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The General Winforms Interview Questions consists the most frequently
asked questions in Winforms. This list of 100+ questions guage your familiarity
with the Winforms platform. The q&a have been collected over a period
of time from various blogs, forums and other similar Winforms sites
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37. GDI Drawing Tips
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| 37.1 Where can I see a
basic tutorial on GDI+?
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| 37.2 What are some things
to remember when drawing in Window Forms?
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| 37.3 When can I not use
AutoScrolling? In other words why would I ever need any other form of
autoscrolling?
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| 37.4 Are AutoScrolling
bounds setup in World or Device coordinates?
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| 37.5 Why am I not being
able to set a Color.Transparent color as a background to my control?
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| 37.6 Why does a Control not
draw transparent even after setting it's BackColor to Transparent?
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| 37.7 How can I get the
screen resolution of my display?
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| 37.8 How do I draw rotated
text?
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| 37.9 Why does my
transparent Panel whose parent has a gradient background flicker a lot when
resized?
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| 37.10 How do I get a count
of the gdi handles currently in use in my application?
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| 37.11 How can I minimize
flickering when drawing a control?
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| 37.12 How do I draw
circles, rectangles, lines and text?
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| 37.13 How do I get a
snapshot of my desktop?
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| 37.14 How can I draw
without handling a paint message?
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37.1 Where can I see a basic tutorial on GDI+?
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Take a look at Mahash Chand's GDI+ Tutorial for Beginners found on C# Corner.
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37.2 What are some things to remember when drawing in Window Forms?
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Check out the Painting techniques using Windows Forms by Fred Balsigerat
gotnetdot.com. It is a good basic discussion of how to get the best performance
from Windows Forms drawing. His hints include leveraging the power of the .Net
Framework by using the proper controls and control styles as well as
consolidating painting code in the OnPaint and OnPaintBackground methods.
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37.3 When can I not use AutoScrolling? In other words why would I ever need any
other form of autoscrolling?
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AutoScrolling does not allow you to dynamically control the scrolling interval.
If you are drawing a complex control such as grid then you want to be able to
scroll based on the current row height, column width etc. With AutoScrolling
you cannot do this. You have to implement Scrolling yourself in such cases.
Autoscrolling is also not very useful if you want multiple views to share a
scrollbar. The most common place where you see this is with a workbook. There
is no direct way in Winforms to hook up your own scrollbars with the
AutoScrolling implementation.
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37.4 Are AutoScrolling bounds setup in World or Device coordinates?
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AutoScrollingMinSize is setup in device coordinates. If you are using a world
coordinate system other than pixels this means that you have to translate
between world and device coordinates before you set these value. You can use
the Graphics.TransformPoints API to do this as shown below.
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g.TransformPoints(CoordinateSpace.Device, CoordinateSpace.World, ptScrollSize);
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All this does is go from the GraphicsUnit that you are using to pixels. For
example if your GraphicsUnit is Inch, then with a Graphics object that
represents a monitor screen you will have translated values that come to about
100 pixels for every logical inch. Using this API ensures that you are
insulated from the underlying device. The values will be much higher for
printers. Another important point to remember is that your Transform in the
painting code (please refer earlier FAQ in this section on AutoScrolling
implementation) will have to be in logical coordinates and will hence have to
translate between logical and device coordinates.
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{
Graphics g = args.Graphics;
g.PageUnit = GraphicsUnit.Inch;
// this will always be in pixels
Point[] ptAutoScrollPos = new Point[]{this.AutoScrollPosition};
// We have to convert the pixel value (device) to inches (logical..world)
g.TransformPoints(CoordinateSpace.World, CoordinateSpace.Device,
ptAutoScrollPos);
// set up a simple translation so that we draw with respect to the doc bounds
// and not the physical bounds
g.TranslateTransform(ptAutoScrollPos[0].X, ptAutoScrollPos[0].Y);
g.DrawEllipse(_pen, _rectEllipse);
}
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37.5 Why am I not being able to set a Color.Transparent color as a background
to my control?
