Re: Casting question

In a message dated 9/26/2005 5:12:44 PM Pacific Standard Time,
taylorrx@xxxxxxxxxxx writes:

Because  for a given set of input, the function in the real code returns an
array  with all B elements. Under other conditions, it may return C, D, M,
what  have you -- all of which derive from A.

But yes, a nice restating of  the whys of the solution.




I asked why you wanted to recast the array because of the foreach  loop:
    foreach (B bOb in  a)
"a" was defined as an  A[] array, but I was able to treat it like it was a
B[] array in my example  code.  True, if you want to index the a variables, they
have to be  explicitly cast as B objects to access the B functionality, so it
isn't really a  B[] array.

Your workaround really created a B[] array and cast it as an A[] array when
it was returned.  You could just as easily returned an object[] array  because
B[] isa A[] isa object[].  You can't create a new "object" object  and then
cast it as an A because all the background information of the A type of  object
is missing.  For the same reason, you can't create an A object  and cast it
as a B object.  You can create a new B or C object and  then cast it as an A or
"object" because all the background information of  the A or "object" type of
object is included.  You define the  actual type of the object when you
create an instance of the object.

Say you add a class C that's an exact copy of my B class.
A[] b = new B[5];-- >the b array is really a B class array.
b[0] = new C();-- > compiles because the object is an A object and  runs!?
Ahh, no, the debugger points to the next line of code, but this is  the line
that really fails with a runtime error because the underlying type of  array
object is B, not A or C.  (Declaring the array new A[5]  runs fine.  A C object
can be cast to A but it can't be cast to  B.)

B b = new A();--> won't even compile type safe checking catches  this error.

B[] b = new A[5];--> won't even compile type safe checking catches  this
error.


A a = new A();
C c = (C) a; -->compiles because it is possible for "a" to cast to type  C,
but generates a runtime error because this "a" instance won't support  it.  The
array definitions work exactly the same way.

Putting "a = new C();" between the previous two lines and it will not only
compile, but it will run.

Put "B b= (B) a;" after the prior three lines and it generates a runtime
error.

Are you beginning to see why you can't cast an A[] array to a B[] array,  but
you can cast a B[] array to an A[] variable and then cast that variable back
to a B[] array?

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