list= new List<ChouJiangItemInfo>
            {
                new ChouJiangItemInfo{ItemId=1,ItemName="起凡至尊礼包",Percentage=25},
                new ChouJiangItemInfo{ItemId=1,ItemName="随机武将灵技能",Percentage=25},
                new ChouJiangItemInfo{ItemId=1,ItemName="貂蝉抱枕",Percentage=35},
                new ChouJiangItemInfo{ItemId=1,ItemName="群雄绝版T恤",Percentage=15}                
            };



var item=(from o in Enumerable.Range(1, 500)
                          let item = list[random.Next(list.Count())]
                          let seed = random.Next(1, 101)
                          where seed < item.Percentage
                          select item).First();

int seed=random.Next(101);
for(var i=0;i<lst.count;i++)
{
     if(seed<=15&&seed>0)   //15-0=15
    {
        return lst[i];
    }  
    else if(seed<=50&&seed>15) //40-15=25
    {
           return lst[i];
    }
   else if(seed<=75&&seed>50)  //65-40=25
   {
        return lst[i];
   }
   else if(seed<=100&&seed>75)//100-75=25
   {
       return lst[i];  
   }  
}