multithreading - Invoking static methods from thread in C# -

i'm trying use this great project since need scan many images process takes lot of time thinking multi-threading it.
however, since class makes actual processing of images uses static methods , manipulating objects ref i'm not sure how right. method call main thread is:

public static void scanpage(ref system.collections.arraylist codesread, bitmap bmp, int numscans, scandirection direction, barcodetype types) {     //added signature, actual class has on 1000 rows     //inside function there calls other     //static functions makes image processing } 

my question if it's safe use use function this:

list<string> filepaths = new list<string>();         parallel.for(0, filepaths.count, =>                 {                     arraylist al = new arraylist();                     barcodeimaging.scanpage(ref al, ...);                 }); 

i've spent hours debugging , of time results got correct did encounter several errors can't seem reproduce.

edit
pasted code of class here: http://pastebin.com/uee6qbhx

there's no way of telling unless know if stores values in local variables or in field (in static class, not method).

all local variables fine , instanced per call, fields not.

a bad example:

public static class testclass {     public static double data;     public static string stringdata = "";      // can, , quite often, return wrong values.     //  example returning result of f(8) instead of f(5)     //  if data changed before stringdata calculated.     public static string changestaticvariables(int x)     {         data = math.sqrt(x) + math.sqrt(x);         stringdata = data.tostring("0.000");         return stringdata;     }      // won't return wrong values, variables     //  can't changed other threads.     public static string nonstaticvariables(int x)     {         var tdata = math.sqrt(x) + math.sqrt(x);         return data.tostring("0.000");     } }