AddMessageFilter Method

Application.AddMessageFilter Method (IMessageFilter)


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

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

public static void AddMessageFilter(
	IMessageFilter value


Type: System.Windows.Forms.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.


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

Legacy Code Example

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.
[SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
public class TestMessageFilter : IMessageFilter
    public bool PreFilterMessage(ref Message m)
        // Blocks all the messages relating to the left mouse button.
        if (m.Msg >= 513 && m.Msg <= 515)
            Console.WriteLine("Processing the messages : " + m.Msg);
            return true;
        return false;


for the immediate caller to call unmanaged code. Associated enumeration: SecurityPermissionFlag.UnmanagedCode

.NET Framework
Available since 1.1
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