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Graphics.ReleaseHdc Method (IntPtr)

Releases a device context handle obtained by a previous call to the GetHdc method of this Graphics.

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

public void ReleaseHdc(
	IntPtr hdc
)

Parameters

hdc
Type: System.IntPtr

Handle to a device context obtained by a previous call to the GetHdc method of 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.

Calls to the GetHdc and ReleaseHdc methods must appear in pairs. During the scope of a GetHdc and ReleaseHdc method pair, you usually make calls only 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 PaintEventArgs e, 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, and it 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.

[System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")]
private static extern bool Rectangle2(
    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 GetHdcForGDI2(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.
    Rectangle2(hdc, 10, 70, 110, 120);

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

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0

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