Try
string[] resourceNames = Assembly.GetExecutingAssembly().GetManifestResourceNames();
It will give you a list of all the existing resources and their full names.
This is to make sure the icon is there.
Then do
System.Drawing.Icon icon1 = new System.Drawing.Icon(Assembly.GetExecutingAssembly().GetManifestResourceStream(ModuleManager.resources.iconfilename));
if
ModuleManager.resources.iconfilename
contains the correct name.