Show Modal Dialog

Form1.cs

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

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

    private async void button1_Click(object sender, EventArgs e)
    {

        Task<int> waitTask = LongRunningTask();
        using (WaitDialog dialog = new WaitDialog(waitTask))
        {
            // UI will be blocked on this ShowDialog until task finishes
            dialog.ShowDialog(this);
            int result = await waitTask;
            MessageBox.Show($"Result: {result}");
        }
    }

    public Task<int> LongRunningTask()
    {
        return Task.Run(() =>
        {
            Thread.Sleep(10000);
            return 10;
        });
    }


    /// <summary>
    /// Required designer variable.
    /// </summary>
    private System.ComponentModel.IContainer components = null;

    /// <summary>
    /// Clean up any resources being used.
    /// </summary>
    /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    #region Windows Form Designer generated code

    /// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InitializeComponent()
    {
        this.button1 = new System.Windows.Forms.Button();
        this.SuspendLayout();
        // 
        // button1
        // 
        this.button1.Location = new System.Drawing.Point(66, 43);
        this.button1.Name = "button1";
        this.button1.Size = new System.Drawing.Size(75, 23);
        this.button1.TabIndex = 0;
        this.button1.Text = "button1";
        this.button1.UseVisualStyleBackColor = true;
        this.button1.Click += new System.EventHandler(this.button1_Click);
        // 
        // Form1
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(284, 261);
        this.Controls.Add(this.button1);
        this.Name = "Form1";
        this.Text = "Form1";
        this.ResumeLayout(false);

    }

    #endregion

    private System.Windows.Forms.Button button1;
}

Program.cs

using System;
using System.Windows.Forms;

static class Program
{
    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        Application.Run(new Form1());
    }
}

WaitDialog.cs

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

public partial class WaitDialog : Form
{
    public WaitDialog(Task waitTask)
    {
        InitializeComponent();

        waitTask.ContinueWith(t =>
        {
            this.DialogResult = DialogResult.OK;
        });
    }

    private Label label1;


    /// <summary>
    /// Required designer variable.
    /// </summary>
    private System.ComponentModel.IContainer components = null;

    /// <summary>
    /// Clean up any resources being used.
    /// </summary>
    /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
    protected override void Dispose(bool disposing)
    {
        if (disposing && (components != null))
        {
            components.Dispose();
        }
        base.Dispose(disposing);
    }

    #region Windows Form Designer generated code

    /// <summary>
    /// Required method for Designer support - do not modify
    /// the contents of this method with the code editor.
    /// </summary>
    private void InitializeComponent()
    {
        this.label1 = new System.Windows.Forms.Label();
        this.SuspendLayout();
        // 
        // label1
        // 
        this.label1.AutoSize = true;
        this.label1.Location = new System.Drawing.Point(39, 97);
        this.label1.Name = "label1";
        this.label1.Size = new System.Drawing.Size(218, 13);
        this.label1.TabIndex = 0;
        this.label1.Text = "Please wait for long running task to complete";
        // 
        // WaitDialog
        // 
        this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
        this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
        this.ClientSize = new System.Drawing.Size(284, 261);
        this.Controls.Add(this.label1);
        this.Name = "WaitDialog";
        this.StartPosition = System.Windows.Forms.FormStartPosition.CenterParent;
        this.Text = "WaitDialog";
        this.ResumeLayout(false);
        this.PerformLayout();

    }

    #endregion
}