Detailed explanation of examples of common C# application functions

<%@ Import Namespace="System" %>
<%@ Import Namespace="System.Collections.Generic" %>
<Script runat="server">
  public int userId = 0;
  protected void Page_Load(object sender, EventArgs e)
  {
    userId =Convert.ToInt32(Request.QueryString["UserID"]);
    Response.Write(userId);
  }
</Script>
<%
  if (userId > 0){
    msg = "欢迎登录!";
  }
  else {
    msg = "未找到用户";
  }
%>
<%= this.msg %>

/// <summary>
/// 获取时间间隔（模拟微博发布文章的时间间隔）
/// </summary>
/// <param name="date"></param>
/// <returns></returns>
public string GetDateStr(DateTime date)
{
  if (date < DateTime.Now)
  {
    TimeSpan ts = DateTime.Now - date;
    if (ts.TotalHours < 1 && ts.TotalMinutes < 1)
    {
      return "1分钟前";
    }
    else if (ts.TotalHours < 1 && ts.TotalMinutes > 0)
    {
      return Convert.ToInt32(ts.TotalMinutes) + "分钟前";
    }
    else if (ts.TotalHours < 4)
    {
      return Convert.ToInt32(ts.TotalHours) + "小时前";
    }
    else if (DateTime.Now.Date == date.Date)
    {
      return date.ToString("HH:mm");
    }
    else
    {
      return date.ToString("yyyy-MM-dd");
    }
  }
  return date.ToString("yyyy-MM-dd");
}

/// <summary>
/// 遍历Url中的参数列表
/// </summary>
/// <returns>如:(?userId=43&userType=2)</returns>
public string GetUrlParam()
{
  string urlParam = "";
  if (Request.QueryString.Count > 0)
  {
    urlParam = "?";
    NameValueCollection keyVals = Request.QueryString;
    foreach (string key in keyVals.Keys)
    {
      urlParam += key + "=" + keyVals[key] + "&";
    }
    urlParam = urlParam.Substring(0, urlParam.LastIndexOf(&#39;&&#39;));
  }
  return urlParam;
}

using System.Text.RegularExpressions;
/// <summary>
/// 清除文本HTML码
/// </summary>
public string RemoveHtmlTag(string htmlStr)
{
  if (string.IsNullOrEmpty(htmlStr))
    return string.Empty;
  return Regex.Replace(htmlStr, @"<[^>]*>", "");
}

using System.Reflection;
/// <summary>
/// 反射 通过类名创建类实例
/// </summary>
public void ReflecTest()
{
  Object objClass = Assembly.GetExecutingAssembly().CreateInstance("MyStudy.BLL.BookInfoBLL"); //参数：类的完全限定名，无需类的后缀名
  if (objClass != null)
  {
    BookInfoBLL bll = (BookInfoBLL)objClass;
  }
}

/// <summary>
/// 货币
/// </summary>
/// <param name="obj"></param>
/// <returns></returns>
public static string ToMoney(object obj)
{
  return String.Format("{0:C}", obj);
}

//1.小数点位数
string str1 = String.Format("{0:F1}", 56789); //result: 56789.0
string str2 = String.Format("{0:F2}", 56789); //result: 56789.00
string str3 = String.Format("{0:N1}", 56789); //result: 56,789.0
string str4 = String.Format("{0:N2}", 56789); //result: 56,789.00
string str5 = String.Format("{0:N3}", 56789); //result: 56,789.000
string str6 = (56789 / 100.0).ToString("#.##"); //result: 567.89
string str7 = (56789 / 100).ToString("#.##"); //result: 567
//2.保留N位,四舍五入 .
decimal d= decimal.Round(decimal.Parse("0.55555"),2);
//3.保留N位四舍五入
Math.Round(0.55555, 2);

Dictionary<int, String> dic = new Dictionary<int, String>();
dic.Add(1,"张三");
dic.Add(2,"李四");
string name = "";
//错误写法，效率底
if (dic.ContainsKey(1))
{
  name = dic[1];
  Console.WriteLine(name);
}
//正确写法，效率提高一倍
if (dic.TryGetValue(1, out name))
{
  Console.WriteLine(name);
}