Call function from pointer - C# Discussion Boards

12-Jul-04 5:23

So the exception is being thrown on the second line? How hard is that question to answer?

You need to debug your code and find out this type of information. Is the Message.HWnd or Message.WParam NULL (IntPtr.Zero) before being passed to GetDCEx?

I never said splitting the lines would make it work - that doesn't matter when the compiler produces the IL. I said it would make things easier to debug. Put a break-point in there and step through your code to see what the inputs and outputs of the methods are. And always perform checks (like making sure that GetDCEx doesn't return IntPtr.Zero before passing it to Graphics.FromHdc) - never assume something just works (especially when dealing with interop since the callee is unmanaged by the CLR).

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Re: 2 questions

bouli12-Jul-04 22:22

bouli

12-Jul-04 22:22

hi,
ok, I'm going to dig deeper.
but it was easier to draw in the NC area with MFC Frown | :(

.NET is very different... I need to check my marks.
Thanks for the help Smile | :)

There is no spoon.

Re: 2 questions

Heath Stewart13-Jul-04 3:01

Call function from pointer

13-Jul-04 3:01

Yes, of course it's very different. .NET is referred to as a "managed environment" because the Common Language Runtime (CLR) manages memory, provides garbage collection (no explicit destruction of objects like in C/C++/MFC), and many other things.

The important thing to remember is that Windows Forms controls encapsulates the Windows APIs, so everything possible in MFC would be possible in .NET, but you have to P/Invoke native APIs (and don't P/Invoke those that are already covered in .NET APIs), define structs and consts, etc. You can't simply call native APIs like you could in MFC, unless you were to use a Managed C++ assembly in a mixed-mode compilation.

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SNTP Client

don7cry11-Jul-04 22:45

don7cry

11-Jul-04 22:45

how to get all TimeZone and send it to Time Server, recieved Time Server reply and add it to database
thanks.

Nho'c Ti`

Re: SNTP Client

Dave Kreskowiak12-Jul-04 1:04

Dave Kreskowiak

12-Jul-04 1:04

What??????????????????????????????

This doesn't make any sense whatsoever. What do you mean by "get all TimeZone" and "send it to Time Server"? What are you trying to do with this information? Are you trying to get a Time value from a server localized to your time zone?

And what's with the database?

RageInTheMachine9532
"...a pungent, ghastly, stinky piece of cheese!" -- The Roaming Gnome

Re: SNTP Client

Steven Campbell12-Jul-04 3:40

Steven Campbell

12-Jul-04 3:40

The RFC for SNTP is located at http://www.faqs.org/rfcs/rfc2030.html. If you do a google search for "sntp component" you will find some 3rd party components that do the dirty work for you.

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joeyespo11-Jul-04 22:45

joeyespo

11-Jul-04 22:45

Hi,

I've searched everywhere! Does anyone here know how to call a function from C# from an IntPtr? I know how to obtain the function pointer:

IntPtr ptr = (typeof(SOMECLASS).GetMethod("SomeMethodName").MethodHandle.GetFunctionPointer())

Now I just need a way to call it ..
The reason I need this is because in MSHTML, there is an interface called IHTMLElementRender, and the function 'DrawToDC' (useful for capturing HTML documents as an image) takes the argument HDC, but is implemented to take a reference variable of an unrelated structure. Therefore I need to call the function by the pointer, as in C++.

Thanks!

- Joe Esposito

Mike Dimmick12-Jul-04 0:37

Mike Dimmick

12-Jul-04 0:37

You've run into one of the limitations of IDL: it was originally for describing structures to be marshalled across processes, but some things we'd like to describe in IDL - handles to GDI and USER objects - are process-relative and have no meaning in another process. So the IDL contains a fake definition.

I've browsed Google Groups for _RemotableHandle. The recommended answer is apparently to use ILDASM to generate IL code for the interface, change the declaration to take an IntPtr, then recompile it. Not nice.

I'd actually recommend using Managed C++ to wrap the interface as declared in mshtml.h.

Stability. What an interesting concept. -- Chris Maunder

Heath Stewart12-Jul-04 3:55

12-Jul-04 3:55

Instead of wrapping this with an MC++ assembly as the first reply states, just interop the interface yourself. Such it's such a simple interface, it won't be hard:

[ComImport, Guid("3050f669-98b5-11cf-bb82-00aa00bdce0b"),
  InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IHTMLElementRender
{
  void DrawToDc(IntPtr hdc);
  void SetDocumentPrinter(
    [MarshalAs(UnmanagedType.BStr)] string printerName, IntPtr hdc);
}

You could then use Marshal.GetComInterfaceForObject to get a pointer to this interface. You then call Marshal.GetObjectForIUnknown and cast to the interface like so:

IntPtr ptr = Marshal.GetComInterfaceForObject(element,
  typeof(IHTMLElementRender)); // Interface type from above
IHTMLElementRender render = (IHTMLElementRender)Marshal.GetObjectForIUnknown(
  ptr);
render.DrawToDc(hdc);
// IUknown::Release will be called when 'render' is GC'd, but you must
// release the IntPtr from GetComInterfaceForObject.
Marshal.Release(ptr);

Please note that this is untested but I've used similar code with different interfaces before with success.

