如何通过字符串插值获取字符串的第一个字符？

Question

static void BuildStrings(List<string> sentences)
{
    string name = "Tom";
    foreach (var sentence in sentences)
        Console.WriteLine(String.Format(sentence, name));
}
static void Main(string[] args)
{

    List<string> sentences = new List<string>();
    sentences.Add("Hallo {0}\n");
    sentences.Add("{0[0]} is the first Letter of {0}\n");

    BuildStrings(sentences);

    Console.ReadLine();
}

//Expected:
//Hallo Tom
//T is the first Letter of Tom

但是我得到了：

System.FormatException: 'Input string was not in a correct format.'

如何在不改变BuildStrings方法的情况下获取“Tom”的第一个字母？

Answer 1

您可以添加自定义格式并使用它代替 [0]。看看这个：Custom format - maximum number of characters

Answer 2

你真的需要做这样的事情：

static void BuildStrings(List<string> sentences)
{
    string name = "Tom";
    foreach (var sentence in sentences)
        Console.WriteLine(String.Format(sentence, name, name[0]));
}

static void Main(string[] args)
{
    List<string> sentences = new List<string>();
    sentences.Add("Hallo {0}\n");
    sentences.Add("{1} is the first Letter of {0}\n");

    BuildStrings(sentences);

    Console.ReadLine();
}

这给了我：

Hallo Tom

T is the first Letter of Tom

Answer 3

这是一个奇怪的要求，没有内置的东西支持这个，但如果你真的必须这样做，你可以编写一个方法来解析索引器，如下所示：

public static string StringFormatExtended(string s, params object[] args)
{
    string adjusted =
        Regex.Replace(s, @"\{(\d+)\[(\d+)\]\}", delegate (Match m)
        {
            int argIndex = int.Parse(m.Groups[1].Value);
            if (argIndex >= args.Length) throw new FormatException(/* Some message here */);

            string arg = args[argIndex].ToString();

            int charIndex = int.Parse(m.Groups[2].Value);
            if (charIndex >= arg.Length) throw new FormatException(/* Some message here */);

            return arg[charIndex].ToString();
        });

    return string.Format(adjusted, args);
}

用法：

static void BuildStrings(List<string> sentences, string name)
{
    foreach (var sentence in sentences)
        Console.WriteLine(StringFormatExtended(sentence, name));
}

static void Main(string[] args)
{
    string name = "Tom";

    List<string> sentences = new List<string>();
    sentences.Add("Hello {0}\n");
    sentences.Add("{0[0]} is the first Letter of {0}");
    sentences.Add("{0[2]} is the third Letter of {0}");

    BuildStrings(sentences, name);

    Console.ReadLine();
}

输出：

Hello Tom

T is the first Letter of Tom
m is the third Letter of Tom

如何通过字符串插值获取字符串的第一个字符？

How to get the first char of a string by string interpolation?

c#

string-interpolation