Did you call MagickNET.Initialize() at the start of your program as explained here? The new release has been publish so you could use this code:
using (MagickImageCollection collection = new MagickImageCollection())
{
MagickReadSettings settings = new MagickReadSettings();
settings.Density = new MagickGeometry(144, 144);
collection.Read(file, settings);
int id = 0;
foreach (MagickImage image in collection)
{
image.Write(destFolder + "img" + id + ".png");
id++;
}
}