DataSet: passed by value, but behaves like by ref

[Christian Ahrenkiel]

Hi,

I've got a DataSet (ds) which was created by a DataAdapter (da) method:

ds.Fill(ds).

This DataSet is passed through various methods (by value). Each method
changes some fields in the DataSet (for converting data in another format).
As this DataSet is _not_ class-wide declarated, each method returns the
changed DataSet and so it is passed through all methods and back to its
initial call.

Example:

private InititalCall()
{
DataSet ds;
// Filling DataSet via DataAdapter...

this.Method1(ds);

/* Although the returned DataSet is _not_ used, the DataSet (ds)
* shows all changes I did in Method1 and Method2!
* Normally I would have suspected, I would need an instruction like
*
* ds = this.Method1(ds)
*
* to apply the changes to my initial DataSet.
* Didn't I passed it by value? Why does it behave like by ref?
* Where is my fault? Is there a special behaviour I do not know yet?
*/
}

private DataSet Method1(DataSet ds)
{
// Some changes...
ds = this.Method2(ds);
return ds;
}

private DataSet Method2(DataSet ds)
{
// Some changes...
return ds;
}

Maybe anyone else is wondering...

Christian

P.S. The code sequence is just an example. I tried to symplify it, but I
am sure, in my real code sequence the DataSet is not class wide
declarated and not passed by ref.

 

[Jon Skeet [C# MVP]]

<snip>

I believe you're getting confused by what "pass by reference" and "pass
by value" mean, and what value types and reference types themselves
are.

See http://www.pobox.com/~skeet/csharp/parameters.html for more
information.

 

[Morten Wennevik]

Hi Christian,

It's the reference to the dataset that is passed by value not the dataset
itself.

DataSet ds1 = new DataSet();

DoSomethingWith(ds1);
....

private void DoSomethingWith(DataSet blrg)
{
// blrg is a completely new reference pointing to the same DataSet as ds1

TweakData(blrg);

// since blrg points to the same dataset as ds1 the changes will be seen
when using ds1 too;

blrg = new DataSet();

// now however blrg points to something new

ClearEverything(blrg);

// but ds1 still points to the old dataset and is not affected
}

If it was by reference ds1 would point to the new DataSet.


Happy coding!
Morten Wennevik [C# MVP]

 

[Christian Ahrenkiel]

I believe you're getting confused by what "pass by reference" and "pass
by value" mean, and what value types and reference types themselves
are.

See http://www.pobox.com/~skeet/csharp/parameters.html for more
information.

Thanks for this link. I always thought a DataSet parameter would create
a copy of the whole DataSet and not a copy of its reference. I wasn't
aware of 'reference types'.

By the way... I know it is offtopic , but I'm curious now and maybe
someone knows: Is Java behaving the same with parameters?

Christian

 

[Jon Skeet [C# MVP]]

By the way... I know it is offtopic , but I'm curious now and maybe
someone knows: Is Java behaving the same with parameters?

Yes, Java is exactly the same, except that *all* parameters are passed
by value in Java - there are no "ref" or "out" modifiers.

 

[Christian Ahrenkiel]

Yes, Java is exactly the same, except that *all* parameters are passed
by value in Java - there are no "ref" or "out" modifiers.

Thanks for your help!
Christian