Disposing and GC

Sat, 29 Jan 2011 18:58:18 +0100

Okay, I know that no one really knows or understands the GC. I still want to touch on it briefly here.
Some people tend to go too far when trying to prevent or cleanup memory leaks. If you remember 2 simple rules, it will save you a lot of headache.
1. Dispose of it when your done with it. The Using statement is my favorite way to accomplish this.
2. Make sure any pointers back to it are set to Nothing.
Do this and the GC will do it's job and you wont have to over-engineer destroying everything.

An example would maybe be with a list:

Dim lstCustom As New List(Of MyCustomClass) 'or whatever you want to put in it.

For i As Integer = 0 to 10

lstCustom.Add(New MyCustomClass(i))

'...do some stuff then clear the list

lstCustom.Clear 'this is (imho) a mistake

lstCustom.Clear only takes the objects out of the list. The objects still exist though. Even though there arent any pointers to the objects, there's really no telling how long they will be floating around before the GC picks em up. The way I like to handle this is to destroy the objects first, then remove them. Like so:

For i As Integer = lstCustom.Count -1 to 0 Step -1

lstCustom(i).Dispose 'naturally this is only good on objects that implement IDisposable

lstCustom(i) = Nothing 'this would handle non-disposable objects

lstCustom.RemoveAt(i) 'you could do this or just wait til after the loop then call .Clear

Index Loops vs For Each

Sat, 29 Jan 2011 18:55:41 +0100

For Each loops are bad, mmmKay?
I avoid them for 2 main reasons:

1. Index loops are faster.
2. You cant modify a list inside a For Each loop.

I'll explain.
This will fail at the remove call:

For Each str As String In lstStr

'test for something

If str = "boosh"

lstStr.Remove(str)

End If

Loop

It fails because you have changed the list count and therefor the index.

This however, will work:

For i As Integer = lstStr.Count -1 To 0 Step -1

'test for something

If lstStr(i) = "boosh"

lstStr.RemoveAt(i)

End If

This one works because we start at the bottom of the list and work up. So if we remove something it only affects where we've already been.

my-blog

Disposing and GC

Index Loops vs For Each