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In general, deadlock is situation where two or more threads waiting to access a common resource are blocked forever.





Understanding a deadlock situation -:



• Suppose we have two threads running at the same time, where, Thread1 has locked the access to a method of class A(by getting a lock over an object of class A), and Thread2 has locked the access to a method of class B(by getting a lock over an object of class B).


• Thread1 calls a locked method of class B, it waits forever, because Thread2 already has the lock on this method i.e. only Thread2 can access the locked method of class B on the locked object.


• At the same time, Thread2 calls a locked method of class A, it also waits forever because Thread1 already has the lock on this method i.e. only Thread1 can access the locked method of class A on the locked object.


• This is a deadlock situation, where both threads would keep on waiting forever, until either of two threads would release the lock on its object.

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Creating a deadlock situation -:

Let us show you an example of a deadlock situation, which arises when two threads are trying to call methods which are already locked by each of them.

//C# Example of Deadlock


using System;
using System.Threading;

class A 
{
//Defining the method M() of class A
public void M(A a, B b)
{	
	//Locking the access to this method of class A
	//by locking the current object i.e. object of A
	//using this keyword
	lock(this)
	{
		Console.Write(Thread.CurrentThread.Name);
		Console.WriteLine(" is in A's M() method, trying to call B's M() method");
		b.M(a,b);	
	}
}
}


class B 
{

//Defining the method M() of class B
public void M(A a, B b)
{
	//Locking the access to this method of class B
	//by locking the current object i.e. object of B
	//using this keyword
	lock(this)
	{
		Console.Write(Thread.CurrentThread.Name);
		Console.WriteLine(" is in B's M() method, trying to call A's M() method");
		a.M(a,b);
	}
}
}


class C
{
//Instance variables
A a = new A(); 
B b = new B(); 
Thread thread;

C(String name)
{
	//Creating a new thread, based on the object of ThreadEx class
	//Which is referred by 'this' reference
	//We are also specifying the entry-point method that will be called to begin the execution of our thread.
	thread = new Thread(this.Run);

	//Setting the name of a thread by using the Name property of Thread
	thread.Name = name;

	//Calling the Start() method, which calls the entry-point method of new thread
	thread.Start();	

	//Main thread calling the method M() of A
	a.M(a,b);  		
}

public void Run()
{
	//New Thread calling the method M() of B
	b.M(a,b);	
}


public static void Main(String []ar)
{
	//Getting the reference of the automatically created Main Thread
	Thread thread = Thread.CurrentThread;

	//Setting the name of the Main thread
	thread.Name = "Main Thread";


	//Creating an object of our C class
	//which is used to construct a new thread
	new C("My Thread1");
}
}

Output is - :

Main Thread is in A's M() method, trying to call B's M() method
My Thread1 is in B's M() method, trying to call A's M() method

Program Analysis

We will create a deadlock situation in the upcoming code by having two threads -

• The automatically created Main Thread, gets a lock on the object of class A.
• The manually created thread called My Thread1, gets a lock on the object of class B.
• The Main Thread calls the locked method on object of A(which is already locked by My Thread1), and at the same time MyThread1 calls the locked method on object B(which is already locked by the Main Thread), causing a deadlock.

Creating a deadlock situation by locking a private object. -:

Let us show you an example of a deadlock situation, similar to the previous example, but this time we will lock methods of class by locking its private object.

//C# Example of Deadlock


using System;
using System.Threading;

class A 
{
//Creating the private object of Object
Object ob = new Object();

//Defining the method M() of class A
public void M(A a, B b)
{	
	//Locking the access to this method of class A
	//by locking its private object of Object type
	lock(ob)
	{
		Console.Write(Thread.CurrentThread.Name);
		Console.WriteLine(" is in A's M() method, trying to call B's M() method");
		b.M(a,b);	//Line1
	}
}
}


class B 
{
//Creating the private object of Object
Object ob = new Object();

//Defining the method M() of class B
public void M(A a, B b)
{
	//Locking the access to this method of class A
	//by locking its private object of Object type
	lock(ob)
	{
		Console.Write(Thread.CurrentThread.Name);
		Console.WriteLine(" is in B's M() method, trying to call A's M() method");
		a.M(a,b);	//Line2
	}
}
}


class C
{
//Instance variables
A a = new A(); //Line3
B b = new B(); //Line4
Thread thread;

C(String name)
{
	//Creating a new thread, based on the object of ThreadEx class
	//Which is referred by 'this' reference
	//We are also specifying the entry-point method that will be called to begin the execution of our thread.
	thread = new Thread(this.Run);	//line5

	//Setting the name of a thread by using the Name property of Thread
	thread.Name = name;

	//Calling the Start() method, which calls the entry-point method of new thread
	thread.Start();		//Line6

	a.M(a,b);  		//Line7	
}

public void Run()
{
	b.M(a,b);		//Line8
}


public static void Main(String []ar)
{
	//Creating an object of our C class
	//which is used to construct a separate thread
	new C("My Thread1"); //Line9
}

}

Output is - :

Main Thread is in A's M() method, trying to call B's M() method
My Thread1 is in B's M() method, trying to call A's M() method

Program Analysis

We will create a deadlock situation in the upcoming code by having two threads -

• The automatically created Main Thread, gets a lock on the private object(instance variable) of class A.
• The manually created thread called My Thread1, gets a lock on the private object(instance variable) of class B.
• The Main Thread calls the locked method on object of A(which is already locked by My Thread1), and at the same time MyThread1 calls the locked method on object B(which is already locked by the Main Thread), causing a deadlock.

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