Interfaces and ref parameters

[Guest]

Hi,

What are the rules regarding ref parameters and interface types?

The reason I ask is that I found myself writing code like this several times:

if (something != null)
{
something.Dispose();
something = null;
}

So I had the "clever" idea of writing a routine like this:

public static void MyDispose(IDisposable disposable)
{
if (disposable != null)
{
disposable.Dispose();
disposable = null;
}
}

I can pass variables of type IDisposable to this routine, but I cannot pass class types that implement IDisposable. E.g. I wanted to do something like:

Bitmap MyBitmap = ....
.....
MyDispose(MyBitmap);

It seems to me that the compiler should be able to figure out what I want (i.e. pass my bitmap to the method, just like it does if its not a ref parameter) then set MyBitmap to null. The compiler disagrees with this plan, and gives an error.

Is there any way to ahieve what I want?

JR

 

[Jon Skeet [C# MVP]]

What are the rules regarding ref parameters and interface types?

The reason I ask is that I found myself writing code like this several times:

if (something != null)
{
something.Dispose();
something = null;
}

So I had the "clever" idea of writing a routine like this:

public static void MyDispose(IDisposable disposable)
{
if (disposable != null)
{
disposable.Dispose();
disposable = null;

As it stands, this line is pointless.

}
}

I can pass variables of type IDisposable to this routine, but I cannot pass class
types that implement IDisposable. E.g. I wanted to do something like:

Bitmap MyBitmap = ....
...
MyDispose(MyBitmap);

You certainly should be able to do that.

It seems to me that the compiler should be able to figure out what I
want (i.e. pass my bitmap to the method, just like it does if its not
a ref parameter) then set MyBitmap to null. The compiler disagrees
with this plan, and gives an error.

Hang on - you *don't* have a ref parameter at the moment...

Is there any way to ahieve what I want?

Assuming that you meant:

public static void MyDispose(ref IDisposable disposable)
{
if (disposable != null)
{
disposable.Dispose();
disposable = null;
}
}

there's a very good reason why you can't do what you want - the
compiler doesn't know that your method doesn't look like this:

public static void MyDispose(ref IDisposable disposable)
{
disposable = new MemoryStream();
}

Your MyBitmap variable would then contain a reference to a
MemoryStream, which is definitely not a good thing!

You *could* adopt a pattern of:

MyBitmap = MyDispose(MyBitmap);

and make MyDispose always return null. However, to be honest, I would
personally just stick with the "using" construct for almost all uses of
IDisposable. I rarely find I need anything else, and it makes it *so*
much easier to get right.

 

[Guest]

----- Jon Skeet [C# MVP] wrote: ----

public static void MyDispose(ref IDisposable disposable

if (disposable != null

disposable.Dispose()
disposable = null;



there's a very good reason why you can't do what you want - the
compiler doesn't know that your method doesn't look like this

public static void MyDispose(ref IDisposable disposable

disposable = new MemoryStream()


True! Thanks for the explaination

JR