Deadlock in a very simple winform app - winforms

Question

I have created a very simple winform app that has a single form and a single button on it. Its click event is wired up as shown below. If you click the button, the app behaves as expected ie the ExecTasks exits. If you replace the lock statement by lock(this), ExecTasks will not exit and the debugger shows a thread in a sleep/wait/join at the code in Exec. Question why does the choice of locking object change this behaviour - why is the form instance not a valid choice ?
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
namespace TestDeadlock
{
public partial class Form1 : Form
{
private object _lock = new object();
public Form1()
{
InitializeComponent();
}
private void Button1Click(object sender, EventArgs e)
{
Task.Factory.StartNew(ExecTasks);
}
private void ExecTasks()
{
lock (_lock) /* replace by lock(this) to see the blocked behavior */
{
var taskList = new List<Task>();
for (var i = 0; i < 2; ++i)
{
taskList.Add(Task.Factory.StartNew(Exec));
}
Task.WaitAll(taskList.ToArray());
}
}
private void Exec()
{
Invoke((Action)delegate{});
}
}
}

Answer 1

I see this blocking behavior. When you call the Invoke method, it result in calling the Control.FindMarshalingControl method which is implemented as follows:
private Control FindMarshalingControl()
{
lock (this)
{
Control parentInternal = this;
....
}
}
Here is a link on a similar issue:
Control.BeginInvoke will also be blocked