Re: 2 questions - C# Discussion Boards

11-Jul-04 23:37

Hi gurus,

I have 2 questions:

1) How can I attach a hwnd to a graphics?
When I do Graphics gr=new Graphics(m.HWnd), I have the message "System.Drawing.Graphics.Graphics(System.IntPtr) is inaccessible due to its protection level" Frown | :(

2) What is the equivalent of the GetSystemMetrics() API in C#?

Best regards.

Thanks.

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Mike Dimmick11-Jul-04 23:56

Mike Dimmick

11-Jul-04 23:56

Graphics gr = Graphics.FromHwnd( m.HWnd )

If the HWnd is coming from a Windows Forms control, use ctl.CreateGraphics() instead.

As for GetSystemMetrics, that depends on what values you want. The simplest thing to do is often to use P/Invoke.

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern int GetSystemMetrics( int metric );

Stability. What an interesting concept. -- Chris Maunder

bouli12-Jul-04 0:25

12-Jul-04 0:25

Hi,

For the graphics, I need to draw a button in the Non client area. I can get the handle of the non client area. I want to draw a button using the ControlPaint.DrawButton method wich need a Graphics as input.

ok for the dll import Smile | :)

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Heath Stewart12-Jul-04 3:14

12-Jul-04 3:14

When you receive the WM_NCPAINT notification message I mentioned to you before, the Message.WParam contains the HRGN (handle to a region) to be painted. You P/Invoke GetDCEx passing your window handle (HWND - the Handle property on all Controls), the HRGN mentioned above, and DCX_WINDOW|DCX_INTERSECTRGN ( =0x81 ). This is covered in the Platform SDK for the WM_NCPAINT message. IntelliSense will not help. You must read about the notification messages and APIs when working with this type of stuff.

The WM_NCPAINT documentation in the Platform SDK even includes a sample of calling GetDCEx.

The return value from GetDCEx is the HDC (handle to a device context) from which you create a Graphics object. Use the static Graphics.FromHdc method to get a Graphics instance for the non-client region.

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bouli12-Jul-04 3:36

12-Jul-04 3:36

Hi Heath,

I have done what you said, but there is nothing. Maybe I have wrongly declared the things...

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern IntPtr GetDCEx(IntPtr hWnd, IntPtr hrgnClip, long flags);

in the WndProc method, I trap the WN_NCPAINT

Graphics gr=Graphics.FromHdc(GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN));

ControlPaint.DrawButton(gr, 50, 1, 5, 5, ButtonState.Normal);

nothing happens. it should draw a button 5x5 at pos (50, 1) in the title bar, but I see nothing. Maybe I don't use it well... Confused | :confused:

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Heath Stewart12-Jul-04 4:02

12-Jul-04 4:02

A DWORD translates to an int (or uint), not a long. The value of DCX_WINDOW|DCX_INTERSECTRGN wouldn't, therefore, by read from the execution stack when calling GetDCEx - only 0 would be read and that wouldn't produce the results you want.

Also, don't bunch your method calls together so much. Try debugging that and you'll see that you can't check return values so it will be hard to know why a particular method failed.

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bouli12-Jul-04 4:10

12-Jul-04 4:10

Hi, I have changed the long to an int
but I still have the error "An unhandled exception of type 'System.OutOfMemoryException' occurred in system.drawing.dll

Additional information: Out of memory."
when I try to do:

Graphics  gr=Graphics.FromHdc(GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN));

I have now declared the functions like this:

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern IntPtr GetDCEx(IntPtr hWnd, IntPtr hrgnClip, int flags);

and the constants:

private const int	DCX_WINDOW=0x0001;
private const int	DCX_INTERSECTRGN=0x0080;

while these constants are declared as long in the WinUser.h from the Platform SDK.
Also, Platform SDK declares the GetDCEx function like this:

HDC GetDCEx(
  HWND hWnd,      // handle to window
  HRGN hrgnClip,  // handle to clipping region
  DWORD flags     // creation options
);

Maybe I'm wrong somewher in the C# translation... Confused | :confused:

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Heath Stewart12-Jul-04 4:18

12-Jul-04 4:18

A native long is 32 bits. A managed long is 64 bits (System.Int64). This is documented in the Platform SDK (and I know from many years of experience). If you want further proof, read Platform Invoke Data Types[^] in the .NET Framework SDK.

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bouli12-Jul-04 4:22

12-Jul-04 4:22

ok for the length between Win32 and .NET.
I have written the declarations as you advised me, but I get an error when I execute the line

Graphics	gr=Graphics.FromHdc(GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN));

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Heath Stewart12-Jul-04 4:24

12-Jul-04 4:24

It would help to know on which operation, which means - as I said before - you have to split them up.

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bouli12-Jul-04 4:44

12-Jul-04 4:44

I have declared the following:

private const int	DCX_WINDOW=0x0001;
private const int	DCX_INTERSECTRGN=0x0080;

[System.Runtime.InteropServices.DllImport("user32.dll")]
private static extern IntPtr GetDCEx(IntPtr hWnd, IntPtr hrgnClip, int flags);

I have trapped the WM_NCPAINT in the WndProc method of my System.Windows.Forms.Form derived window:

protected override void WndProc(ref Message m)
{
	switch (m.Msg)
	{
		case WM_NCPAINT:
		{
			Debug.WriteLine("WM_NCPAINT");

			Graphics		gr=Graphics.FromHdc(GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN));

			ControlPaint.DrawButton(gr, 60, 1, 5, 5, ButtonState.Normal);

			break;
		}
	}
	
	base.WndProc(ref m);
}

and I have the message "An unhandled exception of type 'System.OutOfMemoryException' occurred in system.drawing.dll

Additional information: Out of memory.", at the line

Graphics		gr=Graphics.FromHdc(GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN));

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Heath Stewart12-Jul-04 4:58

12-Jul-04 4:58

As I said before, split the line:

IntPtr hdc = GetDCEx(m.HWnd, m.WParam, DCX_WINDOW|DCX_INTERSECTRGN);
Graphics g = Graphics.FromHdc(hdc);

This is the very problem I mentioned earlier about combing such complex, compound statements together. Now which lines does the exception get thrown?

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bouli12-Jul-04 5:00

12-Jul-04 5:00

I still get the error Frown | :(

it seems the handle is null... When I call the GetDCEx I have 0x0 as result.

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Nick Parker12-Jul-04 5:09

Nick Parker

12-Jul-04 5:09

bouli wrote:
When I call the GetDCEx I have 0x0 as result

Check to make sure you have a valid handle to the window when you call GetDCEx.

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bouli12-Jul-04 5:14

12-Jul-04 5:14

I always get a null handle Confused | :confused:

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Heath Stewart12-Jul-04 5:13

12-Jul-04 5:13

And for the 3rd time, on which line do you get the error after splitting the two methods on two separate lines?

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bouli12-Jul-04 5:16

12-Jul-04 5:16

I get the error because the Handle returned by GetDCEx is null.

I can understand that the handle is null once (at initialization time), but it's always null Confused | :confused:

there is maybe something wrong in my declaration?

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Heath Stewart12-Jul-04 5:23

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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bouli12-Jul-04 22:22

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 | :)

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Heath Stewart13-Jul-04 3:01