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

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                using (var dialog = new FormDialog())
                {
                    var task = DoItNowAsync(dialog);
                    dialog.ShowDialog(this);
                    task.Wait();     // It dosn't work as expected
                    // await task;   // May be so? But it not intension
                }
            }
            catch (Exception ex)
            {
                textBox1.Text = e.ToString();
            }
        }

        private async Task DoItNowAsync(Form dialog)
        {
            await Task.Run(() => { 
                Thread.Sleep(2000);
                throw new ApplicationException("Yep!");
                return true;
            });
            dialog.DialogResult = DialogResult.OK;
        }
    }

private async void button1_Click(object sender, EventArgs e)

await DoItNowAsync(dialog);

public static async Task<DialogResult> ShowDialogAsync(this Form @this)
        {
            await Task.Yield();
            if (@this.IsDisposed)
                return DialogResult.OK;
            return @this.ShowDialog();
        }

public static async Task<DialogResult> ShowDialogAsync(this Form @this)
        {
            await Task.Yield();
            if (@this.IsDisposed)
                return DialogResult.OK;
            return @this.ShowDialog();
        }

1. 
private void button1_Click(object sender, EventArgs e)
vs.
private async void button1_Click(object sender, EventArgs e)

2.
                    var task = DoItNowAsync(dialog);
                    dialog.ShowDialog(this);
                    task.Wait();     // It dosn't work as expected
 vs.
                    var showDialogTask = dialog.ShowDialogAsync();
                    await DoItNowAsync();
                    await showDialogTask;

3.
 private async Task DoItNowAsync(Form dialog)
        {
            await Task.Run(() => { 
                Thread.Sleep(2000);
                throw new ApplicationException("Yep!");
                return true;
            });
            dialog.DialogResult = DialogResult.OK;
        }
vs.
private async Task DoItNowAsync()
        {
            await Task.Run(() => {
                Thread.Sleep(2000);
                throw new ApplicationException("Yep!");
                return true;
            });
        }

private void button1_Click(object sender, EventArgs e)

        private async Task DoItNowAsync(Form dialog)
        {
            await Task.Run(() => { 
                // Thread.Sleep(2000);
                // throw new ApplicationException("Yep!");
                return true;
            });
            dialog.DialogResult = DialogResult.OK;
        }
    }

    public static class SomeExtensions
    {
        public static async Task<DialogResult> ShowDialogAsync(this Form @this)
        {
            await Task.Yield();   // !
            await Task.Yield();
            if (@this.IsDisposed)
                return DialogResult.OK;
            return @this.ShowDialog();
        }
    }

        private async Task DoItNowAsync(Form dialog)
        {
            try
            {
                await Task.Run(() =>
                {
                    Thread.Sleep(2000);
                    throw new ApplicationException("Yep!");
                    return true;
                });
            }
            finally
            {
                dialog.DialogResult = DialogResult.OK;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                using (var dialog = new FormDialog())
                {
                    var task = Task.Run(() =>
                    {
                        Thread.Sleep(4000);
                        throw new ApplicationException("Yep!");
                        return true;
                    });
                    Task.WaitAny(task, Task.Delay(1000));   // This delay is required to prevent flickering (opening and closing dialog for a short operation)
                    if (!task.IsCompleted)
                    {
                        task.ContinueWith((_) => 
                            { 
                                dialog.DialogResult = DialogResult.OK; 
                            });
                        dialog.ShowDialog(this);
                        task.Wait();
                    }
                }
            }
            catch (Exception ex)
            {
                textBox1.Text = ex.ToString();
            }
        }