A simple class that converts a image to a icon in c# without losing image color data, unlike System.Drawing.Icon; ico with png data requires Windows Vista or above

gistfile1.cs

class PngIconConverter
    {
        /* input image with width = height is suggested to get the best result */
        /* png support in icon was introduced in Windows Vista */
        public static bool Convert(System.IO.Stream input_stream, System.IO.Stream output_stream, int size, bool keep_aspect_ratio = false)
        {
            System.Drawing.Bitmap input_bit = (System.Drawing.Bitmap)System.Drawing.Bitmap.FromStream(input_stream);
            if (input_bit != null)
            {
                int width, height;
                if (keep_aspect_ratio)
                {
                    width = size;
                    height = input_bit.Height / input_bit.Width * size;
                }
                else
                {
                    width = height = size;
                }
                System.Drawing.Bitmap new_bit = new System.Drawing.Bitmap(input_bit, new System.Drawing.Size(width, height));
                if (new_bit != null)
                {
                    // save the resized png into a memory stream for future use
                    System.IO.MemoryStream mem_data = new System.IO.MemoryStream();
                    new_bit.Save(mem_data, System.Drawing.Imaging.ImageFormat.Png);

                    System.IO.BinaryWriter icon_writer = new System.IO.BinaryWriter(output_stream);
                    if (output_stream != null && icon_writer != null)
                    {
                        // 0-1 reserved, 0
                        icon_writer.Write((byte)0);
                        icon_writer.Write((byte)0);

                        // 2-3 image type, 1 = icon, 2 = cursor
                        icon_writer.Write((short)1);

                        // 4-5 number of images
                        icon_writer.Write((short)1);

                        // image entry 1
                        // 0 image width
                        icon_writer.Write((byte)width);
                        // 1 image height
                        icon_writer.Write((byte)height);

                        // 2 number of colors
                        icon_writer.Write((byte)0);

                        // 3 reserved
                        icon_writer.Write((byte)0);

                        // 4-5 color planes
                        icon_writer.Write((short)0);

                        // 6-7 bits per pixel
                        icon_writer.Write((short)32);

                        // 8-11 size of image data
                        icon_writer.Write((int)mem_data.Length);

                        // 12-15 offset of image data
                        icon_writer.Write((int)(6 + 16));

                        // write image data
                        // png data must contain the whole png data file
                        icon_writer.Write(mem_data.ToArray());

                        icon_writer.Flush();

                        return true;
                    }
                }
                return false;
            }
            return false;
        }

        public static bool Convert(string input_image, string output_icon, int size, bool keep_aspect_ratio = false)
        {
            System.IO.FileStream input_stream = new System.IO.FileStream(input_image, System.IO.FileMode.Open);
            System.IO.FileStream output_stream = new System.IO.FileStream(output_icon, System.IO.FileMode.OpenOrCreate);

            bool result = Convert(input_stream, output_stream, size, keep_aspect_ratio);

            input_stream.Close();
            output_stream.Close();

            return result;
        }
    }

PowerShell Compatible Version

Although @lextm ported the code to PowerShell (many props, Sir), they did not include all the functionality added later by other contributors such as @kipusoep, @bananbl, and @mdiller.

Here is a usable version with the additional modifications and adapted to work with PowerShell.

I didn't have time to convert all the code to PowerShell syntax like @lextm did, so I added it as a custom type and included the required dependencies and command syntax that can be difficult to figure out with PowerShell. After adding the custom type, I wrapped it in a PowerShell function inspired by @lextm.

Parameters/Parameter Sets should be added later to take full advantage of all the constructors and for adjusting other things like size, etc.

The Preserve Aspect Ratio from @ChuckSavage wasn't included in the other C# code (possibly they accounted for it in another way), so I didn't end up adding it here yet, but maybe later; or someone else could do it.

Compatibility

OS Version	PowerShell Version	.NET Version	❔ ✔️ ❌
Windows 10, ver. 2004, build 19041.572	Windows PowerShell 5.1	4.8	✔️

License

As per @darkfall, this work is licensed under CC BY-SA 4.0

Code

$TypeDefinition = @'
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Collections.Generic;
using System.Drawing.Drawing2D;

/// <summary>
/// Adapted from this gist: https://gist.github.com/darkfall/1656050
/// Provides helper methods for imaging
/// </summary>
public static class ImagingHelper
{
    /// <summary>
    /// Converts a PNG image to a icon (ico) with all the sizes windows likes
    /// </summary>
    /// <param name="inputBitmap">The input bitmap</param>
    /// <param name="output">The output stream</param>
    /// <returns>Wether or not the icon was succesfully generated</returns>
    public static bool ConvertToIcon(Bitmap inputBitmap, Stream output)
    {
        if (inputBitmap == null)
            return false;

        int[] sizes = new int[] { 256, 48, 32, 16 };

        // Generate bitmaps for all the sizes and toss them in streams
        List<MemoryStream> imageStreams = new List<MemoryStream>();
        foreach (int size in sizes)
        {
            Bitmap newBitmap = ResizeImage(inputBitmap, size, size);
            if (newBitmap == null)
                return false;
            MemoryStream memoryStream = new MemoryStream();
            newBitmap.Save(memoryStream, ImageFormat.Png);
            imageStreams.Add(memoryStream);
        }

        BinaryWriter iconWriter = new BinaryWriter(output);
        if (output == null || iconWriter == null)
            return false;

        int offset = 0;

        // 0-1 reserved, 0
        iconWriter.Write((byte)0);
        iconWriter.Write((byte)0);

        // 2-3 image type, 1 = icon, 2 = cursor
        iconWriter.Write((short)1);

