C#：记某次using声明导致的问题

C#8.0引入了using声明（using declaration）特性，极大地简化了原本需要使用using语句（using statement）的代码。但该新特性并不意味着可以随处用using声明来替代旧的using语句写法，本文就是实践中的一次经历。

考虑创建一个压缩文件，其代码实现如下：

static byte[] ZipViaUsingDeclaration(string fileName, byte[] fileContent)
{
    using var memoryStream = new System.IO.MemoryStream();
    using var zipArchive = new System.IO.Compression.ZipArchive(memoryStream, System.IO.Compression.ZipArchiveMode.Create, true);

    var entry = zipArchive.CreateEntry(fileName, System.IO.Compression.CompressionLevel.Fastest);
    using var entryStream = entry.Open();
    entryStream.Write(fileContent, 0, fileContent.Length);
    return memoryStream.ToArray();
}

static byte[] ZipViaUsingStatement(string fileName, byte[] fileContent)
{
    using (var memoryStream = new System.IO.MemoryStream())
    {
        using (var zipArchive = new System.IO.Compression.ZipArchive(memoryStream, System.IO.Compression.ZipArchiveMode.Create, true))
        {
            var entry = zipArchive.CreateEntry(fileName, System.IO.Compression.CompressionLevel.Fastest);
            using (var entryStream = entry.Open())
            {
                entryStream.Write(fileContent, 0, fileContent.Length);
            }
        }
        return memoryStream.ToArray();
    }
}

 

测试表明，第二个方法能正常工作；第一个方法的结果不能解压为原内容。

为了让第一个方法正常工作，需要在退出前显式调用zipArchive.Dispose方法：

static byte[] ZipViaUsingDeclaration(string fileName, byte[] fileContent)
{
    using var memoryStream = new System.IO.MemoryStream();
    using var zipArchive = new System.IO.Compression.ZipArchive(memoryStream, System.IO.Compression.ZipArchiveMode.Create, true);

    var entry = zipArchive.CreateEntry(fileName, System.IO.Compression.CompressionLevel.Fastest);
    using var entryStream = entry.Open();
    entryStream.Write(fileContent, 0, fileContent.Length);

    zipArchive.Dispose();
    return memoryStream.ToArray();
}

 

文章[1] 更深入地讨论了这种现象，值得一读。 

[1] “Be Careful with Using Declarations in C# 8”, Damir, https://www.damirscorner.com/blog/posts/20200103-BeCarefulWithUsingDeclarationsInCSharp8.html