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C# confusion

I'm coming from a 3 year C background with some basic knowlegde on the Win32
API. I just started learning C# a day ago and I have a couple questions that
I need cleared up that just have me very confused.

[quesitons]

1.) Whenever you declare a class - MyClass - is the class alive in memory
for the whole duration of the program? If so, then, since it was not created
via the "new" keyword (like an instance is), is it on the heap or stack?

2.) If you create instances of MyClass, you're really just creating copies
of MyClass, like you would create instances of a struct (non-pointer structs)
in C/C++, correct?

3.) When Myclass is not declared inside another class, or namespace for that
matter, why is it that you cannot make MyClass private? Is it because it
would make the class totally useless since you would not be able to create
any instances of it or use it via the MyClass type?

4.) Still pertaining to the above; if in the programs duration, you never
invoke MyClass or create any instances of it, is it still created in memory
or is it *only* created when you create an instance of it or try to access it
via the MyClass type? if it's the latter, would it make any difference if any
member in the class or the class itself was made static?

4.) static keyword; This one has been hitting me like a hammer on the head.
Ok, from what I can derive from my reading so far is that the concept of the
static keyword on members is like using global variables or passing pointers
around to functions in C/C++, Am I correct? Is that its main and/or sole
purpose?

[/questions]

Knowing C, I think C# isn't too difficult. It seemed pretty straight forward
until I came upon classes and static versus non-static members. Anyway, I
hope someone can enlighten me on the above questions; Thanks in advance.
Dec 12 '05 #1
4 1353
Jesika <Je****@discussions.microsoft.com> wrote:
I'm coming from a 3 year C background with some basic knowlegde on the Win32
API. I just started learning C# a day ago and I have a couple questions that
I need cleared up that just have me very confused.

[quesitons]

1.) Whenever you declare a class - MyClass - is the class alive in memory
for the whole duration of the program? If so, then, since it was not created
via the "new" keyword (like an instance is), is it on the heap or stack?
No instances are created automatically, but the type itself has data
associated with it on the heap (including static variables).
2.) If you create instances of MyClass, you're really just creating copies
of MyClass, like you would create instances of a struct (non-pointer structs)
in C/C++, correct?
You're creating instances of the type, not copies of the type itself.
So for each instance you effectively have a new set of the instance
variables, but there's only one set of the static variables however
many instances you have.
3.) When Myclass is not declared inside another class, or namespace for that
matter, why is it that you cannot make MyClass private? Is it because it
would make the class totally useless since you would not be able to create
any instances of it or use it via the MyClass type?
Exactly.
4.) Still pertaining to the above; if in the programs duration, you never
invoke MyClass or create any instances of it, is it still created in memory
or is it *only* created when you create an instance of it or try to access it
via the MyClass type? if it's the latter, would it make any difference if any
member in the class or the class itself was made static?
That depends on various things. For the most part, types are only
loaded when they're first used, but they may be loaded when they
*might* be used.
4.) static keyword; This one has been hitting me like a hammer on the head.
Ok, from what I can derive from my reading so far is that the concept of the
static keyword on members is like using global variables or passing pointers
around to functions in C/C++, Am I correct? Is that its main and/or sole
purpose?
Static members are associated with the type itself rather than with any
particular instance of the type.

It's really worth reading a book or a tutorial on this - object
orientation isn't a good thing to learn from scratch via newsgroups,
I'm afraid.

You might want to have a look at
http://www.jaggersoft.com/csharp_course/index.html
I haven't looked at it in detail, but its author is someone I very much
respect...
Knowing C, I think C# isn't too difficult. It seemed pretty straight forward
until I came upon classes and static versus non-static members. Anyway, I
hope someone can enlighten me on the above questions; Thanks in advance.


It sounds like you're really just up against object orientation more
than C# itself.

--
Jon Skeet - <sk***@pobox.com>
http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet
If replying to the group, please do not mail me too
Dec 12 '05 #2
"Jesika" <Je****@discussions.microsoft.com> a écrit dans le message de news:
D8**********************************@microsoft.com...

| 1.) Whenever you declare a class - MyClass - is the class alive in memory
| for the whole duration of the program? If so, then, since it was not
created
| via the "new" keyword (like an instance is), is it on the heap or stack?

In C#, only structs whose data is all public and other value types are
created on the stack everything else is created on the heap via the new
call. No call to new, no valid object, just a reference.

| 2.) If you create instances of MyClass, you're really just creating copies
| of MyClass, like you would create instances of a struct (non-pointer
structs)
| in C/C++, correct?

No, you are allocating an area of memory for the data segment for each
instance of your class, but you are using a common memory area allocated for
the code for the class.

| 3.) When Myclass is not declared inside another class, or namespace for
that
| matter, why is it that you cannot make MyClass private? Is it because it
| would make the class totally useless since you would not be able to create
| any instances of it or use it via the MyClass type?

Exactly.

| 4.) Still pertaining to the above; if in the programs duration, you never
| invoke MyClass or create any instances of it, is it still created in
memory
| or is it *only* created when you create an instance of it or try to access
it
| via the MyClass type? if it's the latter, would it make any difference if
any
| member in the class or the class itself was made static?

