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?

Answer 1

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.