如何以共享库的形式异步返回函数的进度

So I want to make a function for download a file in golang using this method below, I build this golang project into C .dll using

go build -buildmode=c-shared -o patcher.dll main.go

I managed to use this function on my C# application to get progress of file download, my current function (DownloadFfile) works if I just print it directly using DownloadFile() , however i want to get the progress asynchronously on my C# application , but i can't get the value directly, so I think I need to return integer of progress from my golang application but if I do that the function just go 1 times (the last result of progress)

The question is how to make my go func DownloadFile being called 1 times on my C# application but I can still keep track the progress ? Any help will be appreciated , thank you.

func DownloadFile(){
    // create client
    client := grab.NewClient()
    req, _ := grab.NewRequest(".", "http://www.golang-book.com/public/pdf/gobook.pdf")

    // start download
    fmt.Printf("Downloading %v...
", req.URL())
    resp := client.Do(req)
    fmt.Printf("  %v
", resp.HTTPResponse.Status)

    // start UI loop
    t := time.NewTicker(500 * time.Millisecond)
    defer t.Stop()

Loop:
    for {
        select {
        case <-t.C:
            fmt.Printf("%.2f%",
                //resp.BytesComplete(),
                //resp.Size,
                100*resp.Progress())

        case <-resp.Done:
            // download is complete
            break Loop
        }
    }

    // check for errors
    if err := resp.Err(); err != nil {
        fmt.Fprintf(os.Stderr, "Download failed: %v
", err)
        os.Exit(1)
    }

    // fmt.Printf("Download saved to ./%v 
", resp.Filename)

    // Output:
    // Downloading http://www.golang-book.com/public/pdf/gobook.pdf...
    //   200 OK
    //   transferred 42970 / 2893557 bytes (1.49%)
    //   transferred 1207474 / 2893557 bytes (41.73%)
    //   transferred 2758210 / 2893557 bytes (95.32%)
    // Download saved to ./gobook.pdf
}

So, after digging out Google, I found the answer, I need to make setter and getter "a like" on Go like below.

var Progress int
var DownloadSpeed int

//export DownloadFile
func DownloadFile(){
    // create client
    client := grab.NewClient()
    req, _ := grab.NewRequest(".", "https://upload.wikimedia.org/wikipedia/commons/d/d6/Wp-w4-big.jpg")

    // start download
    fmt.Printf("Downloading %v...
", req.URL())
    resp := client.Do(req)
    fmt.Printf("  %v
", resp.HTTPResponse.Status)

    // start UI loop
    t := time.NewTicker(500 * time.Millisecond)
    defer t.Stop()

Loop:
    for {
        select {
        case <-t.C:

            //progress = 100*(resp.Progress())

            SetProgressValue(int(resp.Progress() * 100))
            SetDownloadSpeedValue(int(resp.BytesPerSecond()))
            //fmt.Println(progress)
        case <-resp.Done:
            // download is complete
            SetProgressValue(100)
            //fmt.Println(Progress)
            break Loop
        }
    }

    // check for errors
    if err := resp.Err(); err != nil {
        fmt.Fprintf(os.Stderr, "Download failed: %v
", err)
        os.Exit(1)
    }

    fmt.Printf("Download saved to ./%v 
", resp.Filename)
    fmt.Println("Completed")
}

//export ProgressValue
func ProgressValue() int {
    return Progress
}

//export SetProgressValue
func SetProgressValue(val int) {
    Progress = val
}

Then usage in C# :

 void worker_DoWork(object sender, DoWorkEventArgs e) {
    [DllImport(@"M:\GolangProjects\PatcherDLL\patcher.dll", EntryPoint = "ProgressValue")]
     static extern int ProgressValue();

    public partial class MainWindow : Window {
    var task = Task.Factory.StartNew(() => {
                    DownloadFile();
                });

                while (!task.IsCompleted)
                {
                    Thread.Sleep(100);

                    string downloadSpeedFormatted = "";

                    if (DownloadSpeedValue()/1000 > 999)
                    {
                        downloadSpeedFormatted = Math.Round((double) DownloadSpeedValue() / 1000000, 2) + " MB/s";
                    } else
                    {
                        downloadSpeedFormatted = DownloadSpeedValue() / 1000 + " kb/s";
                    }

                    Dispatcher.BeginInvoke(new Action(delegate {
                        progressbar1.Value = ProgressValue();
                        progressPercent1.Text = ProgressValue() + "%";
                        downloadSpeeds.Content = downloadSpeedFormatted;
                    }));
                }
    }
}