I hope this helps.

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joeyespo12-Jul-04 8:34

joeyespo

12-Jul-04 8:34

Thanks! That did exactly what I needed it to!
I did consider using this method, however all of my attempts failed and left me clueless.

It seems that this attribute really makes the difference:
InterfaceType(ComInterfaceType.InterfaceIsIUnknown)

Without it, the program simply crashes when you try to use it.

Also, I have been using type-casts where QueryInterface would be required. It also worked in this case:

IHTMLElementRender render = (IHTMLElementRender)element;
render.DrawToDc(hdc);

Thanks again for your reply Smile | :)

- Joe

Heath Stewart12-Jul-04 9:11

12-Jul-04 9:11

When you perform a type-cast, the CLR performs a QI (QueryInterface). In some cases, however, the compiler will complain if the object (if it's type is known) doesn't and never could implement that interface.

Glad it works for you.

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About error monitoring ?

fu011-Jul-04 21:37

11-Jul-04 21:37

windows provides many win32 functions for error detection on system level.
is there a possibility to detect error on application level?
for example: while running an application, there comes out a warning dialog which might means error. Can i detect it timely ?

Colin Angus Mackay11-Jul-04 22:39

Colin Angus Mackay

11-Jul-04 22:39

fu0 wrote:
there comes out a warning dialog which might means error. Can i detect it timely ?

Confused | :confused:

Then don't write the code that displays the dialog.

I don't think this is the answer you expect, can you be more explicit in your question.

"If a man empties his purse into his head, no man can take it away from him, for an investment in knowledge pays the best interest." -- Joseph E. O'Donnell

The Second EuroCPian Event will be in Brussels on the 4th of September

Can't manage to P/Invoke that Win32 API in .NET? Why not do interop the wiki way!

exhaulted12-Jul-04 2:57

12-Jul-04 2:57

I don't quite know what your asking for but try looking up 2 fatures of .Net.
Exception Handling / Throwing
Trace Switches and the debug class.

Can't really be more helpfull without knowing more about your problem.
Try searching on MSDN for the above subjects and see if that helps.

Kev Pearman MCP

fu012-Jul-04 15:35

12-Jul-04 15:35

my idea is this:
in a sample, an application(not my own) displays a messagebox which might means some error happened.
then my own application could know something happened by this event(messagebox's displaying). however, my application dont want to know any details about the error, it only write into a log like this: when, which application, something happened. And my application dont want to interfere with the application. it works like a shell.
could i realize it ?

exhaulted12-Jul-04 21:48

12-Jul-04 21:48

So you want your application to run and if another application displays a MessageBox you want your application to do something, but you don't want your application to be tied into the other application at all?

If that is the case then i'm afraid i can't help you, i wouldn't even know where to start. If thats not the case then please say so.

Kev Pearman MCP

fu013-Jul-04 15:51

13-Jul-04 15:51

in my original idea, i dont want my application to be tied into other applcation.i thought it would interfere with other appliction's nomarl process.
from these words:but you don't want your application to be tied into the other application at all,i guess :
if my application is tied into other application,will it have the ablitiy to do the work ?
could you make a suggestion about your idea ?

exhaulted13-Jul-04 22:43

13-Jul-04 22:43

Is the "Other Application" something that your developing? If it is you could tie the two together somehow, sending a message to your "Listening" application to prompt it to do something?

Without knowing the entire problem it's difficult to suggest anything specific.

Hope you are able to solve the problem.

Kev Pearman MCP

fu014-Jul-04 2:44

14-Jul-04 2:44

i think the real problem is: "The other applciation" is not my developing . So i want to find a way how to monitor other application 's status to comfirm whethere there occurs an error.
in my opinion, if i can recorgnize a messagebox's displaying i can guess there might be an error occured.

exhaulted14-Jul-04 4:34

DataView binding with Dataset

14-Jul-04 4:34

I don't know if it would even be possible, if the other application doesn't do anything more than display a MessageBox then i wouldn't know how to do it.

Maybe someone who knows a little more about how windows works will be able to help you but this is a bit above me i'm afraid.

Hope you resolve your problem.

Kev Pearman MCP

orpus11-Jul-04 20:16

orpus

11-Jul-04 20:16

Hi..
In my form, I have three combo boxes, one Add button one Save button
and a listview.

Step 1:The comboboxes is being populated by a dataset.
Step 2: On clicking the add button, the listview should get populated with the contents of the combobox.text.
I can repeat Step2 a number of times, each time selecting a different value in the comboBox and adding in the ListView
Step 3: on clicking the Save button, all the data in the listview should be saved in a database.

How will I accomplish this the task? How do i save the Columns and fields of the ListView into a dataset

plz help with code or explanation

Thanks

Breath dot net

Re: DataView binding with Dataset

Heath Stewart12-Jul-04 4:11