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Sometimes the framework will throw an exception if you try to set the bg color
to be transparent. This is because the Control doesn't support transparent
colors. To work around this you should call this method from within the
Control:
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[C#]
this.SetStyle(ControlStyles.SupportsTransparentBackColor, true);
[VB.Net]
Me.SetStyle(ControlStyles.SupportsTransparentBackColor, True)
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37.6 Why does a Control not draw transparent even after setting it's BackColor
to Transparent?
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This is possible if the Control is drawn by the system, rather than by the
framework. This is the case for example with the Label control when it's set to
FlatStyle.System. You should then set to to something else other than System
for your transparent drawing to work.
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37.7 How can I get the screen resolution of my display?
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Use this property: System.Windows.Forms.Screen.PrimaryScreen.Bounds
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37.8 How do I draw rotated text?
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private void pictureBox1_Paint(object sender,
System.Windows.Forms.PaintEventArgs e)
{
Graphics g = e.Graphics;
g.TranslateTransform(100.0f, 100.0f);
g.RotateTransform(-90.0f);
g.DrawString("Vertical Text", Font, Brushes.Blue, 0.0f, 0.0f);
g.ResetTransform();
g.TranslateTransform(100.0f, 100.0f);
g.RotateTransform(-45.0f);
g.DrawString("Slanted Text", new Font(Font, FontStyle.Bold), Brushes.Red, 0.0f,
0.0f);
g.ResetTransform();
}
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37.9 Why does my transparent Panel whose parent has a gradient background
flicker a lot when resized?
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class NonFlickerPanel : Panel
{
public NonFlickerPanel()
{
SetStyle(ControlStyles.DoubleBuffer | ControlStyles.UserPaint |
ControlStyles.AllPaintingInWmPaint,true);
UpdateStyles();
}
}
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37.10 How do I get a count of the gdi handles currently in use in my
application?
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You can do so using the native GetGuiResources api. Here is a sample:
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///
/// uiFlags: 0 - Count of GDI objects
/// uiFlags: 1 - Count of USER objects
/// - Win32 GDI objects (pens, brushes, fonts, palettes, regions, device
contexts, bitmap headers)
/// - Win32 USER objects:
/// - WIN32 resources (accelerator tables, bitmap resources, dialog box
templates, font resources, menu resources, raw data resources, string table
entries, message table entries, cursors/icons)
/// - Other USER objects (windows, menus)
///
[DllImport("User32")]
extern public static int GetGuiResources(IntPtr hProcess, int uiFlags);
public static int GetGuiResourcesGDICount()
{
return GetGuiResources(Process.GetCurrentProcess().Handle, 0);
}
public static int GetGuiResourcesUserCount()
{
return GetGuiResources(Process.GetCurrentProcess().Handle, 1);
}
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'
' uiFlags: 0 - Count of GDI objects
' uiFlags: 1 - Count of USER objects
' - Win32 GDI objects (pens, brushes, fonts, palettes, regions, device
contexts, bitmap headers)
' - Win32 USER objects:
' - WIN32 resources (accelerator tables, bitmap resources, dialog box
templates, font resources, menu resources, raw data resources, string table
entries, message table entries, cursors/icons)
' - Other USER objects (windows, menus)
' _
extern Public static Integer GetGuiResources(IntPtr hProcess, Integer uiFlags)
Public Shared Function GetGuiResourcesGDICount() As Integer
Return GetGuiResources(Process.GetCurrentProcess().Handle,0)
End Function
Public Shared Function GetGuiResourcesUserCount() As Integer
Return GetGuiResources(Process.GetCurrentProcess().Handle,1)
End Function
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37.11 How can I minimize flickering when drawing a control?
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public UserPictureBox() //derived from System.Windows.Forms.Control
{
// This call is required by the Windows.Forms Form Designer.
