如何在 C# 中创建与特定变量具有相同数据类型的新变量？

Question

I want to create a new variable in the same data type of my variable Message . How can I do that?

I tried the following code. but I got an error.

MessageDTO Message;
var Message2 = new Message.GetType();

Error:

Message is a variable but is used like a type.

Here is my code:

public string Send(MessageDTO message)
{
     MessageDTO Message2 = new MessageDTO();
     if (message is Email)
     {
         Message2= new Email();
     }else if(message is SMS)
     {
         Message2= new SMS();
     }
    // rest of code ...
 }

Where

 public class MessageDTO
 {
        public int Id { get; set; }
        public MessageType Type { get; set; }
        public string? Subject { get; set; }
        public string? TextBody { get; set; }
        public string? HTMLBody { get; set; }
        public DateTimeOffset SendDate { get; set; }
        public DateTime SendTime { get; set; }
 }

public class Email : MessageDTO
{
   public string From{ get; set; }
   public string To{ get; set; }
}



 public class SMS : MessageDTO
 {
   public string SenderLine{ get; set; }
   public string[] To{ get; set; }
 }

I want to make my code open for extension, but closed for modification (ie If I add other types of messages, the Send method remains unchanged.). The Send method becomes shorter like this:

public string Send(MessageDTO message)
{
     MessageDTO Message2 = new Activator.CreateInstance(Message.GetType());
     // rest of code ...
 }

Answer 1

Its not clear to me why you need to make a new object in Send() . Knowing that, maybe another solution would be better. But I'll just go with the assumption that it is necessary.

One approach is to have MessageDTO be responsible for creating a new object of the correct type:

public class MessageDTO
{
    public abstract MessageDTO CreateAnother();
}

public class Email : MessageDTO
{
   public override MessageDTO CreateAnother() => new Email();
}

...

public string Send(MessageDTO message)
{
   MessageDTO Message2 = message.CreateAnother();
}

The CreateAnother() function could also do something more sophisticated, depending on what you need. For instance it could clone the existing object, not just create a default new one. (In that case I'd probably name it differently!)