Improve icon selection to pick the best-fitting icon size from ICO files

src/NotifyIconWpf/Interop/SystemInfo.cs

@@ -64,48 +64,20 @@ public static Point ScaleWithDpi(this Point point)
             };
         }
 
-        /// <summary>
-        /// Scale the supplied size to the current DPI settings
-        /// </summary>
-        /// <param name="size"></param>
-        /// <returns>Size</returns>
-        [Pure]
-        public static Size ScaleWithDpi(this Size size)
-        {
-            return new Size
-            {
-                Height = (int)(size.Height / DpiFactorY),
-                Width = (int)(size.Width / DpiFactorX)
-            };
-        }
-
         #region SmallIconSize
 
-        private static Size? _smallIconSize = null;
-
         private const int CXSMICON = 49;
         private const int CYSMICON = 50;
 
         /// <summary>
         /// Gets a value indicating the recommended size, in pixels, of a small icon
         /// </summary>
-        public static Size SmallIconSize
-        {
-            get
+        public static Size SmallIconSize =>
+            new()
             {
-                if (!_smallIconSize.HasValue)
-                {
-                    Size smallIconSize = new Size
-                    {
-                        Height = WinApi.GetSystemMetrics(CYSMICON),
-                        Width = WinApi.GetSystemMetrics(CXSMICON)
-                    };
-                    _smallIconSize = smallIconSize.ScaleWithDpi();
-                }
-
-                return _smallIconSize.Value;
-            }
-        }
+                Height = WinApi.GetSystemMetrics(CYSMICON),
+                Width = WinApi.GetSystemMetrics(CXSMICON)
+            };
 
         #endregion
     }

src/NotifyIconWpf/Util.cs

@@ -4,8 +4,11 @@
 // Contact and Information: http://www.hardcodet.net
 
 using System;
+using System.Collections.Generic;
 using System.ComponentModel;
 using System.Drawing;
+using System.IO;
+using System.Linq;
 using System.Windows;
 using System.Windows.Input;
 using System.Windows.Media;
@@ -161,7 +164,135 @@ public static Icon ToIcon(this ImageSource imageSource)
             }
 
