Winform Interview Questions Page 2
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 How can I make sure there is only one instance of my application running?

Saar Carmi shows how to do this in his sample found on C# Corner. In it, he uses the Process class in System.Diagnostics to implement this functionality using code such as

[C#]
public static Process RunningInstance()
{
   Process current = Process.GetCurrentProcess();
   Process[] processes = Process.GetProcessesByName (current.ProcessName);
   //Loop through the running processes in with the same name
   foreach (Process process in processes)
  {//Ignore the current process
  if (process.Id != current.Id)
  {//Make sure that the process is running from the exe file.  if   Assembly.GetExecutingAssembly(). Location. Replace ("/", "\\") == current.MainModule.FileName)
  { //Return the other process instance.
  return process;
  }  }   } //No other instance was found, return null.
  return null;
 }
[VB.NET]
Public Shared Function RunningInstance() As Process
Dim current As Process = Process.GetCurrentProcess()
Dim processes As Process() = Process.GetProcessesByName(current.ProcessName)

'Loop through the running processes in with the same name
Dim process As Process
For Each process In processes
'Ignore the current process
If process.Id <> current.Id Then
'Make sure that the process is running from the exe file.
If [Assembly].GetExecutingAssembly().Location.Replace("/", "\") = current.MainModule.FileName Then
'Return the other process instance.
Return process
End If
End If
Next process
'No other instance was found, return null.
Return Nothing
End Function 'RunningInstance


How do I determine which operating system is running?



The Environment class in the System namespace has this information.
[C#]
string versionText = Environment.OSVersion.Version.ToString();

[VB.NET]
Dim versionText As String = Environment.OSVersion.Version.ToString()


The Version property has member properties such as Major and Minor that give additional information.

Note that XP is windows version 5.1

1.7 How can I get all IP addresses for my local machine?



[C#]
string s ="";

System.Net.IPAddress[] addressList = Dns.GetHostByName(Dns.GetHostName()).AddressList;

for (int i = 0; i < addressList.Length; i ++)
{
s += addressList[i].ToString() + "\n";
}

textBox1.Text = s;

[VB.NET]
Dim s As String = ""

Dim addressList As System.Net.IPAddress() = Dns.GetHostByName(Dns.GetHostName()).AddressList

Dim i As Integer
For i = 0 To addressList.Length - 1
s += addressList(i).ToString() + ControlChars.Lf
Next i

textBox1.Text = s


My user does not have .NET installed. Will he be able to run my Windows Forms application?


No, the .NET runtime platform has to be on any machine that will run your Windows Forms application. Microsoft has made the .NET runtime platform installation (dotnetredist.exe) available as a free download from Microsoft .NET Framework Redistributable .

How do I get the path to my running EXE?


The Application class has a static member ExecutablePath that has this information.
[C#]
textBox1.Text = Application.ExecutablePath;

[VB.NET]
TextBox1.Text = Application.ExecutablePath


 How can I programmatically obtain the name of the assembly that the code is executing in ?


The following code snippet demonstrates how you can obtain the name of the assembly that the code is executing in:
[C#]
MessageBox.Show(System.Reflection.Assembly.GetEntryAssembly().GetName().Name);

[VB.NET]
MessageBox.Show(System.Reflection.Assembly.GetEnTryAssembly().GetName().Name)




1.11 How can I see what is installed in the Global Assembly on a machine?


Use Windows Explorer to view the C:\WINNT\assembly folder. If the .NET Framework is installed, the Windows Explorer will show a custom view of this folder. Use the detailed view to see the details of each assembly.

1.12 How do I set the company name that is returned by System.Windows.Forms.Application.CompanyName?


This is an assembly attribute. The VS development environment sets it in the AssemblyInfo.cs (vb) file. If you open that file, you will see a block of assembly attributes that you can set, including company and version numbers.





2.1 How do you prevent serialization of certain child controls in your Composite Control?
One solution is to use a panel that has a picturebox placed on it with DockStyle.Fill. This will make the picturebox assume the size of the panel. In addition, set the DockPadding.All property to the width of the desired border. Then in the Panel's OnPaint method, call the baseclass and then paint the desired borders.

