TimeSpan ts1 = TimeSpan.Parse("06:00:00");
TimeSpan ts2 = TimeSpan.Parse("08:00:00");
TimeSpan ts = TimeSpan.Parse("07:00:00");
if(ts>=ts1 && ts<=ts2)
{
    ...
}

------解决方案--------------------
            string str1 = "06:00:00";
           string str2 = "08:00:00";
           string strInput = "07:12:22";
           DateTime dt1 = GetDateTime(str1);
           DateTime dt2 = GetDateTime(str2);
           DateTime dt3 = GetDateTime(strInput);
           if (dt3 > dt2)
           {
               Console.WriteLine(dt3 + "大于" + dt2);
           }
           else
           {
               Console.WriteLine(dt3 + "小于等于" + dt2);
           }
           if (dt3 > dt1)
           {
               Console.WriteLine(dt3 + "大于" + dt1);
           }
           else  
           {  
               Console.WriteLine(dt3 + "小于等于" + dt1);
           }
           Console.ReadKey();
       }

       private static DateTime GetDateTime(string str)
       {
           if(str==null)return default(DateTime);
           return DateTime.Parse(str);
       }