Best practice for nested using statements?

问题:

I have a code block as follows and I'm using 3 nested using blocks.

I found that using try finally blocks I can avoid this but if there are more than two using statements, what is the best approach?

private FileStream fileStream = null;
private Document document = null;
private PdfWriter pdfWriter =  null;

using (fileStream = new FileStream("ABC.pdf", FileMode.Create))
{
    using (document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
    {
        using (pdfWriter = PdfWriter.GetInstance(document, fileStream))
        {
            document.AddAuthor(metaInformation["author"]);
            document.AddCreator(metaInformation["creator"]);
            document.AddKeywords("Report Generation using I Text");
            document.AddSubject("Document subject");
            document.AddTitle("The document title");
        }
    }
}

回答1:

You can remove the indention and curly brackets this way:

using (var fileStream = new FileStream("ABC.pdf", FileMode.Create))
using (var document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
using (var pdfWriter = PdfWriter.GetInstance(document, fileStream))
{
   // code
}

回答2:

A little less verbose way to avoid the indenting:

  using (var fileStream = new FileStream("ABC.pdf", FileMode.Create))
  using (var document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
  using (var pdfWriter = PdfWriter.GetInstance(document, fileStream))
  {
       document.AddAuthor(metaInformation["author"]);
       document.AddCreator(metaInformation["creator"]);
       document.AddKeywords("Report Generation using I Text");
       document.AddSubject("Document subject - Describing the steps creating a PDF document");
       document.AddTitle("The document title - PDF creation using iTextSharp");
   }

As Jon Skeet pointed out, there is no need for these variables to be instance variables, as they are disposed after the using blocks anyway.

You can use local variables as shown in the code above instead.

回答3:

Maybe something conventional; best approach for choosing between two in my opinion would be;

• Using : If you are going to use an instance within a context and need to Dispose it after you are done with it
• try/finally : If you are expecting any issue and have something to do with the exception, catching it before you Dispose the instance you are using.

And as other comments / answers state; you don't need instance level variables;

using (FileStream fileStream = new FileStream("ABC.pdf", FileMode.Create))
using (Document document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom))
using (PdfWriter pdfWriter = PdfWriter.GetInstance(document, fileStream))
{
    // # Implementation here seems like a good approach
}

回答4:

In a single method where you don't need to process or change data; the option suggested by Jan Sommer would be my choice. However, in some circumstances the DisposableList is useful. Particularly, if you have many disposable fields that all need to be disposed of (in which case you cannot use using).

Requires you to remember to add the item to the list. (Although you could also say you have to remember to use using.) Aborts the disposal process if one of the disposes methods throws, leaving the remaining items un-disposed.

public class DisposableList : List<IDisposable>, IDisposable
{
    public void Dispose()
    {
        if (this.Count > 0)
        {
            List<Exception> exceptions = new List<Exception>();

            foreach (var disposable in this)
            {
                try
                {
                    disposable.Dispose();
                }
                catch (Exception e)
                {
                    exceptions.Add(e);
                }
            }
            base.Clear();

            if (exceptions.Count > 0)
                throw new AggregateException(exceptions);
        }
    }

    public T Add<T>(Func<T> factory) where T : IDisposable
    {
        var item = factory();
        base.Add(item);
        return item;
    }
}

Now catch any exceptions from the Dispose calls and will throw a new AggregateException after going through all the items. I've added a helper Add method that allows a simpler usage:

using (var disposables = new DisposableList())
{
        var file = disposables.Add(() => File.Create("test"));
        // ...
        var memory = disposables.Add(() => new MemoryStream());
        // ...
        var cts = disposables.Add(() => new CancellationTokenSource());
        // ... 
}

回答5:

now on C# 8 you have using declarations

    using var fileStream = new FileStream("ABC.pdf", FileMode.Create);
    using var document = new Document(PageSize.A4, marginLeft, marginRight, marginTop, marginBottom);
    using var pdfWriter = PdfWriter.GetInstance(this.document, this.fileStream);