Graphics.GetHdc Method ()


Gets the handle to the device context associated with this Graphics.

Namespace:   System.Drawing
Assembly:  System.Drawing (in System.Drawing.dll)

<SecurityPermissionAttribute(SecurityAction.LinkDemand, Flags := SecurityPermissionFlag.UnmanagedCode)>
Public Function GetHdc As IntPtr

Return Value

Type: System.IntPtr

Handle to the device context associated with this Graphics.

The device context is a Windows structure based on GDI that defines a set of graphical objects and their associated attributes, as well as the graphical modes that affect output. This method returns that device context with the exception of a font. Because a font is not selected, calls to the FromHdc method using a handle returned from the GetHdc method will fail.

Calls to the GetHdc and ReleaseHdc methods must appear in pairs. During the scope of a GetHdc and ReleaseHdc method pair, you usually make only calls to GDI functions. Calls in that scope made to GDI+ methods of the Graphics that produced the hdc parameter fail with an ObjectBusy error. Also, GDI+ ignores any state changes made to the Graphics of the hdc parameter in subsequent operations.

The following code example is designed for use with Windows Forms, and it requires PaintEventArgse, which is a parameter of the Paint event handler. The example illustrates calling a Windows GDI function to perform the same task as a GDI+ Graphics method. The code performs the following actions:

  • Defines the interoperability DllImportAttribute attribute for the Windows DLL file gdi32.dll. This DLL contains the desired GDI function.

  • Defines the Rectangle function in that DLL as external.

  • Creates a red pen.

  • With the pen, draws a rectangle to the screen using the GDI+ DrawRectangle method.

  • Defines an internal pointer type variable hdc and sets its value to the handle to the device context of the form.

  • Draws a rectangle to the screen using the GDI Rectangle function.

  • Releases the device context represented by the hdc parameter.

Public Class GDI
    <System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")> _
    Friend Shared Function Rectangle(ByVal hdc As IntPtr, _
    ByVal ulCornerX As Integer, ByVal ulCornerY As Integer, ByVal lrCornerX As Integer, _
    ByVal lrCornerY As Integer) As Boolean
    End Function
End Class

<System.Security.Permissions.SecurityPermission( _
System.Security.Permissions.SecurityAction.LinkDemand, Flags:= _
System.Security.Permissions.SecurityPermissionFlag.UnmanagedCode)> _
Private Sub GetHdcForGDI1(ByVal e As PaintEventArgs)

    ' Create pen.
    Dim redPen As New Pen(Color.Red, 1)

    ' Draw rectangle with GDI+.
    e.Graphics.DrawRectangle(redPen, 10, 10, 100, 50)

    ' Get handle to device context.
    Dim hdc As IntPtr = e.Graphics.GetHdc()

    ' Draw rectangle with GDI using default pen.
    GDI.Rectangle(hdc, 10, 70, 110, 120)

    ' Release handle to device context.
End Sub


for access to unmanaged code. Related enumeration: UnmanagedCode.

.NET Framework
Available since 1.1
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