C # How does Singleton implement multi-threaded Multi-instance, each thread has only one instance

In the past, the singleton used in the project has always been the simplest C # method. It is Thread Safe, and only one instance can be used in multiple threads. The Code is as follows:
 
Public sealed class Singleton
{
Public static readonly Singleton Instance = new Singleton ();
Private Singleton (){}
}

Recently, the project has new requirements. Multiple instances need to be implemented in multiple threads. It is best to use a singleton in each thread. Therefore, the code is changed to the most basic implementation. This basic implementation method is widely used in almost all Chinese and English documents on the Internet. This method will open multiple instances under multiple threads, but the result... In any case, it will always be a single instance and cannot be understood. The Code is as follows. please correct me. The test environment is Windows 7 64bit, VS2010,. net framework 4.0.
Class Singleton:
1 public class Singleton: ICloneable
2 {
5 private static Singleton instance;
6 public static Singleton Instance
7 {
8 get
9 {
10 if (instance = null)
11 instance = new Singleton ();
12 return instance;
13
14}
15}
16
17 private string instanceID;
18
19 private Singleton ()
20 {
21 // return a random ID to verify whether the instance is the same.
22 instanceID = new Random (). Next (). ToString ();
23}
24
25 public void GetInstanceID ()
26 {
27 System. Threading. Thread. Sleep (3000 );
28 MessageBox. Show ("Instance ID:" + instanceID );
29}
30
31 public object Clone ()
32 {
33 return new Singleton ();
34}
35}

ICloneable is implemented for multiple instances in multiple threads, but it still does not work. The ICloneable interface is not implemented at the beginning, but not at the same time. Www.2cto.com
Call code in Form1:
1 private void button#click (object sender, EventArgs e)
2 {
3 Singleton. Instance. GetInstanceID ();
4 new System. Threading. Tasks. TaskFactory (). StartNew () =>
5 {
6 Singleton. Instance. GetInstanceID ();
7 });
8
9}

As a result, after the IstanceID is obtained under the current thread, the InstanceID in the multi-thread is always the same. It turns out that no new instances are generated in other threads, which is a problem with my implementation method, why? Please correct me.
The running result is as follows: (backgroudnworker and other methods have also been tested, and it is always the same instance)

From Tian Yu