Question

I have a simple web site built with asp.net. It typically only has 1 or 2 users at one time. My question is, is it ok to instantiate a class at the class level or should I be instantiating for each method. Here is an example. I have a class named Host with a name field and mac field. In my code behind for a specific page Is it ok to do this:

public partial class addhosts : Page
{
     private Host host = new Host();
     private HostDal dal = new HostDal();

     protected void myMethod()
     {
          host.Name = "myname"
          host.Mac = "mymac"
     }

     protected void btnSubmit_Click(object sender, EventArgs e)
     {
          dal.AddHost(host)
     }
}         

Answer 1

First, what you are referring to are more typically referred to as global versus local variables.

In the simple case that you have listed, it would be better to create the variable on the submit click. The reason is if a user loads the object, but never calls the submit click, then you have instantiated the host object in memory when there was no need.

However, as many have said, it should not really matter one way or the other here. But, again, this is a simplistic example. Global variables can be dangerous and are often avoided as they can be modified from anywhere in your class. If one method expects a certain value that is then overrode, this can cause difficult to debug issues in more complex examples

Here is a wikipedia article that reiterates my above point:

They are usually considered bad practice precisely because of their
non-locality: a global variable can potentially be modified from
anywhere (unless they reside in protected memory or are otherwise
rendered read-only), and any part of the program may depend on it

To get rid of the globals, you could do this (using object initializers)

protected void btnSubmit_Click(object sender, EventArgs e)
{
      var host = new Host
          {
              Name = "myname",
              Mac = "mymac"
          };
      dal.AddHost(host)
}