GetClientRect获取矩形异常：无法正确覆盖外部窗口客户区

你的代码核心问题出在DPI缩放适配以及ClientToScreen的使用逻辑上，导致获取的客户区坐标与实际屏幕位置不匹配，最终悬浮窗体覆盖了整个目标窗口。

具体问题点 #

1. DPI感知缺失：WinForms程序默认不启用高DPI感知，当系统DPI缩放比例不为100%时，获取的窗口坐标会被错误缩放，导致窗体大小和位置计算偏差。
2. ClientToScreen使用逻辑瑕疵：仅转换客户区的(0,0)点，未通过对角点验证坐标准确性，结合DPI问题进一步放大了位置偏差。

解决方案 #

1. 启用高DPI感知 #

在项目的app.manifest文件中添加DPI感知配置（无manifest时，右键项目→添加→新建项→应用程序清单文件）：

<application xmlns="urn:schemas-microsoft-com:asm.v3">
  <windowsSettings>
    <dpiAware xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">true/PM</dpiAware>
  </windowsSettings>
</application>

该配置让程序正确识别系统DPI缩放，确保坐标计算准确。

2. 修正客户区坐标获取逻辑 #

通过转换客户区的左上角和右下角点，确保位置与大小的准确性：

public static Rectangle GetClientScreenRectangle(IntPtr hWnd)
{
    GetClientRect(hWnd, out RECT clientRect);
    
    // 转换客户区左上角到屏幕坐标
    Point clientTopLeft = new Point(clientRect.Left, clientRect.Top);
    ClientToScreen(hWnd, ref clientTopLeft);
    
    // 转换客户区右下角到屏幕坐标
    Point clientBottomRight = new Point(clientRect.Right, clientRect.Bottom);
    ClientToScreen(hWnd, ref clientBottomRight);
    
    // 计算客户区的实际屏幕位置和大小
    return new Rectangle(
        clientTopLeft.X, 
        clientTopLeft.Y, 
        clientBottomRight.X - clientTopLeft.X, 
        clientBottomRight.Y - clientTopLeft.Y
    );
}

3. 优化窗体初始化逻辑 #

将窗口位置/大小设置移到Form_Load事件中（确保目标窗口完全加载），可选添加定时器实现动态跟随：

public partial class Form1 : Form
{
    private Process? targetProc;
    private IntPtr notepadhWnd;
    private System.Windows.Forms.Timer? _followTimer;

    public struct RECT
    {
        public int Left;
        public int Top;
        public int Right;
        public int Bottom;
    }

    [DllImport("user32.dll", SetLastError = true)]
    public static extern bool GetClientRect(IntPtr hwnd, out RECT lpRect);

    [DllImport("user32.dll")]
    public static extern bool ClientToScreen(IntPtr hWnd, ref Point lpPoint);

    public static Rectangle GetClientScreenRectangle(IntPtr hWnd)
    {
        GetClientRect(hWnd, out RECT clientRect);
        
        Point clientTopLeft = new Point(clientRect.Left, clientRect.Top);
        ClientToScreen(hWnd, ref clientTopLeft);
        
        Point clientBottomRight = new Point(clientRect.Right, clientRect.Bottom);
        ClientToScreen(hWnd, ref clientBottomRight);
        
        return new Rectangle(
            clientTopLeft.X, 
            clientTopLeft.Y, 
            clientBottomRight.X - clientTopLeft.X, 
            clientBottomRight.Y - clientTopLeft.Y
        );
    }

    public Form1()
    {
        InitializeComponent();

        FormBorderStyle = FormBorderStyle.None;
        WindowState = FormWindowState.Normal;
        BackColor = Color.Black;
        StartPosition = FormStartPosition.Manual;
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        targetProc = Process.GetProcessesByName("notepad").FirstOrDefault();
        if (targetProc == null || targetProc.MainWindowHandle == IntPtr.Zero)
        {
            MessageBox.Show("未找到记事本窗口");
            Close();
            return;
        }

        notepadhWnd = targetProc.MainWindowHandle;
        UpdateOverlayPosition();

        // 可选：添加定时器动态跟随目标窗口
        _followTimer = new System.Windows.Forms.Timer();
        _followTimer.Interval = 500;
        _followTimer.Tick += (s, ev) => UpdateOverlayPosition();
        _followTimer.Start();
    }

    private void UpdateOverlayPosition()
    {
        if (notepadhWnd == IntPtr.Zero) return;
        
        Rectangle clientRect = GetClientScreenRectangle(notepadhWnd);
        Location = new Point(clientRect.Left, clientRect.Top);
        Size = new Size(clientRect.Width, clientRect.Height);
    }
}

验证效果 #

运行修改后的程序，确保记事本已打开，此时黑色悬浮窗体将准确覆盖记事本的内容区域（不含标题栏和边框）。

内容的提问来源于stack exchange，提问作者zecuria