向Email插入<Table>时出现赋值错误，如何修复？

你遇到的这个编译错误，根源是字符串拼接时错误混用了赋值运算符。在C#里，+=是用来给变量追加内容的赋值操作，左边必须是一个可以被赋值的目标（比如变量、属性），但你把它直接写在了htmlBody = "..." + ... += "..."这种链式表达式里，编译器根本不知道该把值赋给谁，所以就报了"The left-hand side of an assignment must be a variable, property or indexer"这个错。

直接修复方案：用+串联所有字符串部分 #

把所有要拼接的HTML片段用+连接起来，不要混用=和+=，代码修改后如下：

MailMessage errorOutStanDev = new MailMessage();
errorOutStanDev.IsBodyHtml = true;
SmtpClient SmtpServerThree = new SmtpClient("smtp.xxxx.xxx");
errorOutStanDev.From = new System.Net.Mail.MailAddress("PerformanceUpdate@xxx.xxx");
errorOutStanDev.To.Add(new System.Net.Mail.MailAddress("xxxxxx@xxxx.xxx"));

string htmlBody = "<body>" + 
                  "<p><b> The following errors have occurred more often than others</b></p>" + 
                  avgErrorOutStDev +  // 拼接预先生成的表格开头部分
                  "<tr>" + 
                  "<td>" + sitename + "</td>" + 
                  "<td>" + avg + "</td>" + 
                  "</tr>" + 
                  "</table>" + 
                  "</body>";

更清晰的分步拼接方式（推荐） #

如果HTML内容比较长，分步用+=追加会更易读，也不容易出错：

MailMessage errorOutStanDev = new MailMessage();
errorOutStanDev.IsBodyHtml = true;
SmtpClient SmtpServerThree = new SmtpClient("smtp.xxxx.xxx");
errorOutStanDev.From = new System.Net.Mail.MailAddress("PerformanceUpdate@xxx.xxx");
errorOutStanDev.To.Add(new System.Net.Mail.MailAddress("xxxxxx@xxxx.xxx"));

string htmlBody = "<body>";
htmlBody += "<p><b> The following errors have occurred more often than others</b></p>";
htmlBody += avgErrorOutStDev; // 加入你之前生成的<table>起始部分
htmlBody += "<tr>";
htmlBody += "<td>" + sitename + "</td>";
htmlBody += "<td>" + avg + "</td>";
htmlBody += "</tr>";
htmlBody += "</table>";
htmlBody += "</body>";

额外注意点 #

要确保你的avgErrorOutStDev变量已经包含了<table>的起始标签（比如<table border="1">这类），否则最后加</table>会导致HTML结构不完整，邮件里的表格可能显示不正常。

内容的提问来源于stack exchange，提问作者Pens_Cup87