如何处置MemoryStream对象

Question

以下代码用于拼接现有PDF [另外我们使用TallComponents进行实际拼接,以防您想知道PDFUtility是什么]:

PDFUtility.Document docFinal = new PDFUtility.Document();
PDFUtility.Document docToAdd = null;
byte[] combinedFile;

foreach (byte[] content in fileContents)
{
    MemoryStream fileContentStream = new MemoryStream(content);
    docToAdd = new PDFUtility.Document(fileContentStream);
    docFinal.Pages.AddRange(docToAdd.Pages.CloneToArray());
}
using (MemoryStream stream = new MemoryStream())
{
    docFinal.Write(stream);
    combinedFile = stream.ToArray();
}

这段代码的明显问题是这个命令:

MemoryStream fileContentStream = new MemoryStream(content);

内存流fileContentStream没有被处理掉,可能(我相信)保持资源的时间超过了所需的时间.

显而易见的解决方案是将MemoryStream的创建包装在一个using块中.代码将如下所示:

PDFUtility.Document docFinal = new PDFUtility.Document();
PDFUtility.Document docToAdd = null;
byte[] combinedFile;

foreach (byte[] content in fileContents)
{
    using (MemoryStream stream = new MemoryStream())
    {
        MemoryStream fileContentStream = new MemoryStream(content);
        docToAdd = new PDFUtility.Document(fileContentStream);
        docFinal.Pages.AddRange(docToAdd.Pages.CloneToArray());
    }
}
using (MemoryStream stream = new MemoryStream())
{
    docFinal.Write(stream);
    combinedFile = stream.ToArray();
}

在上面的代码中使用using块导致代码在此行上失败(因为之前已经处理了流):

docFinal.Write(stream);

一种可能的解决方案是跟踪所有MemoryStream实例并在完成它们之后将其丢弃.这是代码:

PDFUtility.Document docFinal = new PDFUtility.Document();
PDFUtility.Document docToAdd = byte[] combinedFile;
List<MemoryStream> streams = new List<MemoryStream>();
foreach (byte[] content in fileContents)
{
    MemoryStream fileContentStream = new MemoryStream(content);
    streams.Add(fileContentStream); //EACH INSTANCE OF A STREAM IS TRACKED
    docToAdd = new PDFUtility.Document(fileContentStream);
    docFinal.Pages.AddRange(docToAdd.Pages.CloneToArray());
}
using (MemoryStream stream = new MemoryStream())
{
    docFinal.Write(stream);
    combinedFile = stream.ToArray();
}
streams.ForEach(s => s.Dispose()); //DISPOSE OF ALL STREAMS HERE

上面的代码有效.我只是推迟Dispose直到最终文件被写出来之后.

但是,这似乎不是"最佳"解决方案.有没有办法实现使用块(从而保证对象被妥善处理？

Answer 1

using块不仅仅是try-finally块的语法糖.

根据使用块的使用方式,最终会得到两种类型的try-finally块.

情况1:

// This code ...
using( var thing = new Thing() ) {
    thing.DoOperation();
}

// ... turns into this scoped try-finally:
{
    var thing = new Thing();
    try {
        thing.DoOperation();
    }
    finally {
        thing.Dispose();
        thing = null;
    }
}

案例二:

// This code ...
var thing = new Thing();
using( thing ) {
    thing.DoOperation();
}

// ... turns into this code
var thing = new Thing();
try {
    thing.DoOperation();
}
finally {
    thing.Dispose();
    // Note the lack of a null assignment.
}

有了这些知识,您可以修改第三个解决方案以使用finally块来确保MemoryStream始终清理对象.

PDFUtility.Document docFinal = new PDFUtility.Document();
PDFUtility.Document docToAdd = byte[] combinedFile;
List<MemoryStream> streams = new List<MemoryStream>();

try 
{
    foreach (byte[] content in fileContents)
    {
        MemoryStream fileContentStream = new MemoryStream(content);
        streams.Add(fileContentStream); //EACH INSTANCE OF A STREAM IS TRACKED
        docToAdd = new PDFUtility.Document(fileContentStream);
        docFinal.Pages.AddRange(docToAdd.Pages.CloneToArray());
    }
    using (MemoryStream stream = new MemoryStream())
    {
        docFinal.Write(stream);
        combinedFile = stream.ToArray();
    }

}
finally 
{
    streams.ForEach(s => s.Dispose()); //DISPOSE OF ALL STREAMS HERE
}