C C格式字符串中最大的字符数是否可以像C PrimTF那样定义?

您想要的不是“本机”支持的C字符串格式,因为 String.ToString string对象的方法只返回字符串本身。

当你打电话

string.Format("{0:xxx}",someobject);

ToString(string format,IFormatProvider formatProvider) 方法被调用,其中“xxx”为 format 参数。

如果您真的需要这样做,您可以使用任何建议的解决方法,或者创建自己的类来实现IFormattable接口。

这不是关于如何使用string.format的答案,而是使用扩展方法缩短字符串的另一种方法。这样,即使没有string.format,也可以直接将最大长度添加到字符串中。

public static class ExtensionMethods
{
    /// <summary>
    ///  Shortens string to Max length
    /// </summary>
    /// <param name="input">String to shortent</param>
    /// <returns>shortened string</returns>
    public static string MaxLength(this string input, int length)
    {
        if (input == null) return null;
        return input.Substring(0, Math.Min(length, input.Length));
    }
}

string Test = "1234567890";
string.Format("Shortened String = {0}", Test.MaxLength(5));
string.Format("Shortened String = {0}", Test.MaxLength(50));

Output: 
Shortened String = 12345
Shortened String = 1234567890

我编写了一个自定义格式化程序,它实现了一个用于设置最大宽度的“L”格式说明符。当我们需要控制格式化输出的大小时(比如当目标是数据库列或Dynamics CRM字段时),这非常有用。

public class StringFormatEx : IFormatProvider, ICustomFormatter
{
    /// <summary>
    /// ICustomFormatter member
    /// </summary>
    public string Format(string format, object argument, IFormatProvider formatProvider)
    {
        #region func-y town
        Func<string, object, string> handleOtherFormats = (f, a) => 
        {
            var result = String.Empty;
            if (a is IFormattable) { result = ((IFormattable)a).ToString(f, CultureInfo.CurrentCulture); }
            else if (a != null) { result = a.ToString(); }
            return result;
        };
        #endregion

        //reality check.
        if (format == null || argument == null) { return argument as string; }

        //perform default formatting if arg is not a string.
        if (argument.GetType() != typeof(string)) { return handleOtherFormats(format, argument); }

        //get the format specifier.
        var specifier = format.Substring(0, 1).ToUpper(CultureInfo.InvariantCulture);

        //perform extended formatting based on format specifier.
        switch(specifier)
        {
            case "L": 
                return LengthFormatter(format, argument);
            default:
                return handleOtherFormats(format, argument);
        }
    }

    /// <summary>
    /// IFormatProvider member
    /// </summary>
    public object GetFormat(Type formatType)
    {
        if (formatType == typeof(ICustomFormatter))
            return this;
        else
            return null;
    }

    /// <summary>
    /// Custom length formatter.
    /// </summary>
    private string LengthFormatter(string format, object argument)
    {
        //specifier requires length number.
        if (format.Length == 1)
        {
            throw new FormatException(String.Format("The format of '{0}' is invalid; length is in the form of Ln where n is the maximum length of the resultant string.", format));
        }

        //get the length from the format string.
        int length = int.MaxValue;
        int.TryParse(format.Substring(1, format.Length - 1), out length);

        //returned the argument with length applied.
        return argument.ToString().Substring(0, length);
    }
}

用法是

var result = String.Format(
    new StringFormatEx(),
    "{0:L4} {1:L7}",
    "Stack",
    "Overflow");

Assert.AreEqual("Stac Overflo", result);

不能用参数而不是格式应用长度参数吗?

String.Format(“->{0}<-”,toFormat.PadRight(10));//->您好<-

为什么不使用Substr来限制字符串的长度呢?

String s = "abcdefghijklmnopqrstuvwxyz";
String.Format("Character limited: {0}", s.Substr(0, 10));

您可能希望在将来的应用程序中使用此帮助器,它在给定的最大长度上限制字符串,并且如果它比给定的限制短,则还需要垫片。

public static class StringHelper
{

    public static string ToString(this string input , int maxLength , char paddingChar = ' ', bool isPaddingLeft = true)
    {
        if (string.IsNullOrEmpty(input))
            return new string('-', maxLength);
        else if (input.Length > maxLength)
            return input.Substring(0, maxLength);
        else
        {
            int padAmount = maxLength - input.Length;
            if (isPaddingLeft)
                return input + new string(paddingChar, padAmount);
            else
                return new string(paddingChar, padAmount) + input;
        }
    }
}

你可以把它当作:

 string inputStrShort = "testString";
 string inputStrLong = "mylongteststringexample";
 Console.WriteLine(inputStrShort.ToString(20, '*', true)); // Output : testString**********
 Console.WriteLine(inputStrLong.ToString(20, '*', true));  // Output : mylongteststringexam