LibTiff.NET ReadDirectory is giving System.ObjectDisposedException Only During Unit Tests

问题

Edit: FYI for future readers, this issue has been fixed as of version 2.3.606.0 of BitMiracle's LibTiff.NET.

I'm using BitMiracle's LibTiff.NET (version 2.3.605.0 and below) in my C# library (compiled at .NET 3.5 | x86) and keep getting this exception when I call ReadDirectory: System.ObjectDisposedException: Cannot write to a closed TextWriter

I realize that this seems to indicate that I have already disposed of my image before making the call...but I have not specifically done so. Is this a bug in the library or am I really missing something here?

Here is my code:

    public static bool IsTiffBiTonal(String tiffFilePath)
    {
        VerifyFileExistence(tiffFilePath);

        using (Tiff tiff = Tiff.Open(tiffFilePath, "r"))
        {
            do
            {
                if (tiff.GetField(TiffTag.BITSPERSAMPLE)[0].ToInt() == 1)
                {
                    continue;
                }
                return false;
            }
            while (tiff.ReadDirectory()); //Error occurs here
        }
        return true;
    }

EDIT: Ok, I have more information after some further testing, this is only happening when I'm running my unit tests! Don't know why that would change anything though.

回答1:

Because of other threads talking about unit testing and getting this same error when trying to write to the console (ObjectDisposedException when outputting to console) I realized that the LibTiff.NET library was trying to write to the error console. After looking through the source code, I found that this code:

        using (TextWriter stderr = Console.Error)
        {
            ...
        }

Because they were wrapping all of the writes to the error out in a using, it was disposing of the Console.Error object after the first write to the error out. This caused my error on the second time around (ReadDirectory does what calling Next on a linked list does). So I removed the using and the problem was fixed!

        TextWriter stderr = Console.Error;
        ...

So, the lesson here: don't dispose of your standard outputs :)

I've asked another question regarding why they were ever allowed to dispose of the standard output in unit tests but not in other situations here: .NET - Why is disposing of standard output only allowed during unit tests?. If you have any answers to the question...please post it there.

来源：https://stackoverflow.com/questions/12114992/libtiff-net-readdirectory-is-giving-system-objectdisposedexception-only-during-u

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