An assembly can be associated with only one unmanaged resource. This means that calling AssemblyBuilder.DefineVersionInfoResource(string, string, string, string, string) or AssemblyBuilder.DefineUnmanagedResource(Byte[]) after either one of the methods was called previously will throw the System.ArgumentException. Multiple unmanaged resources need to be merged with a tool such as the Microsoft ResMerge utility (not supplied with the common language runtime).
Starting with the net_v20sp1_long, this member no longer requires System.Security.Permissions.ReflectionPermission with the System.Security.Permissions.ReflectionPermissionFlag.ReflectionEmit flag. (See Security Issues in Reflection Emit.) To use this functionality, your application should target the net_v35_long or later.