Filtering Win32 messages of a form

I’ve recently came across a nice post on C# Frequently Asked Questions blog. In the post, they’ve shown a way to filter out, or (pre)process Win32 messages of a form.

Let’s have a look at the code, then we’ll go through it.

public class MyMessageFilter : IMessageFilter
{
    public bool PreFilterMessage(ref Message m)
    {
        // Intercept the left mouse button down message
        if (m.Msg == 513)
        {
            Console.WriteLine("WM_LBUTTONDOWN is: " + m.Msg);
            return true;
        }
        return false;
    }
}

And in the form itself:

public class mainForm : System.Windows.Forms.Form
{
    private MyMessageFilter msgFliter = new MyMessageFilter();

    public mainForm()
    {
        // Register message filter
        Application.AddMessageFilter(msgFliter);
    }
}

As you can see, we’ve started by creating a class that implements the IMessageFilter interface. Then we’ve added an instance of that class to the form, and in the constructor registered it. We can use the Application.RemoveMessageFilter() method to stop filtering messages using the specified filter. All the filter class does is handle the WM_LBUTTONDOWN message. By returning true, we specify that we’ve handled the message, and there’s no need of any further handling of it – meaning that no event will be sent. This technique can be used either as a way to handle specific messages, or just ignore certain messages.

Other nice things to try is to catch the message only a certain amount of times, or to hide some of the messages when certain requirements are met.