如何在异步任务中使用拖放

Question

My goal is to drag and drop a file out of my app into a dropzone of a website.

For that I've made this form:

public partial class Form2 : Form
{
    private DataObject File;

    public Form2(string filePath)
    {
        InitializeComponent();

        File = new DataObject();
        File.SetFileDropList(new StringCollection() { filePath });

        labelFileName.Text = Path.GetFileName(filePath);
    }

    private void labelFileName_MouseDown(object sender, MouseEventArgs e)
    {
        labelFileName.DoDragDrop(File, DragDropEffects.Copy);
    }
}

It works if I use it like this:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        
        var form2 = new Form2("C:\\testfile.txt");
        form2.ShowDialog();
    }
}

But drag and drop dosn't work if I use it in an async Task like this:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private async void buttonRunAsync_Click(object sender, EventArgs e)
    {
        await Task.Run(() => OpenForm2());
    }

    private async Task OpenForm2()
    {
        var form2 = new Form2("C:\\testfile.txt");
        form2.ShowDialog();

        // do some stuff based on dialog result
    }
}

I think that's because it's not running in the UI thread but I haven't found a good explanation and also I'm not sure if that's the problem.

Answer 1

Using Invoke like this fixed my problem:

private async Task OpenForm2()
{
    this.Invoke((MethodInvoker)delegate
    {
        var form2 = new Form2("C:\\testfile.txt");
        var result = form2.ShowDialog();
    });
}