Extension methods adding an awaitable ShowAsync method to System.Windows.Forms.Form.

FormExtensions.cs

using System;
using System.Threading.Tasks;
using System.Windows.Forms;

/// <summary>
/// Extension methods for System.Windows.Forms.Form.
/// </summary>
static class FormExtensions
{
    /// <summary>
    /// Shows the form to the user.
    /// </summary>
    /// <param name="form">
    /// The form to show.
    /// </param>
    /// <returns>
    /// A task representing the asynchronous operation.
    /// </returns>
    public static Task<DialogResult> ShowAsync(this Form form)
    {
        return ShowAsync(form, () => form.Show());
    }

    /// <summary>
    /// Shows the form with the specified owner to the user.
    /// </summary>
    /// <param name="form">
    /// The form to show.
    /// </param>
    /// <param name="owner">
    /// Any object that implements System.Windows.Forms.IWin32Window and represents
    /// the top-level window that will own the form.
    /// </param>
    /// <returns>
    /// A task representing the asynchronous operation.
    /// </returns>
    public static Task<DialogResult> ShowAsync(this Form form, IWin32Window owner)
    {
        return ShowAsync(form, () => form.Show(owner));
    }

    /// <summary>
    /// The internal ShowAsync implementation.
    /// </summary>
    /// <param name="form">
    /// The form to show.
    /// </param>
    /// <param name="show">
    /// The action that shows the form.
    /// </param>
    /// <returns>
    /// A task representing the asynchronous operation.
    /// </returns>
    private static Task<DialogResult> ShowAsync(Form form, Action show)
    {
        TaskCompletionSource<DialogResult> tcs = new TaskCompletionSource<DialogResult>();

        FormClosingEventHandler handler = null;
        handler = new FormClosingEventHandler((s, e) =>
        {
            form.FormClosing -= handler;
            tcs.SetResult(form.DialogResult);
        });

        form.FormClosing += handler;

        show();

        return tcs.Task;
    }
}