Using using and getting variable assigned inside using in C#

Copy archive.Volumes instead of just having collection reference it. Then when archive is disposed at the end of the using, your collection won't have been disposed.

You can definitely still read from the variable. There's no problem in terms of definite assignment, or you'd get a compile-time error. For example, this is fine:

using System;
using System.IO;

class Test
{
    static void Main()
    {
        string x;
        using (new MemoryStream())
        {
            x = "hello";
        }
        Console.WriteLine(x);
    }
}

That's absolutely fine.

Now if SevenZipArchive returns a ReadOnlyCollection<string>, I'd usually expect that to still be valid after the archive itself is disposed. However, ReadOnlyCollection<T> is simply a wrapper around another collection... and if that collection is being invalidated by disposing of archive, that would certainly explain things.

Unfortunately, Joel's suggested way of copying the collection is only creating another wrapper - which will ask the first wrapper for the count, in turn asking the original (invalidated) collection.

Here's one approach which should work:

private ReadOnlyCollection<string> ExtractRar(string varRarFileName,
                                              string varDestinationDirectory) {
    ReadOnlyCollection<string> collection;
    using (var archive = new SevenZipArchive(varRarFileName)) {
        collection = new ReadOnlyCollection<string>(archive.Volumes.ToList()); 
        MessageBox.Show(collection.Count.ToString()); // output 10
    }
    MessageBox.Show(collection.Count.ToString()); // output 0
    return collection;
}

Note the extra call to ToList(). That will force the collection to be copied to a List<string> first... truly copied, not just creating a wrapper.

Of course, if you don't really mind if the method returns a List, you could just use:

private List<string> ExtractRar(string varRarFileName,
                                string varDestinationDirectory) {
    List<string> collection;
    using (var archive = new SevenZipArchive(varRarFileName)) {
        collection = archive.Volumes.ToList(); 
        MessageBox.Show(collection.Count.ToString()); // output 10
    }
    MessageBox.Show(collection.Count.ToString()); // output 0
    return collection;
}

... and then when you don't need the extra diagnostics:

private List<string> ExtractRar(string varRarFileName,
                                string varDestinationDirectory) {
    using (var archive = new SevenZipArchive(varRarFileName)) {
        return archive.Volumes.ToList(); 
    }
}

(I'm assuming you're using .NET 3.5 or higher, by the way, to use the ToList extension method.)

I tried something similar and I get a passing test:

[Test] public void CollectionCountShouldBeGreaterThanZero() {
  // arrange
  string tempDir = Path.GetTempPath();
  var fileInfo = new FileInfo(tempDir + Path.DirectorySeparatorChar + "test.zip");
  File.WriteAllBytes(fileInfo.FullName, Resources.TestZipFile);

  SevenZipBase.SetLibraryPath(@"c:\7z.dll");

  // act
  ReadOnlyCollection<string> collection;
  using(var archive = new SevenZipExtractor(fileInfo.FullName))
    collection = archive.ArchiveFileNames;

  // assert
  Assert.IsTrue(collection.Count > 0);
}

The problem is that the field you are referencing is part of archive. The archive object doesn't exist at that point because of the using closure.

You could clone the values inside the using which would give you a copy of the list instead of a reference to the list values and that would do the job.