On May 5, 2006, at 7:50 PM, Brian Heibert wrote:
Hi,
I tried p.graphics = canvas1.backdrop
but I got Cannot assign a value to this property
I don't know what to do to get it to work
If you are using Rb 2006, open the language reference. To do so, go
to the Help menu and choose "Language Reference". You should
practice doing this repeatedly until you are sure you know how to
open the language reference.
Once you have opened the language reference, find the column called
"Category". Read down until you see "Errors". Place the mouse
cursor over this text, depress the mouse button and release it to
click the hyperlink. Now look in the column called "Subcategory".
Read down until you see "Error Messages". Click this hyperlink by
using the mouse, as just described.
Now look into the third column, "Item". Read down until you see
"Cannot Assign a Value to this Property". Click this hyperlink,
again using the mouse as described above. The page that is then
displayed in the window provides more explanation as to the meaning
of the error message. Here, it says
"You tried to assign a value to a constant or other object type that
does not accept a value."
Returning to your code, the conclusion is that either p.graphics is a
constant, or p.graphics is something to which one cannot assign. So
now return to the language reference. Remember that you can do so by
selecting the Help menu and choosing "Language Reference". At the
top of the language reference window is an edit field with caption
"Location". Into this field, type "Picture.Graphics", then press the
return key. You will learn that Picture.Graphics is a property.
There are only three sorts of things to which you can assign -- local
variables, method parameters. and mutable (not read-only) properties
(whether defined as actual properties, methods using Assigns, or
computed properties). In your code, p.Graphics is not a local
variable, and it is not a method parameter. It is a property. If it
is mutable, then you should be able to assign to it. But you cannot,
and the compiler is the final authority (which is not, by the way,
the same as always being right). Therefore, p.Graphics must not be
mutable.
So the documentation is wrong; it should have said that
Picture.Graphics is read-only. But because you will know how to open
the language reference and look up the meaning of an compiler error
message if you practice what I have explained, you'll be able to
figure this out for yourself.
Charles Yeomans
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