        // 4-5 number of images
        iconWriter.Write((short)sizes.Length);

        offset += 6 + (16 * sizes.Length);

        for (int i = 0; i < sizes.Length; i++)
        {
            // image entry 1
            // 0 image width
            iconWriter.Write((byte)sizes[i]);
            // 1 image height
            iconWriter.Write((byte)sizes[i]);

            // 2 number of colors
            iconWriter.Write((byte)0);

            // 3 reserved
            iconWriter.Write((byte)0);

            // 4-5 color planes
            iconWriter.Write((short)0);

            // 6-7 bits per pixel
            iconWriter.Write((short)32);

            // 8-11 size of image data
            iconWriter.Write((int)imageStreams[i].Length);

            // 12-15 offset of image data
            iconWriter.Write((int)offset);

            offset += (int)imageStreams[i].Length;
        }

        for (int i = 0; i < sizes.Length; i++)
        {
            // write image data
            // png data must contain the whole png data file
            iconWriter.Write(imageStreams[i].ToArray());
            imageStreams[i].Close();
        }

        iconWriter.Flush();

        return true;
    }

    /// <summary>
    /// Converts a PNG image to a icon (ico)
    /// </summary>
    /// <param name="input">The input stream</param>
    /// <param name="output">The output stream</param
    /// <returns>Wether or not the icon was succesfully generated</returns>
    public static bool ConvertToIcon(Stream input, Stream output)
    {
        Bitmap inputBitmap = (Bitmap)Bitmap.FromStream(input);
        return ConvertToIcon(inputBitmap, output);
    }

    /// <summary>
    /// Converts a PNG image to a icon (ico)
    /// </summary>
    /// <param name="inputPath">The input path</param>
    /// <param name="outputPath">The output path</param>
    /// <returns>Wether or not the icon was succesfully generated</returns>
    public static bool ConvertToIcon(string inputPath, string outputPath)
    {
        using (FileStream inputStream = new FileStream(inputPath, FileMode.Open))
        using (FileStream outputStream = new FileStream(outputPath, FileMode.OpenOrCreate))
        {
            return ConvertToIcon(inputStream, outputStream);
        }
    }



    /// <summary>
    /// Converts an image to a icon (ico)
    /// </summary>
    /// <param name="inputImage">The input image</param>
    /// <param name="outputPath">The output path</param>
    /// <returns>Wether or not the icon was succesfully generated</returns>
    public static bool ConvertToIcon(Image inputImage, string outputPath)
    {
        using (FileStream outputStream = new FileStream(outputPath, FileMode.OpenOrCreate))
        {
            return ConvertToIcon(new Bitmap(inputImage), outputStream);
        }
    }


    /// <summary>
    /// Resize the image to the specified width and height.
    /// Found on stackoverflow: https://stackoverflow.com/questions/1922040/resize-an-image-c-sharp
    /// </summary>
    /// <param name="image">The image to resize.</param>
    /// <param name="width">The width to resize to.</param>
    /// <param name="height">The height to resize to.</param>
    /// <returns>The resized image.</returns>
    public static Bitmap ResizeImage(Image image, int width, int height)
    {
        var destRect = new Rectangle(0, 0, width, height);
        var destImage = new Bitmap(width, height);

        destImage.SetResolution(image.HorizontalResolution, image.VerticalResolution);

        using (var graphics = Graphics.FromImage(destImage))
        {
            graphics.CompositingMode = CompositingMode.SourceCopy;
            graphics.CompositingQuality = CompositingQuality.HighQuality;
            graphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
            graphics.SmoothingMode = SmoothingMode.HighQuality;
            graphics.PixelOffsetMode = PixelOffsetMode.HighQuality;

            using (var wrapMode = new ImageAttributes())
            {
                wrapMode.SetWrapMode(WrapMode.TileFlipXY);
                graphics.DrawImage(image, destRect, 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, wrapMode);
            }
        }

        return destImage;
    }
}
'@

Add-Type -TypeDefinition $TypeDefinition -ReferencedAssemblies 'System.Drawing','System.IO','System.Collections'

<#
.Synopsis
    Converts .PNG images to icons
.Description
    Converts a .PNG image to an icon
.Example
    ConvertTo-Icon -Path .\Logo.png -Destination .\Favicon.ico
#>
Function ConvertTo-Icon
{
    [CmdletBinding()]
    param(
    # The file
    [Parameter(Mandatory=$true, Position=0,ValueFromPipelineByPropertyName=$true)]
    [Alias('Fullname','File')]
    [string]$Path,
   
    # If provided, will output the icon to a location
    [Parameter(Position=1, ValueFromPipelineByPropertyName=$true)]
    [Alias('OutputFile')]
    [string]$Destination
    )
    
    Begin
    {
        If (-Not 'ImagingHelper' -as [Type])
        {
            Throw 'The custom "ImagingHelper" type is not loaded'
        }
    }
    
    Process
    {
        #region Resolve Path
        $ResolvedFile = $ExecutionContext.SessionState.Path.GetResolvedPSPathFromPSPath($Path)
        If (-not $ResolvedFile)
        {
            return
        }
        #endregion        

        [ImagingHelper]::ConvertToIcon($ResolvedFile[0].Path,$Destination)
    }
    End
    {
    }
}

Hello @darkfall, @mdiller, can I use your code to a DLL I'm developing? You could get power to change the following Code in the DLL if you wanna to change something

Sure!

great, thx, here is btw the respondity for the DLL: CSBeginnerHelp

Thanks for this!

If you get EndOfStreamException when using the Stream output do this before using it:
output.Position = 0;

Unfortunately this cannot be used on Powershell Core, System.Drawing.Bitmap is not available.