FormTestUsingAsync.cs

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

namespace FormTestUsingAsync
{
    public partial class Form1 : Form
    {
        public DataManager DataManager = new DataManager();
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            MessageBox.Show($"{DataManager.DataList[2]}");
        }
    }

    public class DataManager
    {
        private List<int> _list = null;
        public List<int> DataList
        {
            get
            {
                if (_list == null)
                {
                    Init();
                    //InitAsync().Wait();  // Deadlock
                }
                return _list;
            }
        }

        private void Init()
        {
            _list = Task.Run(async () =>
            {
                return await MakeList();
            }).Result;
        }

        private async Task InitAsync()
        {
            _list = await MakeList();
        }

        private async Task<List<int>> MakeList()
        {
            await Task.Delay(1000);

            return new List<int>() { 1, 2, 3, 4, 5 };
        }
    }
}