Question

I have a C# app with a button for dragging and dropping files. I am able to take 6 files from my desktop and drop it onto the button and have it process those 6 files.

However, when I start a thread from the DragDrop event and pass the file path to a new thread started from within the DragDrop event, the file path is incorrect once the thread receives the FilePath parameter.

If I execute my code by dragging 6 text files onto my button (I had to strip a lot of code out of it for this example), I will see the following in my console:

++ Calling testthread with these params: false, TestButton,test.txt,c:\test.txt
++ Calling testthread with these params: false, TestButton,test2.txt,c:\test2.txt
++ Calling testthread with these params: false, TestButton,test3.txt,c:\test3.txt
++ Calling testthread with these params: false, TestButton,test4.txt,c:\test4.txt
++ Calling testthread with these params: false, TestButton,test5.txt,c:\test5.txt
++ Calling testthread with these params: false, TestButton,test6.txt,c:\test6.txt

The above output is correct

The following output is incorrect, notice the FilePath does not match the CleanFileName like it does in the above console output.

++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test.txt FilePath = c:\test2.txt
++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test1.txt FilePath = c:\test3.txt
++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test3.txt FilePath = c:\test4.txt
++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test4.txt FilePath = c:\test5.txt
++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test5.txt FilePath = c:\test5.txt
++ testthread Thread – CallingfromPendingUploads == false ButtonName == TestButton CleanFileName == test6.txt FilePath = c:\test5.txt

As you can see, the FilePath from thread does not match the FilePath that is being passed to the Thread before it starts. All of the FilePaths are off when compared to the filename that is passed to the Thread. And some of the FilePaths are duplicates such as text5.txt.

I have been struggling with this for hours. Can someone please tell me what I am doing wrong?

private void btnClick_DragDrop(object sender, DragEventArgs e)
{
    string[] file = (string[])e.Data.GetData(DataFormats.FileDrop);

    string ButtonName = "TestButton"

    string[] files = new string[10];

    files = (string[])e.Data.GetData(DataFormats.FileDrop);


    foreach (string file in files)
    {
        FileInfo fileInfo = new FileInfo(file);

        Console.WriteLine("++  Filename: " + fileInfo.Name + "   Date of file: " + fileInfo.CreationTime + "   Type of file: " + fileInfo.Extension + "   Size of file: " + fileInfo.Length.ToString());

        string CleanFileName = System.Web.HttpUtility.UrlEncode(fileInfo.Name.ToString());

        //Start  thread
        try
        {
            Console.WriteLine("++ Calling testthread with these params: false, " + ButtonName + "," + CleanFileName + "," + file);

            new Thread(() => testthread(false, ButtonName, CleanFileName, file)).Start();

            Console.WriteLine("++ testthead thread started @ " + DateTime.Now);
         }
         catch (Exception ipwse)
         {
             logger.Debug(ipwse.Message + " " + ipwse.StackTrace);
         }
    }
}

public void testthread(bool CalledfromPendingUploads, string ButtonName, string CleanFileName, string FilePath)
{
    Console.WriteLine("++ testthread Thread - CallingfromPendingUploads == " + CalledfromPendingUploads.ToString() + "  ButtonName == " + ButtonName + "  CleanFileName == " + CleanFileName + "  FilePath = " + FilePath);
}

Answer 1

All of your threads are sharing the same file variable.
If one of the threads only starts running after the UI thread has started the next iteration, it will use the next value of the file variable.

You need to declare a separate variable inside the loop, so that each thread will get its own variable.

For example:

foreach (string dontUse in files)
{
    string file = dontUse;
    ...
}

Since the file variable is now scoped within the loop, each iteration gets a separate variable.