using statement FileStream and / or StreamReader -

问题:

The new Visual Studio 2012 is complaining about a common code combination I have always used. I know it seems like overkill but I have done the following in my code 'just to be sure'.

using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
    using (var sr = new StreamReader(fs))
    {
        // Code here
    }
}

Visual studio is 'warning' me that I am disposing of fs more than once. So my question is this, would the proper way to write this be:

using (var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
{
    var sr = new StreamReader(fs);
    // do stuff here
}

Or should I do it this way (or some other variant not mentioned).

var fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);

using (var sr = new StreamReader(fs))
{
    // Code here
}

I searched several questions in StackOverflow but did not find something that addressed the best practice for this combination directly.

Thank you!

回答1:

The following is how Microsoft recommends doing it. It is long and bulky, but safe:

FileStream fs = null;
try
{
    fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    using (TextReader tr= new StreamReader(fs))
    {
        fs = null;
        // Code here
    }
}
finally
{
    if (fs != null)
        fs.Dispose();
}

This method will always ensure that everything is disposed that should be despite what exceptions may be thrown. For example, if the StreamReader constructor throws an exception, the FileStream would still be properly disposed.

回答2:

Visual studio is 'warning' me that I am disposing of fs more than once.

You are, but that is fine. The documentation for IDisposable.Dispose reads:

If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times.

Based on that, the warning is bogus, and my choice would be to leave the code as it is, and suppress the warning.

回答3:

As Dan's answer only appears to work with StreamWriter, I believe this might be the most acceptable answer. (Dan's answer will still give the disposed twice warning with StreamReader - as Daniel Hilgarth and exacerbatedexpert mentions, StreamReader disposes the filestream)

using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
    string line;
    while ((line = tr.ReadLine()) != null)
    {
        // Do work here
    }
}

This is very similar to Daniel Hilgarth's answer, modified to call dispose via the Using statement on StreamReader as it is now clear StreamReader will call dispose on FileStream (According to all the other posts, documentation referenced)

Update:

I found this post. For what it is worth. Does disposing streamreader close the stream?

回答4:

Yes, the correct way would be to use your first alternative:

using (FileStream fs = new FileStream(filePath, FileMode.Open,
                                      FileAccess.Read, FileShare.ReadWrite)) 
{ 
    TextReader tr = new StreamReader(fs); 
    // do stuff here 
} 

The reason is the following:
Disposing the StreamReader only disposes the FileStream so that's actually the only thing you need to dispose.

Your second option (just the inner "using") is no solution as it would leave the FileStream undisposed if there was an exception inside the constructor of the StreamReader.

回答5:

It's because the way you used StreamReader disposes the stream when it is disposed. So, if you dispose the stream too, it's being disposed twice. Some consider this a flaw in StreamReader--but it's there none-the-less. In VS 2012 (.NET 4.5) there is an option in StreamReader to not dispose of the stream, with a new constructor: http://msdn.microsoft.com/en-us/library/gg712952

回答6:

Two solutions:

A) You trust Reflector or Documentation and you know *Reader and *Writer will close the underlying *Stream. But warning: it won't work in case of a thrown Exception. So it is not the recommended way:

using (TextReader tr = new StreamReader(new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)))
{
    // Code here
}

B) You ignore the warning as documentation states The object must not throw an exception if its Dispose method is called multiple times. It's the recommended way, as it's both a good practice to always use using, and safe in case of a thrown Exception:

[SuppressMessage("Microsoft.Usage", "CA2202:Do not dispose objects multiple times")]
internal void myMethod()
{
    [...]
    using (FileStream fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
    using (TextReader tr = new StreamReader(fs))
    {
        // Code here
    }
}

回答7:

Given all the nonsense this (perfectly legitimate!) question generated, this would be my preference:

FileStream fs = null;
TextReader tr= null;
try
{
    fs = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    tr= new StreamReader(fs);
    // Code here
}
finally
{
    if (tr != null)
        tr.Dispose();
    if (fs != null)
        fs.Dispose();
}

The links below illustrate perfectly legal syntax. IMO, this "using" syntax is far preferable to nested "using". But I admit - it does not solve the original question:

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

• .NET - Replacing nested using statements with single using statement

IMHO...