WinRT C# code that loads some file, and save it back with EXIF metadata

addexif.cs

using System;
using System.Threading.Tasks;
using Windows.Foundation;
using Windows.Graphics.Imaging;
using Windows.Storage;

public static async Task AddExifToFile()
{
    // get an input file
    var inputPath = @"d:\temp\tiger.png";
    var input = await StorageFile.GetFileFromPathAsync(inputPath);
    using var inputStream = await input.OpenAsync(FileAccessMode.Read);
    var decoder = await BitmapDecoder.CreateAsync(inputStream);
    var data = await decoder.GetPixelDataAsync();

    // save to output file
    var folder = await StorageFolder.GetFolderFromPathAsync(Path.GetDirectoryName(inputPath));
    var output = await folder.CreateFileAsync("tiger2.png", CreationCollisionOption.ReplaceExisting);
    using var outputStream = await output.OpenAsync(FileAccessMode.ReadWrite);
    var encoder = await BitmapEncoder.CreateAsync(BitmapEncoder.PngEncoderId, outputStream);
    encoder.SetPixelData(decoder.BitmapPixelFormat, decoder.BitmapAlphaMode, decoder.PixelWidth, decoder.PixelHeight, decoder.DpiX, decoder.DpiX, data.DetachPixelData());

    // add a metadata
    var propertySet = new BitmapPropertySet
    {
        { "System.Title", new BitmapTypedValue("This is a tiger", PropertyType.String) }
        // or 
        //{ "/xmp/dc:title", new BitmapTypedValue("This is a tiger", PropertyType.String) }
    };
    await encoder.BitmapProperties.SetPropertiesAsync(propertySet);

    await encoder.FlushAsync();
}