如何保证异步方法线程安全？

Question

我需要在 Windows 通用应用程序中编写一个方法来写入 SD 卡。在下面的方法中，如何确保两个线程不会尝试同时写入同一个文件？

public async void WriteToCard(string strFileName, IEnumerable<string> listLinesToWrite)
{
    IStorageItem item = await folder.GetItemAsync(strFileName);
    StorageFile file = (StorageFile)item;

    await Windows.Storage.FileIO.WriteLinesAsync(file, listLinesToWrite);
}

Answer 1

您可以保留一个带有 的映射，将ConcurrentDictionary每个文件映射到SemaphoreSlim. 然后，根据您要写入的文件位置获取每个信号量：

private ConcurrentDictionary<string, SemaphoreSlim> fileLocks = new ConcurrentDictionary<string, SemaphoreSlim>();

public async Task WriteToCardAsync(string strFileName, IEnumerable<string> listLinesToWrite)
{
   var semaphoreSlim = fileLocks.GetOrAdd(strFileName, new SemaphoreSlim(1, 1));

   await semaphoreSlim.WaitAsync();
   try
   {
       IStorageItem item = await folder.GetItemAsync(strFileName);
       StorageFile file = (StorageFile)item;

       await Windows.Storage.FileIO.WriteLinesAsync(file, listLinesToWrite);
   }
   finally
   {
       semaphoreSlim.Release();
   }
}

旁注 - 使用async Task而不是async void. 我还将Async后缀添加到方法中。

Answer 2

您可以使用AsyncEx库，其中包含AsyncLock

假设不会有大量文件名，您可以使用 aConcurrentDictionary将文件名与锁关联 - 否则，字典大小可能会无限增长。

private readonly locks = new ConcurrentDictionary<string, AsyncLock>();

public async void WriteToCard(string strFileName, IEnumerable<string> listLinesToWrite)
{
    var lock = locks.GetOrAdd(strFileName, () => new AsyncLock());

    using (await lock.LockAsync())
    {
        IStorageItem item = await folder.GetItemAsync(strFileName);
        StorageFile file = (StorageFile)item;

        await Windows.Storage.FileIO.WriteLinesAsync(file, listLinesToWrite);
    }
}