为什么这会产生空值异常？

Question

我得到的例外是:

MyNamespace.Program的类型初始值设定项引发了异常

内在的例外说

消息:值不能为空.参数名称:collection
来源: ... HashSet ...

这让我相信错误发生在下面指出的线上......

class Program
{
    public static IEnumerable<char> WordCharacters = ExpandCharacterSet("A-Za-z0-9_");
    public static IEnumerable<char> NonWordCharacters = ExpandCharacterSet("^A-Za-z0-9_");
    public static IEnumerable<char> SpaceCharacters = ExpandCharacterSet(" \f\n\r\t\v");
    public static IEnumerable<char> NonSpaceCharacters = ExpandCharacterSet("^ \f\n\r\t\v");
    public static IEnumerable<char> DigitCharacters = ExpandCharacterSet("0-9");
    public static IEnumerable<char> NonDigitCharacters = ExpandCharacterSet("^0-9");
    public static IEnumerable<char> WildCharacters = ExpandCharacterSet("^\n");
    public static IEnumerable<char> AllCharacters = Enumerable.Range(0, 256).Select(Convert.ToChar).Where(c => !char.IsControl(c));

    public static IEnumerable<char> ExpandCharacterSet(string set)
    {
        if (set.Length == 0)
            return "";

        var sb = new StringBuilder();
        int start = 0;
        bool invertSet = false;

        if (set[0] == '[' && set[set.Length - 1] == ']')
            set = set.Substring(1, set.Length - 2);
        if (set[0] == '^')
        {
            invertSet = true;
            set = set.Substring(1);
        }
        set = Regex.Unescape(set);

        foreach (Match m in Regex.Matches(set, ".-.|."))
        {
            if (m.Value.Length == 1)
                sb.Append(m.Value);
            else
            {
                if (m.Value[0] > m.Value[2]) throw new ArgumentException("Invalid character set.");
                for (char c = m.Value[0]; c <= m.Value[2]; ++c)
                    sb.Append(c);
            }
        }

        if (!invertSet) return sb.ToString();

        var A = new HashSet<char>(AllCharacters); // <---- change this to "ABC" and the error goes away
        var B = new HashSet<char>(sb.ToString());
        A.ExceptWith(B);
        return A;
    }

    static void Main(string[] args)
    {
    }
}

但我不知道为什么.当我打印字符时,

Console.WriteLine(string.Concat(AllChars));

它按预期打印每个字符.那么为什么HashSet认为它是空的呢？

Answer 1

我猜测ExpandCharacterSet正用于初始化另一个静态字段.无法保证两个静态字段的初始化顺序,因此可能会在初始化AllChars之前尝试初始化另一个字段.

尝试使用正确的初始化顺序将赋值移动到显式静态构造函数中.

例如,如果您当前的代码有:

public static IEnumerable<char> AllChars = Enumerable.Range(0, 256).Select(Convert.ToChar).Where(c => !char.IsControl(c));
public static IEnumerable<char> Expanded = ExpandCharacterSet(...);

改为:

public static IEnumerable<char> AllChars;
public static IEnumerable<char> Expanded;
static <ClassName> {
    AllChars = Enumerable.Range(0, 256).Select(Convert.ToChar).Where(c => !char.IsControl(c));
    Expanded = ExpandCharacterSet(...);
}