meeting-assistant/MeetingAssistant/Screenshots/ActiveWindowScreenshotCapture.Windows.cs

using System.Drawing;
using System.Drawing.Imaging;
using System.Runtime.InteropServices;

namespace MeetingAssistant.Screenshots;

public sealed class ActiveWindowScreenshotCapture : IActiveWindowScreenshotCapture
{
    private static readonly IntPtr DpiAwarenessContextPerMonitorAwareV2 = new(-4);
    private const int DwmWindowAttributeExtendedFrameBounds = 9;

    public Task<ActiveWindowScreenshot> CaptureAsync(CancellationToken cancellationToken)
    {
        var previousDpiContext = TrySetThreadDpiAwarenessContext(DpiAwarenessContextPerMonitorAwareV2);
        try
        {
            var foregroundWindow = GetForegroundWindow();
            if (foregroundWindow == IntPtr.Zero ||
                !TryGetWindowBounds(foregroundWindow, out var rect))
            {
                throw new InvalidOperationException("Could not determine the active window bounds.");
            }

            using var bitmap = new Bitmap(rect.Width, rect.Height);
            using (var graphics = Graphics.FromImage(bitmap))
            {
                graphics.CopyFromScreen(rect.Left, rect.Top, 0, 0, new Size(rect.Width, rect.Height));
            }

            using var stream = new MemoryStream();
            bitmap.Save(stream, ImageFormat.Png);
            return Task.FromResult(new ActiveWindowScreenshot(stream.ToArray(), GetWindowTitle(foregroundWindow)));
        }
        finally
        {
            if (previousDpiContext != IntPtr.Zero)
            {
                TrySetThreadDpiAwarenessContext(previousDpiContext);
            }
        }
    }

    private static bool TryGetWindowBounds(IntPtr window, out NativeRect rect)
    {
        if (DwmGetWindowAttribute(
                window,
                DwmWindowAttributeExtendedFrameBounds,
                out rect,
                Marshal.SizeOf<NativeRect>()) == 0 &&
            rect.Width > 0 &&
            rect.Height > 0)
        {
            return true;
        }

        return GetWindowRect(window, out rect) &&
            rect.Width > 0 &&
            rect.Height > 0;
    }

    private static IntPtr TrySetThreadDpiAwarenessContext(IntPtr dpiContext)
    {
        try
        {
            return SetThreadDpiAwarenessContext(dpiContext);
        }
        catch (EntryPointNotFoundException)
        {
            return IntPtr.Zero;
        }
    }

    private static string? GetWindowTitle(IntPtr window)
    {
        var length = GetWindowTextLength(window);
        if (length <= 0)
        {
            return null;
        }

        var buffer = new System.Text.StringBuilder(length + 1);
        return GetWindowText(window, buffer, buffer.Capacity) > 0
            ? buffer.ToString()
            : null;
    }

    [DllImport("user32.dll")]
    private static extern IntPtr GetForegroundWindow();

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool GetWindowRect(IntPtr hWnd, out NativeRect lpRect);

    [DllImport("dwmapi.dll", PreserveSig = true)]
    private static extern int DwmGetWindowAttribute(
        IntPtr hwnd,
        int dwAttribute,
        out NativeRect pvAttribute,
        int cbAttribute);

    [DllImport("user32.dll")]
    private static extern IntPtr SetThreadDpiAwarenessContext(IntPtr dpiContext);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern int GetWindowText(IntPtr hWnd, System.Text.StringBuilder lpString, int nMaxCount);

    [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern int GetWindowTextLength(IntPtr hWnd);

    [StructLayout(LayoutKind.Sequential)]
    private readonly struct NativeRect
    {
        public readonly int Left;
        public readonly int Top;
        public readonly int Right;
        public readonly int Bottom;

        public int Width => Right - Left;

        public int Height => Bottom - Top;
    }
}