直接给码：

using System;
using System.IO;
using Telerik.Windows.Zip;

namespace JIMMY
{
    public static class ZipHelper
    {
        /// <summary>
        /// 利用telerik的zip库压缩字符串
        /// </summary>
        /// <param name="str"></param>
        /// <returns></returns>
        public static string CompressString(string str)
        {
            string result = string.Empty;
            try
            {
                MemoryStream memoryStream = new MemoryStream();
                ZipOutputStream zipOutputStream = new ZipOutputStream(memoryStream, ZipCompression.Default);
                StreamWriter writer = new StreamWriter(zipOutputStream);
                writer.Write(str);
                writer.Flush();
                result = Convert.ToBase64String(memoryStream.ToArray());
            }
            catch {  }
            return result;
        }

        /// <summary>
        /// 利用telerik的zip库解压字符串
        /// </summary>
        /// <param name="compressedBase64String"></param>
        /// <returns></returns>
        public static string UnCompressString(string compressedBase64String) 
        {
            string result = string.Empty;
            try
            {
                MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(compressedBase64String));
                ZipInputStream input = new ZipInputStream(memoryStream);
                StreamReader reader = new StreamReader(input);
                result = reader.ReadToEnd();     
            }
            catch { }
            return result;
        }
    }
}

杨过友情提示：该方法只适用于较大字符串，如果字符串很短，压缩后的字符串长度有可能更大。