MailMessage是否需要任何设置来初始化？

In my console program, I want to send email to someone by using System.Net.Mail.MailMessage. However, I cannot initialize it :(

namespace:

using System.Net;
using System.Net.Mail;

code:

class emailHelper
{
    string to = "jane@address.com";
    string from = "ben@address.com";
    MailMessage message = new MailMessage(from, to);// errors here
}

the error message is about :

A field initializer cannot reference the non-static field, method, or property 'programA.emailHelper.from'

A field initializer cannot reference the non-static field, method, or property 'programA.emailHelper.to'

I have no idea that do I miss something to set before using MailMessage.

Any idea?

The error message give you all the information.

A field initializer cannot reference the non-static field, method, or property

So if you want to make it work, you should make your string fields static. Which I doubt is what you want.

class emailHelper
{
    static string to = "jane@address.com";
    static string from = "ben@address.com";
    MailMessage message = new MailMessage(from, to);// errors here
}

Like I said this is not what you want from one helper method, to have static to and from address. Your code should look like this:

public class EmailHelper
{
    public string To {get; set;}
    public string From {get; set;}
    public MailMessage message {get; set;}

    public EmailHelper(string to, string from)
    {
         To = to;
         From = from;

         message = new MailMessage(from, to);
    }

}

In this case you have addresses which are coming out of the helper and you can create multiple MailMessage classes.

As the error message stated

A field initializer cannot reference the non-static field, method, or property

Which means you cant use a field to initialize another field. But you can make use of the constructor here, your class implementation will be like the following:

class emailHelper
{
    string to = "jane@address.com";
    string from = "ben@address.com";
    MailMessage message; // Declare here
    public emailHelper() // Constructor
    {
        message = new MailMessage(from, to);//initialize here
    }
}

Or use a read only property getter like this:

public MailMessage Message
{
    get { return new MailMessage(from, to); }
}

A field initializer cannot reference the nonstatic field, method, or property 'field' Instance fields cannot be used to initialize other instance fields outside a method. If you are trying to initialize a variable outside a method, consider performing the initialization inside the class constructor

Check the Documentation : Compiler Error CS0236

// CS0236.cs
public class MyClass
{
   public int i = 5;
   public int j = i;  // CS0236
   public int k;      // initialize in constructor

   MyClass()
   {
      k = i;
   }

   public static void Main()
   {
   }
}