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Application.AddMessageFilter Method

Adds a message filter to monitor Windows messages as they are routed to their destinations.

Namespace: System.Windows.Forms
Assembly: System.Windows.Forms (in

public static void AddMessageFilter (
	IMessageFilter value
public static void AddMessageFilter (
	IMessageFilter value
public static function AddMessageFilter (
	value : IMessageFilter



The implementation of the IMessageFilter interface you want to install.

Use a message filter to prevent specific events from being raised or to perform special operations for an event before it is passed to an event handler. Message filters are unique to a specific thread.

To prevent a message from being dispatched, the value parameter instance that you pass to this method must override the PreFilterMessage method with the code to handle the message. The method must return false.

Caution noteCaution

Adding message filters to the message pump for an application can degrade performance.

The following code example creates a message filter called TestMessageFilter. This filter blocks all messages relating to the left mouse button. Before you can use a message filter, you must provide an implementation for the IMessageFilter interface.

// Creates a  message filter.
public class TestMessageFilter implements IMessageFilter
    /** @attribute SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)
    public boolean PreFilterMessage(
        Message m)
        // Blocks all the messages relating to the left mouse button.
        if (m.get_Msg() >= 513 && m.get_Msg() <= 515) {
            Console.WriteLine("Processing the messages : " + m.get_Msg());
            return true;
        return false;
    } //PreFilterMessage
} //TestMessageFilter

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

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

.NET Framework

Supported in: 2.0, 1.1, 1.0