             Interop.Size iconSize = SystemInfo.SmallIconSize;
-            return new Icon(streamInfo.Stream, new System.Drawing.Size(iconSize.Width, iconSize.Height));
+
+            using var stream = streamInfo.Stream;
+            var bestIcon = GetBestFitIcon(stream, new System.Drawing.Size(iconSize.Width, iconSize.Height));
+            return bestIcon;
+        }
+
+        /// <summary>
+        /// Finds the best fitting icon from a stream based on the desired size.
+        /// </summary>
+        /// <param name="iconStream">The stream containing the icon data.</param>
+        /// <param name="desiredSize">The desired size of the icon.</param>
+        /// <returns>The best fitting icon as an <see cref="Icon"/> object.</returns>
+        /// <exception cref="InvalidDataException">Thrown if the ICO file header is invalid or contains no images.</exception>
+        /// <exception cref="EndOfStreamException">Thrown if the complete icon image data could not be read.</exception>
+        private static Icon GetBestFitIcon(Stream iconStream, System.Drawing.Size desiredSize)
+        {
+            // Read the icon entries
+            iconStream.Seek(0, SeekOrigin.Begin);
+            using var reader = new BinaryReader(iconStream);
+
+            // Read and validate the ICONDIR header
+            var idReserved = reader.ReadUInt16(); // Reserved (must be 0)
+            var idType = reader.ReadUInt16();     // Resource Type (1 for icons)
+            var idCount = reader.ReadUInt16();          // Number of images
+
+            if (idReserved != 0 || idType != 1)
+                throw new InvalidDataException("Invalid ICO file header.");
+
+            if (idCount == 0)
+                throw new InvalidDataException("The ICO file contains no images.");
+
+            // Read ICONDIRENTRYs
+            var iconEntries = new List<IconEntry>();
+            for (var i = 0; i < idCount; i++)
+            {
+                var entry = new IconEntry
+                {
+                    Width = reader.ReadByte(),
+                    Height = reader.ReadByte(),
+                    ColorCount = reader.ReadByte(),
+                    Reserved = reader.ReadByte(),
+                    Planes = reader.ReadUInt16(),
+                    BitCount = reader.ReadUInt16(),
+                    BytesInRes = reader.ReadUInt32(),
+                    ImageOffset = reader.ReadUInt32()
+                };
+
+                // Adjust for 256x256 icons, which are stored with width and height as 0
+                if (entry.Width == 0) entry.Width = 256;
+                if (entry.Height == 0) entry.Height = 256;
+
+                iconEntries.Add(entry);
+            }
+
+            // Find icons greater than or equal to the desired size
+            IconEntry bestEntry;
+            var largerOrEqualIcons = iconEntries
+                .Where(entry => entry.Width >= desiredSize.Width && entry.Height >= desiredSize.Height)
+                .OrderBy(entry => entry.Width * entry.Height)
+                .ThenBy(entry => entry.Width)
+                .ThenBy(entry => entry.Height)
+                .ToList();
+
+            if (largerOrEqualIcons.Any())
+            {
+                // Select the smallest icon among those larger or equal to the desired size
+                bestEntry = largerOrEqualIcons.First();
+            }
+            else
+            {
+                // No larger icons; select the largest icon smaller than the desired size
+                var smallerIcons = iconEntries
+                    .Where(entry => entry.Width < desiredSize.Width && entry.Height < desiredSize.Height)
+                    .OrderByDescending(entry => entry.Width * entry.Height)
+                    .ThenByDescending(entry => entry.Width)
+                    .ThenByDescending(entry => entry.Height)
+                    .ToList();
+
+                // If no icons are smaller or larger, select any available icon (unlikely case)
+                bestEntry = smallerIcons.Any() ? smallerIcons.First() : iconEntries.FirstOrDefault();
+            }
+
+            if (bestEntry == null)
+                return null;
+
+            // Read the image data of the selected icon
+            var iconImageData = new byte[bestEntry.BytesInRes];
+            iconStream.Seek(bestEntry.ImageOffset, SeekOrigin.Begin);
+            var bytesRead = iconStream.Read(iconImageData, 0, (int)bestEntry.BytesInRes);
+            if (bytesRead != bestEntry.BytesInRes)
+                throw new EndOfStreamException("Could not read the complete icon image data.");
+
+            // Create a new .ico file with the single best-matching image
+            using var destStream = new MemoryStream();
+            using var writer = new BinaryWriter(destStream);
+
+            writer.Write((ushort)0); // idReserved
+            writer.Write((ushort)1); // idType
+            writer.Write((ushort)1); // idCount
+
+            writer.Write(bestEntry.Width == 256 ? (byte)0 : (byte)bestEntry.Width);
+            writer.Write(bestEntry.Height == 256 ? (byte)0 : (byte)bestEntry.Height);
+            writer.Write(bestEntry.ColorCount);
+            writer.Write(bestEntry.Reserved);
+            writer.Write(bestEntry.Planes);
+            writer.Write(bestEntry.BitCount);
+            writer.Write(bestEntry.BytesInRes);
+            writer.Write((uint)(6 + 16)); // Image data offset
+
+            // Write the image data
+            writer.Write(iconImageData);
+
+            destStream.Seek(0, SeekOrigin.Begin);
+            return new Icon(destStream);
+        }
+
+        /// <summary>
+        /// Represents an entry in the icon directory.
+        /// </summary>
+        private class IconEntry
+        {
+            public int Width;
+            public int Height;
+            public byte ColorCount;
+            public byte Reserved;
+            public ushort Planes;
+            public ushort BitCount;
+            public uint BytesInRes;
+            public uint ImageOffset;
         }
 
         #endregion