If I have a method like this:
public void Show()
{
Form1 f = new Form1();
f.ShowDialog();
}
Do I still need to call dispose on the form even though it will go out of scope, which will be eligible for garbage collection.
From some testing, calling this Show() multiple times .. at some point it seems like the GC collects it since I can see the memory spiking then it goes down at some point in time.
From MSDN it seems to say you MUST call dispose when the form is not needed anymore.
In your specific example, no, it's unlikely that it would be particularly useful. Forms do not hold onto a significant amount of resources, so if it takes a little bit longer for some portion of it's code to get cleaned up it isn't going to cause a problem. If that form just happens to be holding onto a control that is used to, say, play a video, then maybe it's actually holding onto some significant number of resources, and if you actually do dispose of those resources in the dispose method then it's worth taking the time to call dispose. For 99% of your forms though, their Dispose method will be empty, and whether you call it or not is unlikely to have any (or any noticeable) effect on your program.
The reason that it's there is primarily to enable the ability to dispose of resources in those 1% of cases where it's important.
It's also worth noting that when a Form is closed its Dispose method is already being called. You would only ever need to add a using or explicit Dispose call if you want to dispose of a Forms resources before that form is closed. (That sounds like a generally bad idea to me). This is easy enough to test. Just create a project with two forms. Have the second form attach an event handler to the Disposing event and show a message box or something. Then when you create an instance of that form and show it (as a dialog or not) you'll see that when you close it the message box will pop up right away, even if you keep the 'Form' instance around and without you ever needing to add a using or Dispose call.