Why does the Tiff.Open method always return NULL and not open a tiff file?

Question

Overview:

I have an issue where 1 out of a hundred PCs just started having an issue with the LibTiff library not wanting to open ANY of my tiff images. I suspect it has to do with a dependency file that has changed. I have seen this issue before but can not remember how I solved the issue (I think I replaced some dll files with ones from another PC that was working). I tried to use Dependency Walker to determine what files were used by the LibTiff.Net dll but could not get Dependency Walker to work, it just had errors opening a bunch of dlls.

Question:

Is it possible to know WHY a file is not being opened successfully? All the method does is return NULL, is there a way to get an error of some type? I know there are no locks on the files (did a reboot, and checked other ways). I know the tiff files are good with no issues (copied them to another PC that uses the same software and it reads it just fine). Could I use something like "GetLastError" to return a reason the Open method is returning Null?

Here is my current usage:

using BitMiracle.LibTiff.Classic;

using (Tiff tImage = Tiff.Open(sFileName, "r"))
{
    if (tImage == null)
    {
      //File could not be opened
      lastError = "File could not be opened.";
      return null;
    }
}

Answer 1

Moved to be an answer from my comment:

It is possible to determine, however you would need to do the checks individually since Tiff.Open does not throw an exception. It either opens the file or returns null.

I would add a if(!File.Exists(sFileName)) throw new FileNotFoundException("File was not found.", sFileName);. That would be an important one as this could occur if the file is stored on a network drive and that drive becomes temporarily unavailable.

See also this question/answer for checking if a file is locked. That would be another big reason why it may not be able to open the file. If the image file is opened by your code or another application and the resource is not released it will remain locked. Here is the code from the linked discussion:

protected virtual bool IsFileLocked(FileInfo file)
{
    FileStream stream = null;

    try
    {
        stream = file.Open(FileMode.Open, FileAccess.Read, FileShare.None);
    }
    catch (IOException)
    {
        //the file is unavailable because it is:
        //still being written to
        //or being processed by another thread
        //or does not exist (has already been processed)
        return true;
    }
    finally
    {
        if (stream != null)
            stream.Close();
    }

    //file is not locked
    return false;
}

In general, you should probably perform your own checks before attempting to open it; or you can just try to open it and check if it returns null, then you can do your checks then. It's really up to you since it doesn't throw its own exceptions.