How to prevent a System.Nullreferenceexception within an if

Question

I’m developing a web application where a photographer does upload of an image. After uploading the image, I will scan the entire Metadata in order to capture some information from directories (Exif, IPTC, XMP, JFIF, PNG, BMP, JPG, etc.), I am using the library Metadata Extractor. Ok, I learned to use it. First I need to receive the directories:

 IEnumerable<MetadataExtractor.Directory> directories = iptcReader.ReturnMetadataFile(Path.Combine(StorageRootPOOL, imagePool.File_name[i]));

Here I already have all the directories of my image, after that I need to declare variables of each directory I need to capture in my case:

var IPTCDirectory = directories.OfType<IptcDirectory>().FirstOrDefault();
var JPEGDirectory = directories.OfType<JpegDirectory>().FirstOrDefault();
var JFIFDirectory = directories.OfType<JfifDirectory>().FirstOrDefault();
var EXIFDirectory = directories.OfType<ExifIfd0Directory>().FirstOrDefault();
var EXIFSubDirectory = directories.OfType<ExifSubIfdDirectory>().FirstOrDefault();

All values of directories ARE NOT NULL even if there is no directory in the image, the problem is in the tags that exist within them. To capture the tags inside the directories I use the following code:

imageFileInfo.Keywords = IPTCDirectory.GetDescription(IptcDirectory.TagKeywords);

imageFileInfo is the class where I will be storing all the data. My problem is, if for example there is no TAG IptcDirectory.TagKeywords I will receive a Nullexception, so far nothing out of the ordinary, but if I will prevent this error with an if for example:

 if (IPTCDirectory.GetDescription(IptcDirectory.TagKeywords) == null)
 {
 }

I get another one NullException, there’s the problem, I wanted to set a default value for tags that don’t exist, for example:

if (IPTCDirectory.GetDescription(IptcDirectory.TagKeywords) == null)
{
      imageFileInfo.Keywords = "valor padrão";
}

Remembering that it is no use I use the following code, because I need to know which tags are null and not which directories, because directories always exist, but some tags nay:

if (EXIFDirectory != null && ExifIfd0Directory != null) {
    //Os diretórios nunca são nulos e sim as tags, então esse exemplo não iria se aplica ao meu problema.
}

Answer 1

Do not under any circumstances what is in the other answer by swallowing the exception. This does not solve any problem, it only worsens the situation. This is even more terrible than capture Exception. And use goto is even worse.

Only catch an exception if you really need to. She is slow and should only be used in exceptional situations, not for flow control.

If you’re getting one NullReferenceException, There’s a programming error there. You have to fix it. Capturing and swallowing the exceptions is not making the code right, the solution is to fix it.

Check with a if which can be null and handles the form you want, if you want to assign a default value, do this. So you may not need if.

You have to see what is generating the exception and check exactly this before the error happens. Let it be very clear whenever you receive NullReferenceException is a programming error that must be solved, it must not be captured. And checking the error before it occurs is the solution. That is why I still believe that the answer that has now been accepted is not appropriate, even if it is not the absurdity that the other proposed.

Maybe that’s what you want:

for (int i = 0; i < numberOfProcessAvailable; i++) {
    var IptcDirectory = directories.OfType<IptcDirectoryBase>().FirstOrDefault();
    imageFileInfo.Keywords = IptcDirectory.TagKeywords == null ? "padrao keywords" : IptcDirectory.GetDescription(IptcDirectory.TagKeywords);
    imageFileInfo.LensModel = IptcDirectory.TagLensModel == null ? "padrao lensmodel" : IptcDirectory.GetDescription(IptcDirectory.TagLensModel);
    imageFileInfo.TagMake = IIptcDirectory.TagMake == null ? "padrao make" : IptcDirectory.GetDescription(IptcDirectory.TagMake);
}

I put in the Github for future reference.

Read this: How to best treat exceptions in Java?. Even if it’s another language, it’s worth the same.

Answer 2

I had understood your question the first time in a different way.

In your case the reason for NullReferenceException is clear-cut and acquaintance by you and is an expected behavior when a tag does not exist. In this case it is in your interest to capture the exception to assign a default value to tags nonexistent.

There is no problem in this case in you use:

try {
    imageFileInfo.Keywords = IPTCDirectory.GetDescription(IptcDirectory.TagKeywords);
} catch (NullReferenceException e) {
    imageFileInfo.Keywords = "valor padrao"
}

Even better to use try catch than if to test if the error exists, so processing is reduced when there is no error.

Observing

You’re getting NullException on the line

if (IPTCDirectory.GetDescription(IptcDirectory.TagKeywords) == null)

probably because the object IptcDirectory does not own the property TagKeywords, the right way to test:

if (IptcDirectory.GetProperty('TagKeywords') == null) {
    imageFileInfo.Keywords = "valor padrao";
} else {
    imageFileInfo.Keywords = IPTCDirectory.GetDescription(IptcDirectory.TagKeywords);
}

Put that code with if is less efficient than try catch.

EDIT

As pointed out by Maniero, actually Exceptions in C# are very expensive, so the structure if else is not less efficient at first (it all depends on how expensive GetProperty is in relation to the making of exceptions)

Answer 3

In this case you should separate each of the instructions into blocks of try catch specific.

for (int i = 0; i < numberOfProcessAvailable; i++) {
    var EXIFDirectory = directories.OfType<ExifDirectoryBase().FirstOrDefault();

    try {
        imageFileInfo.LensModel = Convert.ToString(EXIFDirectory.GetDescription(ExifIfd0Directory.TagLensModel));
    } catch (NullReferenceException e){ //tratar excecao }

    try {    
        imageFileInfo.Make = Convert.ToString(EXIFDirectory.GetDescription(ExifIfd0Directory.TagMake));
    } catch (NullReferenceException e){ //tratar excecao }

    try {
        imageFileInfo.Model = Convert.ToString(EXIFDirectory.GetDescription(ExifIfd0Directory.TagModel));
    } catch (NullReferenceException e){ //tratar excecao }
}

That way all the blocks would always run.

PS.: You could treat this problem also by using the instruction goto, but this usually makes the code less readable and more complicated.

Ps2.: I had not made explicit because I thought it was obvious.. But exceptions should always be dealt with and be as specific as possible