Magick.NET reads every entry of an ICO or CUR file and writes multi-size icons from one image with a single setting. Native ImageMagick is bundled in the NuGet package.
ImageSharp is pure managed code and gives per-entry control (PNG or BMP storage, bit depth, cursor hotspot). ICO and CUR support starts in version 4.0. Check the Six Labors Split License for commercial use.
System.Drawing.Icon is part of .NET on Windows: load the entry closest to a size, convert it to a Bitmap, or extract the icon of an EXE or DLL. It can’t build new ICO files.
ImageSharpRead and write Windows ICO icons and CUR cursors in C# with ImageSharp 4: list entries, export each size to PNG, build a multi-size ICO, read cursor hotspots.
Magick.NETRead and write Windows ICO icons in C# with Magick.NET (ImageMagick for .NET): list icon sizes, convert ICO to PNG, create a multi-size favicon.ico, with code.
System.Drawing.IconLoad Windows ICO files in C# with System.Drawing.Icon: pick the entry closest to a size, convert an icon to a PNG bitmap, extract icons from EXE and DLL files.