这是罗马数字的to-and-from。不常用，但可能很方便。用法:

if ("IV".IsValidRomanNumeral())
{
   // Do useful stuff with the number 4.
}

Console.WriteLine("MMMDCCCLXXXVIII".ParseRomanNumeral());
Console.WriteLine(3888.ToRomanNumeralString());

源:

    public static class RomanNumeralExtensions
    {
        private const int NumberOfRomanNumeralMaps = 13;

        private static readonly Dictionary<string, int> romanNumerals =
            new Dictionary<string, int>(NumberOfRomanNumeralMaps)
            {
                { "M", 1000 }, 
                { "CM", 900 }, 
                { "D", 500 }, 
                { "CD", 400 }, 
                { "C", 100 }, 
                { "XC", 90 }, 
                { "L", 50 }, 
                { "XL", 40 }, 
                { "X", 10 }, 
                { "IX", 9 }, 
                { "V", 5 }, 
                { "IV", 4 }, 
                { "I", 1 }
            };

        private static readonly Regex validRomanNumeral = new Regex(
            "^(?i:(?=[MDCLXVI])((M{0,3})((C[DM])|(D?C{0,3}))"
            + "?((X[LC])|(L?XX{0,2})|L)?((I[VX])|(V?(II{0,2}))|V)?))$", 
            RegexOptions.Compiled);

        public static bool IsValidRomanNumeral(this string value)
        {
            return validRomanNumeral.IsMatch(value);
        }

        public static int ParseRomanNumeral(this string value)
        {
            if (value == null)
            {
                throw new ArgumentNullException("value");
            }

            value = value.ToUpperInvariant().Trim();

            var length = value.Length;

            if ((length == 0) || !value.IsValidRomanNumeral())
            {
                throw new ArgumentException("Empty or invalid Roman numeral string.", "value");
            }

            var total = 0;
            var i = length;

            while (i > 0)
            {
                var digit = romanNumerals[value[--i].ToString()];

                if (i > 0)
                {
                    var previousDigit = romanNumerals[value[i - 1].ToString()];

                    if (previousDigit < digit)
                    {
                        digit -= previousDigit;
                        i--;
                    }
                }

                total += digit;
            }

            return total;
        }

        public static string ToRomanNumeralString(this int value)
        {
            const int MinValue = 1;
            const int MaxValue = 3999;

            if ((value < MinValue) || (value > MaxValue))
            {
                throw new ArgumentOutOfRangeException("value", value, "Argument out of Roman numeral range.");
            }

            const int MaxRomanNumeralLength = 15;
            var sb = new StringBuilder(MaxRomanNumeralLength);

            foreach (var pair in romanNumerals)
            {
                while (value / pair.Value > 0)
                {
                    sb.Append(pair.Key);
                    value -= pair.Value;
                }
            }

            return sb.ToString();
        }
    }

日期时间扩展

例子:

DateTime firstDayOfMonth = DateTime.Now.First();
DateTime lastdayOfMonth = DateTime.Now.Last();
DateTime lastFridayInMonth = DateTime.Now.Last(DayOfWeek.Friday);
DateTime nextFriday = DateTime.Now.Next(DayOfWeek.Friday);
DateTime lunchTime = DateTime.Now.SetTime(11, 30);
DateTime noonOnFriday = DateTime.Now.Next(DayOfWeek.Friday).Noon();
DateTime secondMondayOfMonth = DateTime.Now.First(DayOfWeek.Monday).Next(DayOfWeek.Monday).Midnight();

而与MVC工作，有很多if语句，我只关心或真或假，打印null，或字符串。在另一种情况下，我想到了:

public static TResult WhenTrue<TResult>(this Boolean value, Func<TResult> expression)
{
    return value ? expression() : default(TResult);
}

public static TResult WhenTrue<TResult>(this Boolean value, TResult content)
{
    return value ? content : default(TResult);
}

public static TResult WhenFalse<TResult>(this Boolean value, Func<TResult> expression)
{
    return !value ? expression() : default(TResult);
}

public static TResult WhenFalse<TResult>(this Boolean value, TResult content)
{
    return !value ? content : default(TResult);
}

它允许我改变<%= (someBool) ?print y:字符串。将%>空为<%= someBool。WhenTrue("print y") %>。

我只在我的视图中使用它，在这里我混合了代码和HTML，在代码文件中编写“更长的”版本更清楚。

空格规范化非常有用，特别是在处理用户输入时:

namespace Extensions.String
{
    using System.Text.RegularExpressions;

    public static class Extensions
    {
        /// <summary>
        /// Normalizes whitespace in a string.
        /// Leading/Trailing whitespace is eliminated and
        /// all sequences of internal whitespace are reduced to
        /// a single SP (ASCII 0x20) character.
        /// </summary>
        /// <param name="s">The string whose whitespace is to be normalized</param>
        /// <returns>a normalized string</returns>
        public static string NormalizeWS( this string @this )
        {
            string src        = @this ?? "" ;
            string normalized = rxWS.Replace( src , m =>{
                  bool isLeadingTrailingWS = ( m.Index == 0 || m.Index+m.Length == src.Length ? true : false ) ;
                  string p                 = ( isLeadingTrailingWS ? "" : " " ) ;
                  return p ;
                }) ;

            return normalized ;

        }
        private static Regex rxWS = new Regex( @"\s+" ) ;
    }
}

有时需要有类的实例，不管是否有效，但不是null

public static T Safe<T>(this T obj) where T : new()
{
    if (obj == null)
    {
        obj = new T();
    }

    return obj;
}

用法如下:

MyClass myClass = Provider.GetSomeResult();
string temp = myClass.Safe().SomeValue;

而不是:

MyClass myClass = Provider.GetSomeResult();
string temp = "some default value";
if (myClass != null)
{
        temp = myClass.SomeValue;
}

如果是口是心非的话，我很抱歉，但我没有找到。

我发现自己一遍又一遍地这样做……

public static bool EqualsIgnoreCase(this string a, string b)
{
    return string.Equals(a, b, StringComparison.OrdinalIgnoreCase);
}

.．.然后是StartsWithIgnoreCase, EndsWithIgnoreCase和ContainsIgnoreCase。