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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 PinmemberRoss Brigoli21-Jan-09 18:52 
GeneralMesage Filters PinmemberRoryHerman18-Sep-08 22:55 
GeneralWonderful Article Pinmemberanupamaroy14-Dec-05 19:08 
Questionpossible to use with a wedge barcode scanner? Pinmemberyogiberr22-Nov-05 9:45 
AnswerRe: possible to use with a wedge barcode scanner? [modified] PinmemberDapo Onawole25-May-07 7:26 
QuestionDisposing the IMessageFilter? PinmemberJediMasterYoder10-Jun-05 12:01 
QuestionVB6 Implementation? PinmemberTAlvord@kavlico.com6-Dec-04 8:32 
GeneralThx a lot PinmemberSteven Wilssens28-Apr-04 3:20 
GeneralJust a note PinmemberLee Alexander16-Oct-03 7:24 
It's all very well using IMessageFilter but be aware that it won't work if the control is part of a COM control. We had a problem where we were using it and it worked fine in the WindowsForms application, but never got called whilst running under an MFC application via COM. The reason for this being the hosting application is in charge of the message pump. We got around this by creating hook a policy. Under COM the policy invoked was one that used windows hooks, and under pure windows forms we used the IMessageFilter approach.
Just my two pennies worth Poke tongue | ;-P
GeneralRe: Just a note PineditorHeath Stewart20-Nov-03 3:18 
GeneralRe: Just a note PinmemberLee Alexander23-Nov-03 23:21 
GeneralRe: Just a note PineditorHeath Stewart24-Nov-03 5:37 
GeneralRe: Just a note PinmemberLee Alexander28-Nov-03 1:12 
GeneralRe: Just a note PinmemberSantosh Rao17-Aug-05 21:18 
GeneralRe: Just a note PinmemberLee Alexander17-Aug-05 22:37 
GeneralRe: Just a note PinmemberSantosh Rao17-Aug-05 23:32 
GeneralRe: Just a note PinmemberSantosh Rao17-Aug-05 23:29 

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