为什么 Char.IsDigit 对于无法解析为 int 的字符返回

问题描述

我经常使用字符.IsDigit 来检查 char 是否是一个数字，这在 LINQ 查询中特别方便以预先检查 int.Parse 如下:"123".All(Char.IsDigit).

但是有些字符是数字，但不能像 ۵ 那样解析为 int.

//真bool isDigit = Char.IsDigit('۵');var文化 = CultureInfo.GetCultures(CultureTypes.SpecificCultures);整数;//错误的bool isIntForAnyCulture = 文化.Any(c => int.TryParse('۵'.ToString(), NumberStyles.Any, c, out num));

这是为什么?我的 int.Parse-通过 Char.IsDigit 进行预检查是否不正确?

有 310 个字符是数字:

ListdigitList = Enumerable.Range(0, UInt16.MaxValue).Select(i => Convert.ToChar(i)).Where(c => Char.IsDigit(c)).ToList();

以下是 .NET 4 (ILSpy) 中 Char.IsDigit 的实现:

public static bool IsDigit(char c){如果 (char.IsLatin1(c)){返回 c >= '0' &&c <= '9';}返回 CharUnicodeInfo.GetUnicodeCategory(c) == UnicodeCategory.DecimalDigitNumber;}

那么为什么会有属于 DecimalDigitNumber-category("十进制数字字符，即 0 到 9 范围内的字符...")在任何文化中都不会被解析为 int 吗?

这是因为它正在检查 Unicode数字，十进制数字"类别中的所有数字，如下所列:

http://www.fileformat.info/info/unicode/类别/Nd/list.htm

这并不意味着它是当前语言环境中的有效数字字符.事实上，使用int.Parse()，你只能解析正常的英文数字，​​而不管区域设置如何.

例如，这不起作用:

int test = int.Parse("٣", CultureInfo.GetCultureInfo("ar"));

即使 ٣ 是有效的阿拉伯数字字符，并且ar"是阿拉伯语区域设置标识符.

Microsoft 文章 如何:解析 Unicode 数字" 指出那个:

.NET Framework 解析为十进制的唯一 Unicode 数字是 ASCII 数字 0 到 9，由代码值 U+0030 到 U+0039 指定..NET Framework 将所有其他 Unicode 数字解析为字符.

但是，请注意，您可以使用 char.GetNumericValue() 将 unicode 数字字符转换为双精度数字.

返回值是 double 而不是 int 的原因是这样的:

Console.WriteLine(char.GetNumericValue('¼'));//打印 0.25

您可以使用类似的方法将字符串中的所有数字字符转换为它们的 ASCII 等价物:

public string ConvertNumericChars(string input){StringBuilder 输出 = new StringBuilder();foreach(输入中的字符ch){如果 (char.IsDigit(ch)){双值 = char.GetNumericValue(ch);if ((value >= 0) && (value <= 9) && (value == (int)value)){output.Append((char)('0'+(int)value));继续;}}output.Append(ch);}返回 output.ToString();}

I often use Char.IsDigit to check if a char is a digit which is especially handy in LINQ queries to pre-check int.Parse as here: "123".All(Char.IsDigit).

But there are chars which are digits but which can't be parsed to int like ۵.

// true
bool isDigit = Char.IsDigit('۵'); 

var cultures = CultureInfo.GetCultures(CultureTypes.SpecificCultures);
int num;
// false
bool isIntForAnyCulture = cultures
    .Any(c => int.TryParse('۵'.ToString(), NumberStyles.Any, c, out num)); 

Why is that? Is my int.Parse-precheck via Char.IsDigit thus incorrect?

There are 310 chars which are digits:

List<char> digitList = Enumerable.Range(0, UInt16.MaxValue)
   .Select(i => Convert.ToChar(i))
   .Where(c => Char.IsDigit(c))
   .ToList(); 

Here's the implementation of Char.IsDigit in .NET 4 (ILSpy):

public static bool IsDigit(char c)
{
    if (char.IsLatin1(c))
    {
        return c >= '0' && c <= '9';
    }
    return CharUnicodeInfo.GetUnicodeCategory(c) == UnicodeCategory.DecimalDigitNumber;
}

So why are there chars that belong to the DecimalDigitNumber-category("Decimal digit character, that is, a character in the range 0 through 9...") which can't be parsed to an int in any culture?

It's because it is checking for all digits in the Unicode "Number, Decimal Digit" category, as listed here:

http://www.fileformat.info/info/unicode/category/Nd/list.htm

It doesn't mean that it is a valid numeric character in the current locale. In fact using int.Parse(), you can ONLY parse the normal English digits, regardless of the locale setting.

For example, this doesn't work:

int test = int.Parse("٣", CultureInfo.GetCultureInfo("ar"));

Even though ٣ is a valid Arabic digit character, and "ar" is the Arabic locale identifier.

The Microsoft article "How to: Parse Unicode Digits" states that:

The only Unicode digits that the .NET Framework parses as decimals are the ASCII digits 0 through 9, specified by the code values U+0030 through U+0039. The .NET Framework parses all other Unicode digits as characters.

However, note that you can use char.GetNumericValue() to convert a unicode numeric character to its numeric equivalent as a double.

The reason the return value is a double and not an int is because of things like this:

Console.WriteLine(char.GetNumericValue('¼')); // Prints 0.25

You could use something like this to convert all numeric characters in a string into their ASCII equivalent:

public string ConvertNumericChars(string input)
{
    StringBuilder output = new StringBuilder();

    foreach (char ch in input)
    {
        if (char.IsDigit(ch))
        {
            double value = char.GetNumericValue(ch);

            if ((value >= 0) && (value <= 9) && (value == (int)value))
            {
                output.Append((char)('0'+(int)value));
                continue;
            }
        }

        output.Append(ch);
    }

    return output.ToString();
}

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