从字符串的右侧获取 x 个字符

Visual Basic 具有 Left，Right 和 Mid 函数，它们返回字符串的 Left，Right 和 Middle 中的字符。这些方法在 C#中不存在，但可以用 Substring() 实现。它们可以作为扩展方法实现，如下所示：

   public static class StringExtensions
   {
      /// <summary>
      /// VB Left function
      /// </summary>
      /// <param name="stringparam"></param>
      /// <param name="numchars"></param>
      /// <returns>Left-most numchars characters</returns>
      public static string Left( this string stringparam, int numchars )
      {
         // Handle possible Null or numeric stringparam being passed
         stringparam += string.Empty;
    
         // Handle possible negative numchars being passed
         numchars = Math.Abs( numchars );
    
         // Validate numchars parameter        
         if (numchars > stringparam.Length)
            numchars = stringparam.Length;
    
         return stringparam.Substring( 0, numchars );
      }
    
      /// <summary>
      /// VB Right function
      /// </summary>
      /// <param name="stringparam"></param>
      /// <param name="numchars"></param>
      /// <returns>Right-most numchars characters</returns>
      public static string Right( this string stringparam, int numchars )
      {
         // Handle possible Null or numeric stringparam being passed
         stringparam += string.Empty;
    
         // Handle possible negative numchars being passed
         numchars = Math.Abs( numchars );
    
         // Validate numchars parameter        
         if (numchars > stringparam.Length)
            numchars = stringparam.Length;
    
         return stringparam.Substring( stringparam.Length - numchars );
      }
    
      /// <summary>
      /// VB Mid function - to end of string
      /// </summary>
      /// <param name="stringparam"></param>
      /// <param name="startIndex">VB-Style startindex, 1st char startindex = 1</param>
      /// <returns>Balance of string beginning at startindex character</returns>
      public static string Mid( this string stringparam, int startindex )
      {
         // Handle possible Null or numeric stringparam being passed
         stringparam += string.Empty;
    
         // Handle possible negative startindex being passed
         startindex = Math.Abs( startindex );
    
         // Validate numchars parameter        
         if (startindex > stringparam.Length)
            startindex = stringparam.Length;
         
         // C# strings are zero-based, convert passed startindex
         return stringparam.Substring( startindex - 1 );
      }
    
      /// <summary>
      /// VB Mid function - for number of characters
      /// </summary>
      /// <param name="stringparam"></param>
      /// <param name="startIndex">VB-Style startindex, 1st char startindex = 1</param>
      /// <param name="numchars">number of characters to return</param>
      /// <returns>Balance of string beginning at startindex character</returns>
      public static string Mid( this string stringparam, int startindex, int numchars)
      {
         // Handle possible Null or numeric stringparam being passed
         stringparam += string.Empty;
    
         // Handle possible negative startindex being passed
         startindex = Math.Abs( startindex );
    
         // Handle possible negative numchars being passed
         numchars = Math.Abs( numchars );
    
         // Validate numchars parameter        
         if (startindex > stringparam.Length)
            startindex = stringparam.Length;
    
         // C# strings are zero-based, convert passed startindex
         return stringparam.Substring( startindex - 1, numchars );

       }
    }

此扩展方法可以使用如下：

string myLongString = "Hello World!";
string myShortString = myLongString.Right(6);  // "World!"
string myLeftString = myLongString.Left(5);    // "Hello"
string myMidString1 = myLongString.Left(4);    // "lo World"
string myMidString2 = myLongString.Left(2,3);    // "ell"