Instances are created in memory, classes don't consume memory unless static
data members are initialised upon first reference of the class.

| 4.) static keyword; This one has been hitting me like a hammer on the
head.
| Ok, from what I can derive from my reading so far is that the concept of
the
| static keyword on members is like using global variables or passing
pointers
| around to functions in C/C++, Am I correct? Is that its main and/or sole
| purpose?

A static property/method/field is accessible without having to create an
instance of the class. They can be used as "global" variables/methods, but
you have to pass all data required to the method that you are calling unless
you use the data stored in static fields of the class.

| Knowing C, I think C# isn't too difficult. It seemed pretty straight
forward
| until I came upon classes and static versus non-static members. Anyway, I
| hope someone can enlighten me on the above questions; Thanks in advance.

C# is very much like a mixture of C++ and Java, but you do have to be aware
of "faux amis" which don't work the same :-)

Joanna

--
Joanna Carter [TeamB]
Consultant Software Engineer
Dec 12 '05 #3

Jesika wrote:
I'm coming from a 3 year C background with some basic knowlegde on the Win32
API. I just started learning C# a day ago and I have a couple questions that
I need cleared up that just have me very confused.

[quesitons]

1.) Whenever you declare a class - MyClass - is the class alive in memory
for the whole duration of the program? If so, then, since it was not created
via the "new" keyword (like an instance is), is it on the heap or stack?
You have to distinguish here between three things. First, you are
essentially correct (with caveats following): if you never say "new
MyClass" then you will never create an instance of MyClass on the heap,
so any instance members you declare in MyClass will never exist. That
said, you could declare some things in MyClass as "static", meaning
that there is only one copy for MyClass as a whole, shared amongst all
instances. I believe that static members are stored in a separate area
of memory: not the stack and not the heap, since they are brought into
existence the first time anything in MyClass is referenced by your
program, and are never garbage collected. Ever. Third, remember that
classes consist of data and code, so the _code_ associated with MyClass
will in fact take up some memory, but again it is static in nature so
it lives neither on the stack nor on the heap.

The memory allocated for instance members of classes is always stored
in the heap. Local variables and parameters to methods are stored on
the stack.
2.) If you create instances of MyClass, you're really just creating copies
of MyClass, like you would create instances of a struct (non-pointer structs)
in C/C++, correct?
You're creating in the heap copies of the instance members you declared
for MyClass, yes. Note that the code for MyClass any any static members
are not copies over and over again on the heap. They reside in static
memory.
3.) When Myclass is not declared inside another class, or namespace for that
matter, why is it that you cannot make MyClass private? Is it because it
would make the class totally useless since you would not be able to create
any instances of it or use it via the MyClass type?
Exactly. The compiler won't let you declare something that you can't
use.
4.) Still pertaining to the above; if in the programs duration, you never
invoke MyClass or create any instances of it, is it still created in memory
or is it *only* created when you create an instance of it or try to access it
via the MyClass type? if it's the latter, would it make any difference if any
member in the class or the class itself was made static?
If you never reference anything about MyClass, I believe that the code
still occupies memory space. I don't know whether the IL (Intermediate
Language) is JITted (just-in-time compiled) or not. Anyone? At what
granulatiry does the JITter work? Does it always compile entire
assemblies? Or just entire classes? Or method-by-method as needed?

However, you are essentially correct: if you never make any reference
to MyClass, even its static members will never be initialized. If you
never say "new MyClass", then MyClass will never occupy any heap space.
4.) static keyword; This one has been hitting me like a hammer on the head.
Ok, from what I can derive from my reading so far is that the concept of the
static keyword on members is like using global variables or passing pointers
around to functions in C/C++, Am I correct? Is that its main and/or sole
purpose?


Semantically, "static" provides a way to associate a variable with the
class as a whole, rather than a separate copy of the variable for each
instance. A classic application of this is the Singleton design
pattern. C also has a "static" keyword which, although it doesn't
translate directly, had the same sort of memory behaviour: static
variables persisted across method calls, and so were stored neither on
the heap nor on the stack.

Static is sort of like having a global variable, but it is more
controlled and safer. For one thing, you can make it local to your
class, strictly for internal use, so it's "global" to the class, but
not global to your program. It's also necessary in certain
circumstances, for example creating a cache to speed up certain methods
in your class. You want the cache to be shared amongst all instances of
the class rather than having each instance have its own cache.

However, "static" is _not_ like passing pointers around in C / C++.
Passing pointers as arguments means putting the pointer on the stack.
Static members live in static memory, not stack memory, so they're very
different from pointer arguments.

Dec 12 '05 #4
"Joanna Carter [TeamB]" wrote:
"Jesika" <Je****@discussions.microsoft.com> a écrit dans le message de news:
D8**********************************@microsoft.com...

| 1.) Whenever you declare a class - MyClass - is the class alive in memory
| for the whole duration of the program? If so, then, since it was not
created
| via the "new" keyword (like an instance is), is it on the heap or stack?

In C#, only structs whose data is all public and other value types are
created on the stack everything else is created on the heap via the new
call. No call to new, no valid object, just a reference.


not true. it is not the 'new' keyword that decides what goes where.

read Jon's article for more information
http://www.yoda.arachsys.com/csharp/memory.html
Dec 12 '05 #5

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