Question

I have this code first the constructor and button click event:

using System.Text;
using System.Windows.Forms;
using System.Xml;
using System.Xml.Linq;
using System.Xml.XPath;
using System.IO;
using System.Collections;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        DirectoryInfo dirinf = new DirectoryInfo(@"C:\");
        List<FileSystemInfo> fsi = new List<FileSystemInfo>();

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            backgroundWorker1.RunWorkerAsync();
            button1.Enabled = false;
        }

Then i have a search function and a backgroundowrker DoWork event:

public void Search(string strExtension,
                            DirectoryInfo di,
                            List<FileSystemInfo> pResult)
        {
            try
            {

                foreach (FileInfo fi in di.GetFiles())
                {
                    if (InvokeRequired)
                    {
                        BeginInvoke(new Action(() => label2.Text = fi.Name));
                    }
                    if (fi.Name == "MessageLog.xsl")
                    {
                        foreach (FileInfo fii in di.GetFiles())
                        {
                        if (fii.Extension == strExtension)
                            pResult.Add(fii);
                        }
                        if (InvokeRequired)
                        {
                            BeginInvoke(new Action(() => textBox1.AppendText("Number Of History Files Found: ===> " + pResult.Count.ToString() + Environment.NewLine)));
                        }

                    }
                }

                    foreach (DirectoryInfo diChild in di.GetDirectories())
                        Search(strExtension, diChild, pResult);

            }
            catch (Exception e)
            {
            }
        }

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            Search(".xml", dirinf, fsi);
            for (int i = 0; i < fsi.Count; i++)
            {
                if (InvokeRequired)
                {
                    BeginInvoke(new Action(() => textBox1.AppendText(fsi[i - 1].FullName + Environment.NewLine)));
                }

            }
        }

When its getting to the part in the DoWork event:

for (int i = 0; i < fsi.Count; i++)
                {
                    if (InvokeRequired)
                    {
                        BeginInvoke(new Action(() => textBox1.AppendText(fsi[i - 1].FullName + Environment.NewLine)));
                    }

                }

After one or two itertions its throwing exception in the Program.cs on the line:

Application.Run(new Form1());

Exception has been thrown by the target of an invocation

Solved it by reporting to the backgroundworker completed event:

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
        {
            Search(".xml", dirinf, fsi);
            backgroundWorker1.ReportProgress(100);

        }

        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
        {
            for (int i = 0; i < fsi.Count; i++)
            {                
                    textBox1.AppendText(fsi[i].FullName + Environment.NewLine);
            }
        }

Working just good.

Answer 1

You have already solved the issue (correctly, this should be done in the Completed event).

What remains is an explanation:

You are using the variable i , that is local to the DoWork method, inside the Action lambda. This means all code uses 1 shared (boxed) version of the variable. It’s called ‘closing over the loop var’.

The symptoms fit: the lambdas are executed async, when the 2nd or 3rd starts executing the main loop has already made i == fsi.Count. The (inner) exception should be “Index out of range”.

for (int i = 0; i < fsi.Count; i++)
{
    if (InvokeRequired)
    {
        BeginInvoke(new Action(() => 
            textBox1.AppendText(fsi[i - 1].FullName   // captured 'i'
            + Environment.NewLine)));
    }
}

It can be fixed this way:

for (int i = 0; i < fsi.Count; i++)
{
    if (InvokeRequired)   
    {
        int iCopy = i;     // 1 instance per loop

        BeginInvoke(new Action(() => 
            textBox1.AppendText(fsi[iCopy - 1].FullName  
            + Environment.NewLine)));
    }
}