Merge pull request #3124 from AvaloniaUI/fix-win7-dpi-error

Fallback to GDI when ShCore.dll isnt found (win7/8).

src/Windows/Avalonia.Win32/Interop/UnmanagedMethods.cs

@@ -1031,6 +1031,9 @@ namespace Avalonia.Win32.Interop
         [DllImport("shcore.dll")]
         public static extern long GetDpiForMonitor(IntPtr hmonitor, MONITOR_DPI_TYPE dpiType, out uint dpiX, out uint dpiY);
 
+        [DllImport("gdi32.dll")]
+        public static extern int GetDeviceCaps(IntPtr hdc, DEVICECAP nIndex);
+
         [DllImport("shcore.dll")]
         public static extern void GetScaleFactorForMonitor(IntPtr hMon, out uint pScale);
 
@@ -1147,6 +1150,12 @@ namespace Avalonia.Win32.Interop
             }
         }
 
+        public enum DEVICECAP
+        {
+            HORZRES = 8,
+            DESKTOPHORZRES = 118
+        }
+
         public enum PROCESS_DPI_AWARENESS
         {
             PROCESS_DPI_UNAWARE = 0,

src/Windows/Avalonia.Win32/ScreenImpl.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using Avalonia.Platform;
+using Avalonia.Win32.Interop;
 using static Avalonia.Win32.Interop.UnmanagedMethods;
 
 namespace Avalonia.Win32
@@ -28,9 +29,28 @@ namespace Avalonia.Win32
                         (IntPtr monitor, IntPtr hdcMonitor, ref Rect lprcMonitor, IntPtr data) =>
                         {
                             MONITORINFO monitorInfo = MONITORINFO.Create();
-                            if (GetMonitorInfo(monitor,ref monitorInfo))
+                            if (GetMonitorInfo(monitor, ref monitorInfo))
                             {
-                                GetDpiForMonitor(monitor, MONITOR_DPI_TYPE.MDT_EFFECTIVE_DPI, out var x, out _);
+                                var dpi = 1.0;
+
+                                var shcore = LoadLibrary("shcore.dll");
+                                var method = GetProcAddress(shcore, nameof(GetDpiForMonitor));
+                                if (method != IntPtr.Zero)
+                                { 
+                                    GetDpiForMonitor(monitor, MONITOR_DPI_TYPE.MDT_EFFECTIVE_DPI, out var x, out _);
+                                    dpi = (double)x;
+                                }
+                                else
+                                {
+                                    var hdc = GetDC(IntPtr.Zero);
+
+                                    double virtW = GetDeviceCaps(hdc, DEVICECAP.HORZRES);
+                                    double physW = GetDeviceCaps(hdc, DEVICECAP.DESKTOPHORZRES);
+
+                                    dpi = (96d * physW / virtW);
+
+                                    ReleaseDC(IntPtr.Zero, hdc);
+                                }
 
                                 RECT bounds = monitorInfo.rcMonitor;
                                 RECT workingArea = monitorInfo.rcWork;
@@ -40,7 +60,7 @@ namespace Avalonia.Win32
                                     new PixelRect(workingArea.left, workingArea.top, workingArea.right - workingArea.left,
                                         workingArea.bottom - workingArea.top);
                                 screens[index] =
-                                    new WinScreen((double)x / 96.0d, avaloniaBounds, avaloniaWorkArea, monitorInfo.dwFlags == 1,
+                                    new WinScreen(dpi / 96.0d, avaloniaBounds, avaloniaWorkArea, monitorInfo.dwFlags == 1,
                                         monitor);
                                 index++;
                             }