Generate transparent PNG c#

You can do something like this:

Bitmap bmp = new Bitmap(300, 300);
Graphics g = Graphics.FromImage(bmp);

g.Clear(Color.Transparent);
g.FillRectangle(Brushes.Red, 100, 100, 100, 100);

g.Flush();
bmp.Save("test.png", System.Drawing.Imaging.ImageFormat.Png);

Take a look at Can you make an alpha transparent PNG with C#?