Install-Package System.Drawing.Common -Version 4.5.1

// 加水印
var watermarkedStream = new MemoryStream();
using (var img = Image.FromStream(File.OpenRead(@"D:\_\WatermarkDemo.png")))
{
 using (var graphic = Graphics.FromImage(img))
 {
  var font = new Font("微軟雅黑", 30, FontStyle.Bold, GraphicsUnit.Pixel);
  var color = Color.FromArgb(128, 255, 255, 255);
  var brush = new SolidBrush(color);
  var point = new Point(img.Width - 130, img.Height - 50);

  graphic.DrawString("水印在此", font, brush, point);
  img.Save(watermarkedStream, ImageFormat.Png);
 }
}

Install-Package SharpDX.Direct2D1 -Version 4.2.0
Install-Package SharpDX.Mathematics -Version 4.2.0

using D2D = SharpDX.Direct2D1;
using DWrite = SharpDX.DirectWrite;
using SharpDX;
using SharpDX.IO;
using WIC = SharpDX.WIC;

MemoryStream AddWatermark(Stream fileName, string watermarkText)
{
  using (var wic = new WIC.ImagingFactory2())
  using (var d2d = new D2D.Factory())
  using (var image = CreateWicImage(wic, fileName))
  using (var wicBitmap = new WIC.Bitmap(wic, image.Size.Width, image.Size.Height, WIC.PixelFormat.Format32bppPBGRA, WIC.BitmapCreateCacheOption.CacheOnDemand))
  using (var target = new D2D.WicRenderTarget(d2d, wicBitmap, new D2D.RenderTargetProperties()))
  using (var bmpPicture = D2D.Bitmap.FromWicBitmap(target, image))
  using (var dwriteFactory = new SharpDX.DirectWrite.Factory())
  using (var brush = new D2D.SolidColorBrush(target, new Color(0xff, 0xff, 0xff, 0x7f)))
  {
    target.BeginDraw();
    {
      target.DrawBitmap(bmpPicture, new RectangleF(0, 0, target.Size.Width, target.Size.Height), 1.0f, D2D.BitmapInterpolationMode.Linear);
      target.DrawRectangle(new RectangleF(0, 0, target.Size.Width, target.Size.Height), brush);
      var textFormat = new DWrite.TextFormat(dwriteFactory, "微軟雅黑", DWrite.FontWeight.Bold, DWrite.FontStyle.Normal, 30.0f);
      target.DrawText(watermarkText, textFormat, new RectangleF(target.Size.Width - 130, target.Size.Height - 50, int.MaxValue, int.MaxValue), brush);
    }
    target.EndDraw();

    var ms = new MemoryStream();
    SaveD2DBitmap(wic, wicBitmap, ms);
    return ms;
  }
}

void SaveD2DBitmap(WIC.ImagingFactory wicFactory, WIC.Bitmap wicBitmap, Stream outputStream)
{
  using (var encoder = new WIC.BitmapEncoder(wicFactory, WIC.ContainerFormatGuids.Png))
  {
    encoder.Initialize(outputStream);
    using (var frame = new WIC.BitmapFrameEncode(encoder))
    {
      frame.Initialize();
      frame.SetSize(wicBitmap.Size.Width, wicBitmap.Size.Height);

      var pixelFormat = wicBitmap.PixelFormat;
      frame.SetPixelFormat(ref pixelFormat);
      frame.WriteSource(wicBitmap);

      frame.Commit();
      encoder.Commit();
    }
  }
}

WIC.FormatConverter CreateWicImage(WIC.ImagingFactory wicFactory, Stream stream)
{
  using (var decoder = new WIC.PngBitmapDecoder(wicFactory))
  {
    var decodeStream = new WIC.WICStream(wicFactory, stream);
    decoder.Initialize(decodeStream, WIC.DecodeOptions.CacheOnLoad);
    using (var decodeFrame = decoder.GetFrame(0))
    {
      var converter = new WIC.FormatConverter(wicFactory);
      converter.Initialize(decodeFrame, WIC.PixelFormat.Format32bppPBGRA);
      return converter;
    }
  }
}

File.WriteAllBytes(@"D:\_\Demo2.png", AddWatermark(File.OpenRead(@"D:\_\WatermarkDemo.png"), "水印在此").ToArray());