Re: Assigning "this"
- From: "Fred Mellender" <nospamPlease_fredm@xxxxxxxxxxxxxxx>
- Date: Tue, 16 Oct 2007 19:15:16 GMT
Perhaps the OP is searching for the Smalltalk "become" operation, wherein
A.become(B) would swap all references to A with a reference to B, and
vice-versa. There is no such function in C# (or in the CLR, so far as I
know), although it would not be too hard to implement. See
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/104551.
The semantics might be problematic in C# unless A and B were of the same
class. Owing to Smalltalk's runtime type checking this is not a problem
(but of limited utility, admittedly, when the objects are of different
classes).
"Peter Duniho" <NpOeStPeAdM@xxxxxxxxxxxxxxxx> wrote in message
news:13ha1olot2hkb3b@xxxxxxxxxxxxxxxxxxxxx
Trecius wrote:
Hello, Newsgroupians:
I have a large class with a lot of member variables. I also have a
function in the class that I would like to change ALL Of the member
variables. I am trying to assign "this" to the result, but I always get
the error message, "Cannot assign to '<this>' because it is read-only."
I've been searching on the Internet, and I notice some C# code is
violating this rule. Perhaps their code is wrong.
Or maybe you're misreading the code. Can you provide links to the code
that you believe is wrong?
For simplicity, suppose I have a Point class that two members: x and y.
An unfortunate choice of example, IMHO. .NET already has a Point type,
and it's a struct. Structs are value types and work in a very different
way from classes. This could be confusing in this context.
That said, let's go with your example nevertheless, with the assumption
that we will ignore .NET's Point struct and assume we have a whole new
Point type that is in fact a class...
Suppose I have two functions called DoubleSizes() and Double().
DoubleSizes() is defined as...
public Point DoubleSizes()
{
return new Point(this.x * 2, this.y * 2);
}
Now, for the Double() function, I'd like to have the following...
public Point Double()
{
this = this.DoubleSizes();
return this;
}
First, note that when assigning something to "this", you aren't copying
values from one instance to another. You would be replacing the "this"
reference altogether. Hopefully you can see why, in a class, it doesn't
make sense to try to replace the instance reference from within the
instance itself.
Beyond that, what is the point of the Point.Double() method? In what
situation do you want to modify every member variable of a class while
returning a new instance of the class? Why would you not simply create a
new instance and use that in place of the old one? You can use something
like MemberwiseClone() to create an exact value-for-value duplicate of a
class, if you're looking for getting two different instances (for example,
you want to modify one without changing the other).
But this doesn't work. Instead, I need to set the result to a temporary
variable and iterate through my variables... EXA:
Point temp = this.DoubleSizes();
this.x = temp.x;
this.y = temp.y;
return this;
Yes, if you want the original instance to be the same instance after the
operation, you will have to modify each and every member variable
individually. But IMHO this is suggestive of a more basic design issue.
If your design is such that you feel you want to replace the class from
within based on an operation that returns a different, new instance, I
would suggest there's a problem with the design itself.
Again, this is a small example. In my case, I have about twenty
variables that I'd like to reassign. Is it just possible to reassign
"this?" Again, I've seen some C# code changing the value of this, but
are they in violation of a compile rule?
Again, if you could post links to the examples of code you say does this,
that would be helpful. I think it's likely the code isn't doing what you
think it's doing.
In addition, it would be helpful if you could try to clarify what it is
exactly you're trying to do. The code you're posting doesn't fit exactly
with the words you're writing. In particular, you seem to just want to
copy values from one instance to another, but the code you've posted isn't
doing that (even if the compiler let you). It's replacing the instance
reference altogether.
Do you want to replace the reference? If so, how would you expect that to
work? If not, then why are you trying to write code that would (if it
compiled) replace the reference? Are you confused about the difference
between a struct and a class?
Pete
.
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