C# static methods & attributes: Object reference not set to an instance of an object

Question

I'm using static methods and attributes, when I call a static method, I get a NullReferenceException.

sample class:

internal class Utils
{
    private static Regex[] _allRegexes = { _regexCategory };
    private static Regex _regexCategory = new Regex(@"(?<name>c(ategory){0,1}):(?<value>([^""\s]+)|("".+""))\s*", RegexOptions.IgnoreCase);

    public static string ExtractKeyWords(string queryString)
    {
        if (string.IsNullOrWhiteSpace(queryString))
            return null;   

        _allRegexes[0];//here: _allRegexes[0]==null throw an exception
    }
}

cause:

_allRegexes[0]==null

I can't figure it out why this happens, I think _allRegexes should be initialized when I call that method.

Can anybody explain it?

allRegexes by itself isn't a valid statement...?

Marc Gravell
– Marc Gravell

2012-08-19 08:59:46 +00:00
Commented Aug 19, 2012 at 8:59 — Marc Gravell
– Marc Gravell, Commented Aug 19, 2012 at 8:59

CodesInChaos · Accepted Answer · 2012-08-19 09:18:02Z

2

Static fields get initialized in declaration order. This means _regexCategory is null when you initialize _allRegexes.

The static field variable initializers of a class correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration.

(Quoted from C# Language Specification Version 4.0 - 10.5.5.1 Static field initialization)

This leads to _allRegexes becoming an array that contains a single null element, i.e. new Regex[]{null}.

This means you can fix your code by putting _regexCategory before _allRegexes in your class.

edited Aug 19, 2012 at 9:18

answered Aug 19, 2012 at 9:11

CodesInChaos

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3 Comments

Anirudha Over a year ago

sorry for that..It was in chaos..haha..actually I ans the same thing but was downvoted..?

Scott 混合理论 Over a year ago

what if add a static constructor??

CodesInChaos Over a year ago

@Kevin The static constructor will run after the fields get initialized. The field initializers with still run in textual order.

oleksii · Accepted Answer · 2012-08-19 09:11:39Z

This code works without NRE

internal class Utils
{
    private static Regex _regexCategory = new Regex(
        @"(?<name>c(ategory){0,1}):(?<value>([^""\s]+)|("".+""))\s*", 
        RegexOptions.IgnoreCase);
    private static Regex[] _allRegexes = { _regexCategory };


    public static string ExtractKeyWords(string queryString)
    {
        if (string.IsNullOrWhiteSpace(queryString))
            return null;

        //change it to your needs, I just made it compile
        return _allRegexes[0].Match(queryString).Value;
    }
}    

class Program
{
    static void Main(string[] args)
    {
        string result = Utils.ExtractKeyWords("foo");
    }
}

I believe the problem is in the order in which parameters are getting initialized.

Anirudha · Accepted Answer · 2012-08-19 09:24:07Z

1

It should be

    private static Regex _regexCategory = new Regex(@"(?<name>c(ategory){0,1}):(?<value>([^""\s]+)|("".+""))\s*", RegexOptions.IgnoreCase);
    private static Regex[] _allRegexes = { _regexCategory };

In your code the IL will load _regexCategory into _allRegexes which is NULL because the IL had never initialized it..

It initalizes when you instantiate _regexCategory with new keyword

edited Aug 19, 2012 at 9:24

answered Aug 19, 2012 at 8:58

Anirudha

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