IDeviceContext interface

IDeviceContext is the pair of members that hand out a Windows device context for drawing with GDI calls.

It is implemented by Graphics. The device context is a GDI HDC: a handle that GDI functions such as SetPixelV, TextOut or BitBlt accept. A program takes it with GetHdc, draws with GDI and gives it back with ReleaseHdc. Between the two calls the Graphics object is locked. Calling any other method of the Graphics object fails with ObjectBusy until ReleaseHdc is called.

The interface has two members. Graphics also has methods of its own with the names GetHDC and ReleaseHDC; the one that takes the handle as an argument is described on the Graphics page.

GetHdc

Syntax: object.GetHdc As LongPtr

Returns the device context of the object. The handle is 0 if the call fails.

ReleaseHdc

Syntax: object.ReleaseHdc

Gives the device context back, which unlocks the object. It must be called after each successful GetHdc, and before the next use of the object. The handle is not passed in: the object remembers the one it gave out.

The GDI calls draw on the same surface as GDI+. A GDI colour is a COLORREF, the value &H00BBGGRR, so a call that takes a Visual Basic colour such as vbRed draws red. The example uses this declaration:

Declare PtrSafe Function SetPixelV Lib "gdi32" (ByVal hdc As LongPtr, ByVal x As Long, ByVal y As Long, ByVal crColor As Long) As Long

Dim user As GdiPlusUser = GdiPlusUser()
Dim bmp As Bitmap = Bitmap(8!, 8!)
Dim g As Graphics = Graphics(bmp)
g.Clear Color(255, 255, 255, 255)

Dim dc As IDeviceContext = g
Dim hdc As LongPtr = dc.GetHdc
SetPixelV hdc, 2, 3, &HFF&                    ' a COLORREF: red
SetPixelV hdc, 4, 3, &HFF0000                 ' a COLORREF: blue
dc.ReleaseHdc

Dim c As Color = bmp.Pixel(2, 3)
Debug.Print Hex$(c.R) & " " & Hex$(c.G) & " " & Hex$(c.B)    ' FF 0 0
c = bmp.Pixel(4, 3)
Debug.Print Hex$(c.R) & " " & Hex$(c.G) & " " & Hex$(c.B)    ' 0 0 FF

See Also