Is there a better deterministic disposal pattern than nested “using”s?

问题

In C#, if I want to deterministically clean up non-managed resources, I can use the "using" keyword. But for multiple dependent objects, this ends up nesting further and further:

using (FileStream fs = new FileStream("c:\file.txt", FileMode.Open))
{
    using (BufferedStream bs = new BufferedStream(fs))
    {
        using (StreamReader sr = new StreamReader(bs))
        {
            // use sr, and have everything cleaned up when done.
        }
    }
}

In C++, I'm used to being able to use destructors to do it like this:

{    
    FileStream fs("c:\file.txt", FileMode.Open);
    BufferedStream bs(fs);
    StreamReader sr(bs);
    // use sr, and have everything cleaned up when done.
}

Is there a better way in C# to do this? Or am I stuck with the multiple levels of nesting?

回答1:

You don't have to nest with multiple usings:

using (FileStream fs = new FileStream("c:\file.txt", FileMode.Open))
using (BufferedStream bs = new BufferedStream(fs))
using (StreamReader sr = new StreamReader(bs))
{
    // all three get disposed when you're done
}

回答2:

You can put using statements together before the opening braces like so:

  using (StreamWriter w1 = File.CreateText("W1"))
  using (StreamWriter w2 = File.CreateText("W2"))
  {
      // code here
  }

http://blogs.msdn.com/ericgu/archive/2004/08/05/209267.aspx

回答3:

You could use this syntax to condense things down a bit:

using (FileStream fs = new FileStream("c:\file.txt", FileMode.Open))
using (BufferedStream bs = new BufferedStream(fs))
using (StreamReader sr = new StreamReader(bs))
{
}

This is one of those rare occasions where not using { } for all blocks makes sense IMHO.

回答4:

Instead of nesting using statements, you can just write out the .Dispose calls manually - but you'll almost certainly miss one at some point.

Either run FxCop or something else that can make sure that all IDisposable-implementing type instances have a .Dispose() call, or deal with the nesting.

回答5:

I have implemented solutions like Michael Meadows's before, but his StreamWrapper code doesn't take into account if the Dispose() methods called on the member variables throw an exception for one reason or another, the subsequent Dispose()es will not be called and resources could dangle. The safer way for that one to work is:

        var exceptions = new List<Exception>();

        try
        {
            this.sr.Dispose();
        }
        catch (Exception ex)
        {
            exceptions.Add(ex);
        }

        try
        {
            this.bs.Dispose();
        }
        catch (Exception ex)
        {
            exceptions.Add(ex);
        }

        try
        {
            this.fs.Dispose();
        }
        catch (Exception ex)
        {
            exceptions.Add(ex);
        }

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

回答6:

you can omit the curly braces, like:

using (FileStream fs = new FileStream("c:\file.txt", FileMode.Open))
using (BufferedStream bs = new BufferedStream(fs))
using (StreamReader sr = new StreamReader(bs))
{
        // use sr, and have everything cleaned up when done.
}

or use the regular try finally approach:

FileStream fs = new FileStream("c:\file.txt", FileMode.Open);
BufferedStream bs = new BufferedStream(fs);
StreamReader sr = new StreamReader(bs);
try
{
        // use sr, and have everything cleaned up when done.
}finally{
   sr.Close(); // should be enough since you hand control to the reader
}

回答7:

This makes for a much larger net plus in lines of code, but a tangible gain in readability:

using (StreamWrapper wrapper = new StreamWrapper("c:\file.txt", FileMode.Open))
{
    // do stuff using wrapper.Reader
}

Where StreamWrapper is defined here:

private class StreamWrapper : IDisposable
{
    private readonly FileStream fs;
    private readonly BufferedStream bs;
    private readonly StreamReader sr;

    public StreamWrapper(string fileName, FileMode mode)
    {
        fs = new FileStream(fileName, mode);
        bs = new BufferedStream(fs);
        sr = new StreamReader(bs);
    }

    public StreamReader Reader
    {
        get { return sr; }
    }

    public void Dispose()
    {
        sr.Dispose();
        bs.Dispose();
        fs.Dispose();
    }
}

With some effort, StreamWrapper could be refactored to be more generic and reusable.

回答8:

It should be noted that generally when creating stream based off another stream the new stream will close the one being passed in. So, to further reduce your example:

using (Stream Reader sr = new StreamReader( new BufferedStream( new FileStream("c:\file.txt", FileMode.Open))))
{
    // all three get disposed when you're done
}

回答9:

for this example let us assume you have:

a file named 1.xml under c:\

a textbox named textBox1, with the multi-line properties set ON.

const string fname = @"c:\1.xml";

StreamReader sr=new StreamReader(new BufferedStream(new FileStream(fname,FileMode.Open,FileAccess.Read,FileShare.Delete)));
textBox1.Text = sr.ReadToEnd();

回答10:

The using statement is syntactic sugar that converts to:

   try
   {
      obj declaration
      ...
   }
   finally
   {
      obj.Dispose();
   }

You can explicitly call Dispose on your objects, but it won't be as safe, since if one of them throws an exception, the resources won't be freed properly.

来源：https://stackoverflow.com/questions/75722/is-there-a-better-deterministic-disposal-pattern-than-nested-usings

标签

c++

using

raii