方法一：
/**//// <summary>
    /// 判斷字符是否英文半角字符或標(biāo)點(diǎn)
    /// </summary>
    /// <remarks>
    /// 32    空格
    /// 33-47    標(biāo)點(diǎn)
    /// 48-57    0~9
    /// 58-64    標(biāo)點(diǎn)
    /// 65-90    A~Z
    /// 91-96    標(biāo)點(diǎn)
    /// 97-122    a~z
    /// 123-126  標(biāo)點(diǎn)
    /// </remarks>
    public static bool IsBjChar(char c)
    {
      int i = (int)c;
      return i >= 32 && i <= 126;
    }

    /**//// <summary>
    /// 判斷字符是否全角字符或標(biāo)點(diǎn)
    /// </summary>
    /// <remarks>
    /// <para>全角字符 - 65248 = 半角字符</para>
    /// <para>全角空格例外</para>
    /// </remarks>
    public static bool IsQjChar(char c)
    {
      if (c == '/u3000') return true;

      int i = (int)c - 65248;
      if (i < 32) return false;
      return IsBjChar((char)i);
    }

    /**//// <summary>
    /// 將字符串中的全角字符轉(zhuǎn)換為半角
    /// </summary>
    public static string ToBj(string s)
    {
      if (s == null || s.Trim() == string.Empty) return s;

      StringBuilder sb = new StringBuilder(s.Length);
      for (int i = 0; i < s.Length; i++)
      {
        if (s[i] == '/u3000')
          sb.Append('/u0020');
        else if (IsQjChar(s[i]))
          sb.Append((char)((int)s[i] - 65248));
        else
          sb.Append(s[i]);
      }

      return sb.ToString();
    }

方法二：
添加 Microsoft Visual Basic.NET Runtime 引用。
Console.WriteLine(Microsoft.VisualBasic.Strings.StrConv("ａｂｃｄｅｆｇ", VbStrConv.Narrow, 0));
http://www.survivalescaperooms.com/ami/archive/2006/12/11/588435.html