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Graphics.GetHdc Method

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

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

public IntPtr GetHdc ()
public final IntPtr GetHdc ()
public final function GetHdc () : IntPtr
Not applicable.

Return Value

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 GDIRectangle function.

  • Releases the device context represented by the hdc parameter.

public class GDI
{
    [System.Runtime.InteropServices.DllImport("gdi32.dll")]
    internal static extern bool Rectangle(
       IntPtr hdc,
       int ulCornerX, int ulCornerY,
       int lrCornerX, int lrCornerY);
}

[System.Security.Permissions.SecurityPermission(
System.Security.Permissions.SecurityAction.LinkDemand, Flags = 
System.Security.Permissions.SecurityPermissionFlag.UnmanagedCode)]            
private void GetHdcForGDI1(PaintEventArgs e)
{

    // Create pen.
    Pen redPen = new Pen(Color.Red, 1);

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

    // Get handle to device context.
    IntPtr hdc = e.Graphics.GetHdc();

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

    // Release handle to device context.
    e.Graphics.ReleaseHdc(hdc);
}

Windows 98, Windows Server 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0

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