c#基础功能

using System.Net;

internal class Program
{
    private static void Main(string[] args)
    {         
    // toint32 会进行四舍五入
    //math.floor 向下取整，不进行四舍五入，

          var str = "38.124584999999999";
　　      var dec = Convert.ToDecimal(str);
          var t222 = Convert.ToInt32(Math.Floor(dec));

//组成字符串
        var m1 = string.Concat(1, 2,3,4,121);
        //判断是否有空格的字符串
        string.IsNullOrWhiteSpace(" ");
        string i1 ="abc";string i2 = "ABC";
        //比较结果0是相等
        int r1 = string.Compare(i1, i2,ignoreCase:true);
        //百分比
        var a = (decimal)5;
        var b = 3;
        var c = (a / b).ToString("p");
        //时间戳转时间
        var t1 = StampToDateTime("1686037513000");
        //url转成base64
        var result = GetUrlToBase64(@"http://59.249.230.64:80/group1/M00/9B/66/O_nmQGR-5T2AGYEeAAKraJLaWBc10.jpeg");
        //发布环境才走
        #if !DEBUG
          var ttt = "ssss";
        #endif
        //分组去每组最大的积分
        var customerList = new List<Customer>{
              new Customer {Name = "a", Score =30},
              new Customer {Name = "a", Score =35},
              new Customer {Name = "b", Score =20},
              new Customer {Name = "c", Score =50},
              new Customer {Name = "b", Score =60},
              new Customer {Name = "a", Score =80},
          };
        List<Customer> customers = customerList .GroupBy(t => t.Name).SelectMany(a => a.Where(b => b.Score == a.Max(c => c.Score))).ToList();
        foreach (var data in customers)
        {
            Console.WriteLine(data.Name + "," + data.Score);
        }
        Console.ReadLine();
    }
    /// <summary>
    /// 获取网络图片转换为base64
    /// </summary>
    /// <param name="url">网络图片路径   例：http://123.123.123.123/a.jpg</param>
    /// <returns></returns>
    public static (bool, string) GetUrlToBase64(string url)
    {
        try
        {
            WebClient mywebclient = new WebClient();
            byte[] Bytes = mywebclient.DownloadData(url);
            string Base64string = Convert.ToBase64String(Bytes);
            return (true, Base64string);
        }
        catch (Exception e)
        {
            return (false, null);
        }
    }
    /// <summary>
    /// 时间戳转时间带毫秒
    /// </summary>
    /// <param name="timeStamp"></param>
    /// <returns></returns>
    public static DateTime StampToDateTime(string timeStamp)
    {
        DateTime dtStart = TimeZone.CurrentTimeZone.ToLocalTime(new DateTime(1970, 1, 1));
        long lTime = long.Parse(timeStamp + "0000");
        TimeSpan toNow = new TimeSpan(lTime);
        return dtStart.Add(toNow);
    }

}
public class Customer
{
    public string Name;
    public int Score;
}

 

//对比
string a = "开始";
string b = "开始";
if (string.Compare(a, b, true) == 0)
{ }
//值对比
if (string.Equals(a, b)) { }

1、使用str.Length == 0做空串比较

   string str="";    if (str.Length == 0){      Console.WriteLine("为空！");}

或者使用if(str==String.Empty)其次使用if(str=="")。

2、使用string.Empty给一个空字符串变量赋初始值

string.Empty不占用内存空间，而str==""的方式是具体的实现，是占用空间的。

string str=string.Empty;

3、避免不必要的字符串ToUpper、ToLower类操作而使用String.Compare

String.Compare可实现忽略字符串大小写，而ToUpper、ToLower这类方法均会重新生成字符串对。

    string str1="公众号：dotNet开发跳槽";    string str2="公众号：DOTNet开发跳槽";    //推荐使用    if (string.Compare(str1,str2,true)==0)      Console.WriteLine("相等！");    //不推荐    if (str1.ToUpper()==str2.ToUpper())       Console.WriteLine("相等！");

4、正确使用StringBuilder进行字符串拼接操作

    如果要构造一个较长的字符串，尤其是拼接超过10次时（经验值），就应使用StringBuilder做字符串拼接操作。

 Stopwatch timer = new Stopwatch(); string s = null; timer.Start(); for (int i = 0; i < 66666; i++) {     s += (i + 1) + "、Dotnet开发跳槽 "; } timer.Stop(); decimal micro = timer.Elapsed.Ticks / 10m; Console.WriteLine($"拼串耗时：{micro}"); StringBuilder sb = new StringBuilder(); timer.Restart(); for (int i = 0; i < 66666; i++) {     sb.Append(i + 1);     sb.Append("、Dotnet开发跳槽 "); } string t = sb.ToString(); timer.Stop(); micro = timer.Elapsed.Ticks / 10m; Console.WriteLine($"StringBuilder耗时：{micro}");

另外避免滥用StringBuilder类似str1+str2+str3+str4的字符串拼接操作会被编译为 String.Concat(str1,str2,str3, str4)，效率反而高于StringBuilder。

 

5、创建StringBuilder应指定初始大小

    StringBuilder默认的初始大小为16，一旦超过即需要Resize一次并增加GC压力。建议根据经验值为其指定初始大小，比如最大有128个中文字符，那么初始为256。

StringBuilder sb = new StringBuilder(256);