Here are both VB and C# projects that illustrate how you might go about this. The derived PicturePanel class has properties that allow you to set the bordersize and color as well as the image that is to be displayed. This sample retrieves the image from an embedded



2.2 How do I get hold of the currently focused Control?



The .Net framework libraries does not provide you an API to query for the focused Control. You have to invoke a windows API to do so:



[C#]

public class MyForm : Form
{

[DllImport("user32.dll", CharSet=CharSet.Auto, CallingConvention=CallingConvention.Winapi)]

iternal static extern IntPtr GetFocus();

private Control GetFocusedControl()

{

Control focusedControl = null;

// To get hold of the focused control:

IntPtr focusedHandle = GetFocus();

if(focusedHandle != IntPtr.Zero)

// Note that if the focused Control is not a .Net control, then this will return null.

focusedControl = Control.FromHandle(focusedHandle);

return focusedControl;

}

}


[VB.Net]
Public Class Form1
' Declare the GetFocused method here:
Public Shared Function GetFocus() As IntPtr
End Function


Private Function GetFocusedControl() As Control

Dim focusedControl As Control = Nothing

' To get hold of the focused control:
Dim focusedHandle As IntPtr = GetFocus()

If IntPtr.Zero.Equals(focusedHandle) Then

' Note that if the focused Control is not a .Net control, then this will return null.

focusedControl = Control.FromHandle(focusedHandle)

End If

Return focusedControl

End Function

End Class
2.3 Why do calling Focus() on a control not set focus on it?

Note that when you call this method the control should be visible, otherwise the focus will not be set. Hence calling this in say Form_Load on a control in the form will be ineffective. You should instead consider give that control an appropriate TabIndex, so that it will be the first focused control.

2.4 How do I listen to windows messages in my Control?
In a derived class you should override WndProc as follows (listening to the WM_KEYUP message, for example):

[C#]

public class MyCombo : ComboBox

{
private const int WM_KEYUP = 0x101;

protected override void WndProc(ref System.Windows.Forms.Message m)
{

if(m.Msg == WM_KEYUP)
{
return; //ignore the keyup
}
base.WndProc(ref m);
}
}
[VB.NET]

Public Class MyTextBox
Inherits TextBox



Private WM_KEYUP As Integer = &H101







Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)



If m.Msg = WM_KEYUP Then



Return 'ignore the keyup



End If



MyBase.WndProc(m)



End Sub 'WndProc



End Class 'MyTextBox





2.5 How do I programatically change the color of a control?



Use the properties BackColor and ForeColor.



button1.BackColor = Color.White;



button1.ForeColor = Color.Blue;



2.6 How can I change the Border color of my control?



Override the OnPaint. Here is some code for a derived Button.



[C#]



public class MyButton : Button



{



protected override void OnPaint(PaintEventArgs e)



{



base.OnPaint(e);



int borderWidth = 1;



Color borderColor = Color.Blue;



ControlPaint.DrawBorder(e.Graphics, e.ClipRectangle, borderColor,



borderWidth, ButtonBorderStyle.Solid, borderColor, borderWidth,



ButtonBorderStyle.Solid, borderColor, borderWidth, ButtonBorderStyle.Solid,



borderColor, borderWidth, ButtonBorderStyle.Solid);



}



}







[VB.NET]



Public Class MyButton



Inherits Button





Protected Overrides Sub OnPaint(e As PaintEventArgs)



MyBase.OnPaint(e)



Dim borderWidth As Integer = 1



Dim borderColor As Color = Color.Blue



ControlPaint.DrawBorder(e.Graphics, e.ClipRectangle, borderColor, borderWidth, ButtonBorderStyle.Solid, borderColor, borderWidth, ButtonBorderStyle.Solid, borderColor, borderWidth, ButtonBorderStyle.Solid, borderColor, borderWidth, ButtonBorderStyle.Solid)



End Sub 'OnPaint



End Class 'MyButton





2.7 Why should I provide a Non-Client border to my Control derived class?



Providing a border in the non-client region of your control rather than in the ClientRectangle has very many advantages:

· When you include a scrollbar in your control, the scrollboar will appear inside the border, rather than to the outside if you drew the border in the client area.

· When you allow custom painting of the control, your user will not draw over the NC border.

· Your own client painting code will be simplified in that you will not have to bother about taking the border into account while painting the client area. The next faq will tell you how to include a non-client border.