine("From AddDocument method " + DocumentsElement.Count);
            }
        }

The issue is that at the end of the entire process, DocumentsElement is missing some values., however the "File.AppendAllText" is correctly writing the XElement value into a text file and all files are there. There are multiple threads that access this AddDocument method.

e.g. DocumentsElement.Count = 4495
No. Text files = 4500 (correct count).

Is there some synchronization issue that I'm missing?

The calling code is in my main app, Process(ConcurrentDictionary<long, byte> idList)

private ConcurrentDictionary<string, DocumentLoadFile> loadfiles;

private async Task Process(ConcurrentDictionary<long, byte> idList)
{
     var tasks = idList.Select(async (id) =>
     {
          var metadata = FetchMetadataForId(id);
          var loadfile = loadfiles[metadata.BatchId];
          
          loadfile.AddDocument(metadata.DocumentId);

          if(metadata.HasAttachment)
              ProcessAttachment(metadata.AttachmentId)
     });

     await Task.WhenAll(tasks);

     //dostuff
}


private void ProcessAttachment(long attId)
{
          var metadata = FetchMetadataForId(attId);
          var loadfile = loadfiles[metadata.BatchId];
          
          loadfile.AddDocument(metadata.DocumentId);

}



</details>


# 答案1
**得分**: 2

你正在向文件 "unconditionally" 添加内容，而字典会跳过重复项。根据 [`ConcurrentDictionary&lt;TKey, TValue&gt;.TryAdd(TKey, TValue)`][1] 文档：

> 尝试将指定的键和值添加到 `ConcurrentDictionary&lt;TKey, TValue&gt;` 中。
> 如果成功将键/值对添加到 `ConcurrentDictionary&lt;TKey, TValue&gt;`，则返回 `true`；如果键已经存在，则返回 `false`。

只有在字典中不存在该项时才尝试添加到文件中：

```csharp
var documentElement = new XElement("Document", new XAttribute("DocID", docId));
added = DocumentsElement.TryAdd(docId, documentElement);
if (added)
{
    lock (lockObject)
    {
        string path = $"{_batchNumber}";
        if (!Directory.Exists(path)) { Directory.CreateDirectory(path); }
        File.AppendAllText($"{path}/{docId}.txt", documentElement.ToString());
    }
}

var documentElement = new XElement("Document", new XAttribute("DocID", docId));
added = DocumentsElement.TryAdd(docId, documentElement);
if (added)
{
    lock (lockObject)
    {
        string path = $"{_batchNumber}";
        if(!Directory.Exists(path)) { Directory.CreateDirectory(path); }
        File.AppendAllText($"{path}/{docId}.txt", documentElement.ToString());
    }
}

答案2

得分: 1

首先建议您进行以下更改（并理解），这将使代码更加简洁，并在存在重复项时提供通知：

```csharp
public void AddDocument(string docId)
{
    try
    {
        lock (lockObject)
        {
            var documentElement = new XElement("Document", new XAttribute("DocID", docId));
            if (!DocumentsElement.TryAdd(docId, documentElement))
                Debug.WriteLine("重复项：{0}", docId);

            // 文件I/O较慢。您可以考虑将其外包给队列并并行执行。
            string path = $"{_batchNumber}";
            if(!Directory.Exists(path)) { Directory.CreateDirectory(path); }
            File.AppendAllText($"{path}/{docId}.txt", documentElement.ToString());
            Debug.WriteLine("成功添加文档。");
            Debug.WriteLine("来自AddDocument方法的文档计数：" + DocumentsElement.Count);
        }
    }
    catch (Exception ex)
    {
         Debug.WriteLine("添加文档失败。");
    }
}

public void AddDocument(string docId)
{
    try
    {
        lock (lockObject)
        {
            var documentElement = new XElement("Document", new XAttribute("DocID", docId));
            if (!DocumentsElement.TryAdd(docId, documentElement))
                Debug.WriteLine("Duplicate : {0}", docId);

            // File I/O is slow. You may consider outsourcing this
            // to a queue and execute in parallel.
            string path = $"{_batchNumber}";
            if(!Directory.Exists(path)) { Directory.CreateDirectory(path); }
            File.AppendAllText($"{path}/{docId}.txt", documentElement.ToString());
            Debug.WriteLine("Document added successfully.");
            Debug.WriteLine("From AddDocument method " + DocumentsElement.Count);
        }
    }
    catch (Exception ex)
    {
         Debug.WriteLine("Failed to add document.");
    }
}

</details>