在Application.AddMessageFilter之后从未调用过IMessageFilter.PreFilterMessage

Question

在调用Application.AddMessageFilter之后，永远不会调用PreFilterMessage方法。它只是在一个简单的WPF应用程序中(如下所示)。我是不是漏掉了什么？

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        var _filter = new PreMessageFilter();
        Application.AddMessageFilter(_filter);
    }
}

[SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
public class PreMessageFilter : IMessageFilter
{
    public bool PreFilterMessage(ref Message m)
    {
        Console.WriteLine(m.ToString());
        return true;
    }
}

Answer 1

我在WinForms下成功地使用了IMessageFilter，但我不确定它在WPF下是否能正常工作。

如果你需要在WPF中截取一些消息，还有另一种方法。

这种方式不同于MessageFilter，因为它不能过滤消息，只能监听消息循环。

让我们来听一听WM_KEYDOWN消息。

显示完整的应用程序代码，以便于重现

MyMessageHook.cs

using System;
using System.Windows.Input;
using System.Windows.Interop;

namespace WPFMessageHookExample
{
    public class MyKeyEventArgs : EventArgs
    {
        public Key Key { get; private set; }
        public MyKeyEventArgs(Key key) { Key = key; }
    }

    public class MyMessageHook : IDisposable
    {
        private const int WM_KEYDOWN = 0x0100;

        private readonly HwndSourceHook _hook;
        private static HwndSource _hwndSource;

        public event EventHandler<MyKeyEventArgs> KeyDown;
        public MyMessageHook(HwndSource hwndSource)
        {
            _hook = new HwndSourceHook(WndProc);
            _hwndSource = hwndSource ?? throw new ArgumentNullException(nameof(hwndSource));
            _hwndSource.AddHook(_hook);
        }

        private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
        {
            switch (msg)
            {
                case WM_KEYDOWN:
                    KeyDown?.Invoke(this, new MyKeyEventArgs(KeyInterop.KeyFromVirtualKey((int)wParam)));
                    break;
            }
            return IntPtr.Zero;
        }

        #region IDisposable
        private bool disposed;
        protected virtual void Dispose(bool disposing)
        {
            if (disposed) return;
            if (disposing)
            {
                _hwndSource.RemoveHook(_hook);
            }
            disposed = true;
        }
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }
        ~MyMessageHook()
        {
            Dispose(false);
        }
        #endregion
    }
}

MainWindow.xaml.cs

using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows;
using System.Windows.Interop;

namespace WPFMessageHookExample
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private MyMessageHook messageHook;
        private string _myText;

        public string MyText
        {
            get => _myText;
            set
            {
                _myText = value;
                OnPropertyChanged();
            }
        }
        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
        }

        private void Window_SourceInitialized(object sender, EventArgs e)
        {
            HwndSource hwnd = HwndSource.FromHwnd(new WindowInteropHelper(this).Handle);
            messageHook = new MyMessageHook(hwnd);
            messageHook.KeyDown += MessageHook_KeyDown;
        }

        private void MessageHook_KeyDown(object sender, MyKeyEventArgs e)
        {
            MyText += e.Key + ", ";
        }

        private void Window_Closing(object sender, CancelEventArgs e)
        {
            messageHook.Dispose();
        }

        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
            => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

MainWindow.xaml

<Window x:Class="WPFMessageHookExample.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WPFMessageHookExample"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800" SourceInitialized="Window_SourceInitialized" Closing="Window_Closing">
    <Grid>
        <TextBox Margin="5" VerticalScrollBarVisibility="Auto" Text="{Binding MyText}" IsReadOnly="True" TextWrapping="Wrap"/>
    </Grid>
</Window>

请注意，TextBox是只读的。

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