c# - 如果变量为空，为什么我没有得到空异常？

Question

这是代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Net;
using System.Text.RegularExpressions;
using System.IO;
using unfreez_wrapper;
using Shell32;


namespace DownloadImages
{
    public partial class Form1 : Form
    {
        string rainMapToRead;
        string UrlsPath;
        int counter;
        UnFreezWrapper uf;
        string localFilename;
        string stringForSatelliteMapUrls;
        string satelliteMapToRead;
        List<string> StartTags;
        List<string> LastTags;
        List<string> Maps;

        ExtractImages ei;

        public Form1()
        {
            InitializeComponent();




                using (WebClient client = new WebClient())
                {
                    client.DownloadFile("http://www.sat24.com/foreloop.aspx?type=1&continent=europa#",localFilename + "rainMap.html");
                    client.DownloadFile("http://www.sat24.com/en/eu?ir=true", localFilename + "satelliteMap.html");
                }

                rainMapToRead = File.ReadAllText(localFilename + "rainMap.html");
                satelliteMapToRead = File.ReadAllText(localFilename + "satelliteMap.html");

localFileName 位于目录路径之前。但现在我没有定义它，所以它是空的。但即使它为空，rainMapToRead 也不为空，并且能够找到并读取“rainMap.html”

我的意思是，如果变量 localFilename 为 null，则文件被下载到？ C: ？达:？如果它为空，默认位置是什么？

Answer 1

因为这个 (null + "hello") 在 C# 中是完全合法的表达式。

如果你看How to: Concatenate Multiple Strings (C# Programming Guide) ，你可以找到以下语句:

In string concatenation operations, the C# compiler treats a null string the same as an empty string, but it does not convert the value of the original null string.