InitializeComponent();
// Activates double buffering
SetStyle(ControlStyles.UserPaint, true);
SetStyle(ControlStyles.AllPaintingInWmPaint, true);
SetStyle(ControlStyles.DoubleBuffer, true);
// TODO: Add any initialization after the InitForm call
}
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37.12 How do I draw circles, rectangles, lines and text?
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Handle the Paint event for your control or form.
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private void Form1_Paint(object sender, System.Windows.Forms.PaintEventArgs e)
{
Graphics g = e.Graphics;
Pen pen = new Pen(Color.White, 2);
SolidBrush redBrush = new SolidBrush(Color.Red);
g.DrawEllipse(pen, 100,150,100,100);
g.DrawString("Circle", this.Font, redBrush, 80, 150);
g.FillRectangle(redBrush, 140, 35, 20, 40);
g.DrawString("Rectangle", this.Font, redBrush, 80, 50);
g.DrawLine(pen, 114, 110, 150, 110);
g.DrawString("Line", this.Font, redBrush, 80, 104);
}
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37.13 How do I get a snapshot of my desktop?
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Here is some code that will do it.
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[C#]
internal class NativeMethods
{
[DllImport("user32.dll")]
public extern static IntPtr GetDesktopWindow();
[System.Runtime.InteropServices.DllImport("user32.dll")]
public static extern IntPtr GetWindowDC(IntPtr hwnd);
[System.Runtime.InteropServices.DllImport("gdi32.dll")]
public static extern UInt64 BitBlt
(IntPtr hDestDC,
int x,
int y,
int nWidth,
int nHeight,
IntPtr hSrcDC,
int xSrc,
int ySrc,
System.Int32 dwRop);
}
// Save the screen capture into a jpg
public void SaveScreen()
{
Image myImage = new Bitmap(Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height);
Graphics gr1 = Graphics.FromImage(myImage);
IntPtr dc1 = gr1.GetHdc();
IntPtr dc2 = NativeMethods.GetWindowDC(NativeMethods.GetDesktopWindow());
NativeMethods.BitBlt(dc1, 0, 0, Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height, dc2, 0, 0, 13369376);
gr1.ReleaseHdc(dc1);
myImage.Save("screenshot.jpg", ImageFormat.Jpeg);
}
[VB.Net]
Friend Class NativeMethods
_
Public Shared Function GetDesktopWindow() As IntPtr
End Function
_
Public Shared Function GetWindowDC(ByVal hwnd As IntPtr) As IntPtr
End Function
_
Public Shared Function BitBlt(ByVal hDestDC As IntPtr, ByVal x As Integer,
ByVal y As Integer, ByVal nWidth As Integer, ByVal nHeight As Integer, ByVal
hSrcDC As IntPtr, ByVal xSrc As Integer, ByVal ySrc As Integer, ByVal dwRop As
System.Int32) As UInt64
End Function
End Class 'NativeMethods
'Save the screen capture into a jpg
Private Sub SaveScreen()
Dim myImage = New Bitmap(Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height)
Dim gr1 As Graphics = Graphics.FromImage(myImage)
Dim dc1 As IntPtr = gr1.GetHdc()
Dim dc2 As IntPtr = NativeMethods.GetWindowDC(NativeMethods.GetDesktopWindow())
NativeMethods.BitBlt(dc1, 0, 0, Screen.PrimaryScreen.Bounds.Width,
Screen.PrimaryScreen.Bounds.Height, dc2, 0, 0, 13369376)
gr1.ReleaseHdc(dc1)
myImage.Save("screenshot.jpg", ImageFormat.Jpeg)
End Sub 'SaveScreen
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37.14 How can I draw without handling a paint message?
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o draw on any hwnd, you need to get a Graphics object from that hwnd. Once you
have the Graphics object, you can then use its methods to draw. Of course,
since this drawing is not done in the Paint handler, it will not be
automatically refreshed when the control is redrawn for any reason.
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Graphics g = Graphics.FromHwnd(this.Handle);
SolidBrush brush = new SolidBrush(Color.Red);
Rectangle rectangle = new Rectangle(25, 25, 100, 100);
g.FillRectangle(brush, rectangle);
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