Null reference error in foreach statement

Question

I am getting a null reference error in the below code:

string artistName =     mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationArtist);
string albumName =     mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationAlbum);
string songTitle =     mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationFullName);
foreach (char c in artistName)
{
    artistName = artistName.Replace("?", "");
    artistName = artistName.Replace("*", "");
    artistName = artistName.Replace("/", "");
    artistName = artistName.Replace(":", "");
}
foreach (char c in albumName)
{
    albumName = albumName.Replace("?", "");
    albumName = albumName.Replace("*", "");
    albumName = albumName.Replace("/", "");
    albumName = albumName.Replace(":", "");
}

I have copied this code from another persons project and though I mostly understand what is happening I cannot figure out the error. The error is on the for each (char c in artistName) line. Thanks for any help.

What do you see when you put a breakpoint on string artistName = ...? — Tieson T.
– Tieson T., Commented Aug 13, 2013 at 5:50
I know but how would I do this so artist name isnt null? I basically need to remove illegal characters to use the strings in a directory. I am in way over my head making this program but I kinda need it. — Tom
– Tom, Commented Aug 13, 2013 at 5:51

Jon Skeet · Accepted Answer · 2013-08-13 05:51:47Z

6

Firstly, this code is bad to start with. There's no point in iterating over a string - but then performing the same replacement in it each time.

However, the cause of your problem can only be that artistName is null - it's possible that the other two strings are null too, of course.

You quite possibly want something like:

string artistName = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationArtist);
string albumName = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationAlbum);
string songTitle = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationFullName);

artistName = RemoveSymbols(artistName);
albumName = RemoveSymbols(albumName);

...

private static string RemoveSymbols(string input)
{
    if (input == null)
    {
        return input;
    }
    return input.Replace("?", "")
                .Replace("*", "")
                .Replace("/", "")
                .Replace(":", "");
}

You'll still have null references after this if any particular annotation isn't found, but it won't throw an exception. You'll just need to work out what you want to do with those missing values. (For example, you might want to use a hardcoded "Unknown" value, or maybe an empty string.)

answered Aug 13, 2013 at 5:51

Jon Skeet

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2 Comments

Tom Over a year ago

Thanks its stopped the error, unfortunately now my program freezes with no errors :/

Jon Skeet Over a year ago

@Tom: I suggest you work on diagnosing why that is, and then ask a new question with appropriate details if you get stuck.

Damith · Accepted Answer · 2013-08-13 06:20:42Z

0

you can add validation before removing special characters like below

artistName= RemoveSymbols(artistName);
albumName= RemoveSymbols(albumName);

private static string RemoveSymbols(string input)
{
    if(!String.IsNullOrEmpty(input))
       return input;

    return  Regex.Replace(input, "[?*/:]", string.Empty);
}

Now you will not get Null reference error but you need to find why you receive null value from mov.get_Annotation method

edited Aug 13, 2013 at 6:20

answered Aug 13, 2013 at 6:01

Damith

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Comments

NeverHopeless · Accepted Answer · 2013-08-13 06:42:03Z

0

C# provides a benefit of coalesce operator ??. Try using like this:

string artistName = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationArtist);
string albumName  = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationAlbum);
string songTitle  = mov.get_Annotation((int)QTAnnotationsEnum.qtAnnotationFullName);

artistName = Regex.Replace(artistName ?? "" , @"[\*\:\/\?]" , "");
albumName  = Regex.Replace(albumName  ?? "" , @"[\*\:\/\?]" , "");
songTitle  = Regex.Replace(songTitle  ?? "" , @"[\*\:\/\?]" , "");

edited Aug 13, 2013 at 6:42

answered Aug 13, 2013 at 6:14

NeverHopeless

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