C# Online Compiler | .NET Fiddle

using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
					
public class Program
{
	public static void Main()
	{
		Console.WriteLine("Hello World");
		MyClass myClass1 =new MyClass();
		MyClass myClass2 =new MyClass();
		MyClass myClass3 =new MyClass();
		
		Task<int> task1 = Task<int>.Factory.StartNew(()=>myClass1.DoWork());
		Task<int> task2 = Task<int>.Factory.StartNew(()=>myClass2.DoWork());
		Task<int> task3 = Task<int>.Factory.StartNew(()=>myClass3.DoWork());
		
		int i1= task1.Result;
		int i2 = task2.Result;
		int i3 = task3.Result;
		
		Console.WriteLine(i1);
		Console.WriteLine(i2);
		Console.WriteLine(i3);
	}
}

public class MyClass{
	public static List<int> lst = new List<int>();
	
	//DoWork with no lock will cause the application to fail
	public int DoWork(){
		
		Monitor.Enter (lst);
		try{
			Console.WriteLine("Work started");
			Thread.Sleep(300);
			lst.Add(100);
			Thread.Sleep(100);
			lst.Add(200);
			Thread.Sleep(100);
			lst.Add(300);
			Thread.Sleep(100);
			lst.Add(400);
			Thread.Sleep(100);
			lst.Add(100);
			Thread.Sleep(200);
			lst.Add(200);
			Thread.Sleep(100);
			lst.Add(300);
			Thread.Sleep(100);
			lst.Add(400);
			Thread.Sleep(100);
			Console.WriteLine("Work Complete");

		}catch(Exception ex){
			Console.WriteLine(ex);
		}
		finally{
			Monitor.Exit(lst);
		}
		return lst.Count;
	}
}

 
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
                    
public class Program
{
    public static void Main()
    {
        Console.WriteLine("Hello World");
        MyClass myClass1 =new MyClass();
        MyClass myClass2 =new MyClass();
        MyClass myClass3 =new MyClass();
        
        Task<int> task1 = Task<int>.Factory.StartNew(()=>myClass1.DoWork());
        Task<int> task2 = Task<int>.Factory.StartNew(()=>myClass2.DoWork());
        Task<int> task3 = Task<int>.Factory.StartNew(()=>myClass3.DoWork());
        
        int i1= task1.Result;
        int i2 = task2.Result;
        int i3 = task3.Result;
        
        Console.WriteLine(i1);
        Console.WriteLine(i2);

Cached Result

Hello World
Work started
Work Complete
Work started
Work Complete
Work started
Work Complete
8
16
24