if (listBox1.Items.Count == 20)
                listBox1.Items.RemoveAt(0);
            listBox1.Items.Add("a");

------解决方案--------------------
http://msdn.microsoft.com/zh-cn/library/7977ey2c.aspx

看看这个Queue<T>类及下面的示例

------解决方案--------------------
队列不明白么？first in first out……不好好上学跑出来的罪过啊。

------解决方案--------------------

C# code

string yourfilepath="你要存储的文件路径";
            StreamReader reader = new StreamReader(yourfilepath);
            StreamWriter writer;
            int count = 0;
         //   string[] aa=new string[]{System.Environment.NewLine};
            count = reader.ReadToEnd().Split(new string[] { System.Environment.NewLine }, StringSplitOptions.RemoveEmptyEntries).Length;
            //或 while(reader .ReadLine())count++;
            reader.Close();
            if (count > 10000)
            {
                int index = yourfilepath.LastIndexOf("+");
                int n ;
                int.TryParse(index == -1 ? "0" : (yourfilepath.Substring(index == -1 ? 0 : index + 1)),out n);//序号
                int indextemp = yourfilepath.Substring(0, index == -1 ? 0 : index).LastIndexOf("+");
                string temp = yourfilepath.Substring(0, indextemp == -1 ? (yourfilepath.Length - 1) : indextemp);
                string newfilepath = (n == 0 ? string.Empty : temp) + DateTime.Today.ToLongDateString() + "+" + (n + 1).ToString();
                writer = new StreamWriter(newfilepath);
            }
            else writer = new StreamWriter(yourfilepath);
            writer.WriteLine("你要存储的信息"）;
            writer.Flush(); 
            writer.Close();