BitmapSource to expand while maintaining the DPI.

BitmapSourceExtensions.cs

public static BitmapSource Resize(this BitmapSource source, int resolution)
{
    var scale = resolution / source.DpiX;

    // サイズ変更
    var transformedBitmap = new TransformedBitmap();
    transformedBitmap.BeginInit();
    transformedBitmap.Source = source;
    transformedBitmap.Transform = new ScaleTransform(scale, scale);
    transformedBitmap.EndInit();

    // 一旦バイト列へ変換し
    int stride = transformedBitmap.PixelWidth * (transformedBitmap.Format.BitsPerPixel + 7) / 8;
    byte[] data = new byte[stride * transformedBitmap.PixelHeight];
    transformedBitmap.CopyPixels(data, stride, 0);

    // DPIを指定サイズに変更したイメージにコピーする
    WriteableBitmap target = new WriteableBitmap(
        transformedBitmap.PixelWidth,
        transformedBitmap.PixelHeight,
        resolution, resolution,
        transformedBitmap.Format, null);
    target.WritePixels(
        new Int32Rect(0, 0, transformedBitmap.PixelWidth, transformedBitmap.PixelHeight),
        data, stride, 0);
    target.Freeze();

    return target;
}