.net - NullReferenceException WIA C# -

when run code below error

nullreferenceexception unhandled..

below code

private void showscannerdialog()         {             this.scanner = null;             this.imageitem = null;              this.getscanner();              wia.commondialog dialog = new wia.commondialog();              items imageitems = dialog.showselectitems(this.scanner,  wiaimageintent.textintent, wiaimagebias.minimizesize, false, true, false);             if (imageitems != null)             {                                     foreach (item item in imageitems)                 {                     imageitem = item;                     break;                 }             }         } 

thanks

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// complete code

using system; using system.collections.generic; using system.componentmodel; using system.data; using system.drawing; using system.linq; using system.text; using system.windows.forms; using system.io; using system.collections; using system.drawing.imaging; using system.runtime.interopservices;  namespace windowsformsapplication1 {     public partial class form1 : form     {          string scannername;          private device scanner;         private item imageitem;           private const int adf = 1;          private const int flatbed = 2;          private const int device_name_property_id = 7;         private const int document_handling_property_id = 3088;            public form1()         {             initializecomponent();         }          private void form1_load(object sender, eventargs e)         {             showscannerdialog();         }         public static string[] getscannerlist()         {             arraylist scannerlist = new arraylist();              devicemanager devicemanager = new devicemanager();                if (devicemanager.deviceinfos.count == 0)             {                 return new string[0]; // return empty string array             }              foreach (deviceinfo deviceinfo in devicemanager.deviceinfos)             {                 if (deviceinfo.type == wiadevicetype.scannerdevicetype)                 {                     device device = deviceinfo.connect();                       scannerlist.add(getdeviceproperty(device, device_name_property_id));                      }             }             return (string[])scannerlist.toarray(typeof(string));         }           public int init(string scannername)         {             this.scannername = scannername;              this.showscannerdialog();             if (this.imageitem == null)             {                 return 0;             }             else             {                 return 1;             }         }                 public int scan(string filepath)         {              tiff tiff = new tiff(filepath);              bool adf = false;             int numscans = 0;             arraylist tempfiles = new arraylist();              // determine if scanner set use adf             string dochandlingselect = getdeviceproperty(this.scanner, document_handling_property_id);             if (dochandlingselect != "")             {                 try                 {                     if ((int.parse(dochandlingselect) & adf) == adf)                     {                         adf = true;                     }                 }                 catch { }             }              while (true)             {                  string tempfile = path.gettempfilename();                 tempfiles.add(tempfile);                    file.delete(tempfile);                  imagefile wiafile = (imagefile)imageitem.transfer(formatid.wiaformattiff);                  wiafile.savefile(tempfile);                 image tempimage = image.fromfile(tempfile);                 tiff.addimage(tempimage);                 tempimage.dispose();                  numscans++;                   if (!adf)                 {                     dialogresult result =                         messagebox.show("do wish scan page , save same file?",                         "scanner message", messageboxbuttons.yesno);                     if (result == dialogresult.no)                     {                         break;                     }                      this.showscannerdialog();                     if (this.imageitem == null)                     {                         break;                     }                 }             }              tiff.close();               foreach (string f in tempfiles.toarray(typeof(string)))             {                 file.delete(f);             }             marshal.releasecomobject(imageitem);               if (numscans == 0)             {                 throw new exception("nothing scanned.");             }              return 1;         }           private void getscanner()         {              devicemanager devicemanager = new devicemanager();              foreach (deviceinfo deviceinfo in devicemanager.deviceinfos)             {                 if (deviceinfo.type == wiadevicetype.scannerdevicetype)                 {                     device device = deviceinfo.connect();                     if (this.scannername == getdeviceproperty(device, device_name_property_id))                     {                         this.scanner = device;                     }                 }             }            }          private void showscannerdialog()         {             this.scanner = null;             this.imageitem = null;                this.getscanner();               wia.commondialog dialog = new wia.commondialog();              items imageitems = dialog.showselectitems(this.scanner, wiaimageintent.textintent, wiaimagebias.minimizesize, false, true, false);             if (imageitems != null)             {                  foreach (item item in imageitems)                 {                     imageitem = item;                     break;                 }             }         }          private static string getdeviceproperty(device device, int propteryid)         {             string retval = "";                   foreach (property prop in device.properties)                 {                     if (prop.propertyid == propteryid)                     {                         retval = prop.get_value().tostring();                         break;                     }                 }                 return retval;          }        } } 

this.scanner set null when pass dialog.showselectitems(). function may trying use without checking if null.