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Using IMessageFilter to create a generic filter for operating system events

, 15 Oct 2003
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An article on implementing IMessageFilter to create a message filter for operating system messages


The IMessageFilter class in .NET framework allows an application to capture a windows OS message before it is dispatched to a control or form. A class that implements the IMessageFilter interface can be added to the application's message pump to filter out a message or perform other operations before the message is dispatched to a form or control. Disabling the keys at the forms level leads to lot of code repetition and something that doesn’t look very elegant. The solution provides a generic solution to the problem by separating the implementation from the UI.


The requirement that lead to this solution was, conditional Enabling/Disabling specified keys on a windows form.

Using the code

On click of button3 the keys passed are disabled and enabled again on click of button4. Although the solution addresses the Key_Pressed event only, it can be extended to filter any operating system event.

private void button3_Click(object sender, System.EventArgs e) 
  mobjMessageFilter = new MessageFilter(); 
  mobjMessageFilter.KeysToRestrict=new int[]{
private void button4_Click(object sender, System.EventArgs e) 

The MessageFilter Class

MessgeFilter class implements the IMessageFilter interface and gives the concrete implementation of the Message Filter.

public class MessageFilter:IMessageFilter

The VirtualKeys enumeration

This Enumeration contains virtual key values sent by the operating system when a key is pressed.

public enum VirtualKeys : int 
VK_LBUTTON = 0x01, 
VK_RBUTTON = 0x02, 
VK_CANCEL = 0x03, 
VK_MBUTTON = 0x04, 
VK_XBUTTON1 = 0x05, 
VK_XBUTTON2 = 0x6, 
VK_BACK = 0x08, 
VK_TAB = 0x09, 
VK_RETURN = 0x0D, 
VK_SHIFT = 0x10, 
VK_CONTROL = 0x11, 
VK_ESCAPE = 0x1B, 
VK_SPACE = 0x20, 
VK_LEFT = 0x25, 
VK_UP = 0x26, 
VK_RIGHT = 0x27, 
VK_DOWN = 0x28, 
VK_DELETE = 0x2E, 
VK_F1 = 0x70, 
VK_F2 = 0x71, 
VK_F3 = 0x72, 
VK_F4 = 0x73, 
VK_F5 = 0x74, 
VK_F6 = 0x75, 
VK_F7 = 0x76, 
VK_F8 = 0x77, 
VK_F9 = 0x78, 
VK_F10 = 0x79, 
VK_F11 = 0x7A, 
VK_F12 = 0x7B, 
VK_F13 = 0x7C, 
VK_F14 = 0x7D, 

The WindowsMessages enumeration

This enumeration contains the WindowsMessage value for various Operating System events. Included code contains an exhaustive listing of windows messages.

private enum WindowsMessages: int 
WM_KEYDOWN = 0x0100 

And now to the implementation of the MessageFilter class. It in fact is very simple, all we have to do is give an implementation for the PreFilterMessage method, I have created a property int[] KeysToRestrict to accept the values of the keys to be filtered. And here is what the method looks like,

private enum WindowsMessages: int
  WM_KEYDOWN = 0x0100

public bool PreFilterMessage(ref Message objMessage)
  //Check if the Message is a windows Key down message
  if(objMessage.Msg == Convert.ToInt32(WindowsMessages.WM_KEYDOWN))
    for(int i=0;i<marrKeysToRestrict.Length;i++)
      //Check if the Message's WParam(the key value) is 
in the restricted keys list
      if( objMessage.WParam.ToInt32() == 
      //Disable message propagation
      return true;
 return false;

The method returns true if the Message objects WParam property is equal to any element in the array thus disabling the key.


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Comments and Discussions

QuestionIMessageFilter does not work on Dialog boxes Pin
Ross Brigoli21-Jan-09 18:52
memberRoss Brigoli21-Jan-09 18:52 
GeneralMesage Filters Pin
RoryHerman18-Sep-08 22:55
memberRoryHerman18-Sep-08 22:55 
GeneralWonderful Article Pin
anupamaroy14-Dec-05 19:08
memberanupamaroy14-Dec-05 19:08 
Questionpossible to use with a wedge barcode scanner? Pin
yogiberr22-Nov-05 9:45
memberyogiberr22-Nov-05 9:45 
AnswerRe: possible to use with a wedge barcode scanner? [modified] Pin
Dapo Onawole25-May-07 7:26
memberDapo Onawole25-May-07 7:26 
I understand ur question and I have the same problem at the moment. Were u able to get thru this?

-- modified at 13:22 Friday 25th May, 2007
QuestionDisposing the IMessageFilter? Pin
JediMasterYoder10-Jun-05 12:01
memberJediMasterYoder10-Jun-05 12:01 
QuestionVB6 Implementation? Pin
TAlvord@kavlico.com6-Dec-04 8:32
memberTAlvord@kavlico.com6-Dec-04 8:32 
GeneralThx a lot Pin
Steven Wilssens28-Apr-04 3:20
memberSteven Wilssens28-Apr-04 3:20 
GeneralJust a note Pin
Lee Alexander16-Oct-03 7:24
memberLee Alexander16-Oct-03 7:24 
GeneralRe: Just a note Pin
Heath Stewart20-Nov-03 3:18
editorHeath Stewart20-Nov-03 3:18 
GeneralRe: Just a note Pin
Lee Alexander23-Nov-03 23:21
memberLee Alexander23-Nov-03 23:21 
GeneralRe: Just a note Pin
Heath Stewart24-Nov-03 5:37
editorHeath Stewart24-Nov-03 5:37 
GeneralRe: Just a note Pin
Lee Alexander28-Nov-03 1:12
memberLee Alexander28-Nov-03 1:12 
GeneralRe: Just a note Pin
Santosh Rao17-Aug-05 21:18
memberSantosh Rao17-Aug-05 21:18 
GeneralRe: Just a note Pin
Lee Alexander17-Aug-05 22:37
memberLee Alexander17-Aug-05 22:37 
GeneralRe: Just a note Pin
Santosh Rao17-Aug-05 23:32
memberSantosh Rao17-Aug-05 23:32 
GeneralRe: Just a note Pin
Santosh Rao17-Aug-05 23:29
memberSantosh Rao17-Aug-05 23:29 

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