how to replace some content in mail template before send in C#?

Question

I have create a mail template ,but some value will be dynamic from cs file , I try to using ##username in my mail template , and using mailbody.Replace("##UserName", "jack"); to replace the username value in mail template ，but it's still show ##UserName in my mail, please how to replace some content in mail template before send in C# ?

my mail template : mailtemplate.html :

    <div>
        <table>
            <tr>
                <td> ##UserName<span style='font-size:24px'><span></td>
            </tr>
           
        </table>
    </div>
</div>

my sendmail.aspx.cs :

 System.Net.Mail.MailMessage mm = new System.Net.Mail.MailMessage();
 string mailbody = string.Empty;
 using (StreamReader reader = new StreamReader(HttpContext.Current.Server.MapPath("mailtemplate.html")))
 {
      mailbody = reader.ReadToEnd();
  }
mailbody.Replace("##UserName", "jack");
mm.Body = mailbody;

Answer 1

You are just using your "original" mailbody.

You need to do sth like this

mailbody = mailbody.Replace("##UserName", "jack");

Then mailbody will have the replaced text....if not you are just replacing, but throwing away the result...

In C# strings are inmutable, so you get a NEW one, with desired replacement

So this

mailbody = mailbody.Replace("##UserName", "jack");
mm.Body = mailbody;

or this

var replacedBody = mailbody.Replace("##UserName", "jack");
mm.Body = replacedBody;

or this

mm.Body = mailbody.Replace("##UserName", "jack");;

should work. You just need to work with the returned value of Replace not with the original string (what you are doing)

Take a look here at the docs. Replace returns a string, which is the result string you are looking for