CreateDC Fails when not fed 'properly'
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Dale Atki
#1 / 5
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 CreateDC Fails when not fed 'properly' I am exploring GDI with the eventual goal of printing, and I have run in to
an 'odd' problem. Basically the CreateDC call in failing to return anything
other than 0, unless I feed it with the DeviceName and DriverName from the
default printer.
For example
CreateDC(Printer.DriverName, Printer.DeviceName, vbNullString, vbNull)
returns a valid device context, but
CreateDC(Printers(0).DriverName, Printers(0).DeviceName, vbNullString,
vbNull)
always returns 0
further
DevName = Printer.DeviceName
DrvName = Printer.DriverName
CreateDC(DrvName, DevName, vbNullString, vbNull)
also always returns 0
Ideas?
Dale
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| Sat, 01 Jan 2005 23:09:30 GMT |
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Dale Atki
#2 / 5
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CreateDC Fails when not fed 'properly' I seem to have sorted something out, that seems to work, basically by
changing the declaration of CreateDC to
Declare Function CreateDC Lib "gdi32.dll" Alias "CreateDCA" (ByVal
lpszDriver As String, ByVal lpszDevice As String, ByVal lpszOutput As Long,
lpInitData As Any) As Long
And passing 0s to lpszOutput and Clng(0) by Val to lpInitData. Anyone know
why this is required?
http://216.26.168.92/vbapi/ref/c/createdc.html
Dale
Quote: > I am exploring GDI with the eventual goal of printing, and I have run in to > an 'odd' problem. Basically the CreateDC call in failing to return anything > other than 0, unless I feed it with the DeviceName and DriverName from the > default printer. > For example > CreateDC(Printer.DriverName, Printer.DeviceName, vbNullString, vbNull)
> returns a valid device context, but
> CreateDC(Printers(0).DriverName, Printers(0).DeviceName, vbNullString,
> vbNull)
> always returns 0
> further
> DevName = Printer.DeviceName
> DrvName = Printer.DriverName
> CreateDC(DrvName, DevName, vbNullString, vbNull)
> also always returns 0
> Ideas?
> Dale
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| Sat, 01 Jan 2005 23:34:17 GMT |
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Doug Ros
#3 / 5
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CreateDC Fails when not fed 'properly' The problem is vbNull is not what you think it is (it's a Variant that
contains a special value indicating null, or something like that).
vbNullString was the right choice for the unused third parameter though.
I recommend the following declarations:
Declare Function CreateDC Lib "gdi32.dll" Alias "CreateDCA" (ByVal
lpszDriver As String, ByVal lpszDevice As String, ByVal lpszOutput As
String, ByRef lpInitData As DEVMODE) As Long
Dim hDC As Long
Dim tDevMode As DEVMODE
hDC = CreateDC("DriverName", "DeviceName", vbNullString, tDevMode)
Use this if you don't want to use the last parameter:
Declare Function CreateDCNull Lib "gdi32.dll" Alias "CreateDCA" (ByVal
lpszDriver As String, ByVal lpszDevice As String, Optional ByVal lpszOutput
As String = vbNullString, Optional ByVal lpInitData As Long = 0&) As Long
Dim hDC As Long
hDC = CreateDCNull("DriverName", "DeviceName")
Quote: > I seem to have sorted something out, that seems to work, basically by > changing the declaration of CreateDC to > Declare Function CreateDC Lib "gdi32.dll" Alias "CreateDCA" (ByVal
> lpszDriver As String, ByVal lpszDevice As String, ByVal lpszOutput As
Long,
> lpInitData As Any) As Long
> And passing 0s to lpszOutput and Clng(0) by Val to lpInitData. Anyone know
> why this is required?
> http://216.26.168.92/vbapi/ref/c/createdc.html
> Dale
> > I am exploring GDI with the eventual goal of printing, and I have run in
> to
> > an 'odd' problem. Basically the CreateDC call in failing to return
> anything
> > other than 0, unless I feed it with the DeviceName and DriverName from
the
> > default printer.
> > For example
> > CreateDC(Printer.DriverName, Printer.DeviceName, vbNullString, vbNull)
> > returns a valid device context, but
> > CreateDC(Printers(0).DriverName, Printers(0).DeviceName, vbNullString,
> > vbNull)
> > always returns 0
> > further
> > DevName = Printer.DeviceName
> > DrvName = Printer.DriverName
> > CreateDC(DrvName, DevName, vbNullString, vbNull)
> > also always returns 0
> > Ideas?
> > Dale
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| Sun, 02 Jan 2005 01:57:32 GMT |
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Dale Atki
#4 / 5
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CreateDC Fails when not fed 'properly' Quote: > The problem is vbNull is not what you think it is (it's a Variant that
> contains a special value indicating null, or something like that).
> vbNullString was the right choice for the unused third parameter though.
I thought it might not be, but then why did it work the first way?
Dale
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| Sun, 02 Jan 2005 02:26:33 GMT |
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Doug Ros
#5 / 5
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CreateDC Fails when not fed 'properly' Luck? The docs say it's a constant for the value 1 and is used with the
VarType function. It indicates the variable passed to VarType "contains no
valid data" (something I believe only a Variant can do).
Now, I would guess that CreateDC checks the the lpInitData parameter against
NULL (0). Since you passed vbNull, I assume it then attempts to access a
DEVMODE structure at address 0x00000001, which should throw an access
violation. Since you didn't report this happening, I'm at a loss to explain
it. Maybe CreateDC is smarter than I think...
Quote: > > The problem is vbNull is not what you think it is (it's a Variant that > > contains a special value indicating null, or something like that). > > vbNullString was the right choice for the unused third parameter though. > I thought it might not be, but then why did it work the first way?
> Dale
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| Sun, 02 Jan 2005 03:06:00 GMT |
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