NullReferenceException WIA C#

问题

When I run the code below I get this error

NullreferenceException was unhandled..

below is my 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

---

// 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 an 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 the scanner is set to use an 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 you wish to scan another page and save it to the 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 was 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;

        } 


    }
}

回答1:

this.scanner is set to null when you pass it to dialog.ShowSelectItems(). The function may be trying to use it without checking if it is null.

来源：https://stackoverflow.com/questions/5098738/nullreferenceexception-wia-c-sharp