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A finally block encloses the statements that are always executed whether an exception was thrown by the program or not.






A finally block can be defined at either of two places -



• A finally block can be defined right after the try-catch block.
• A finally block can be defined right after the try block(if catch block is not defined)

Conditions when a finally block runs -

• If an exception is not thrown out of try block, finally block still runs.

//C# If an exception is not thrown out of try block, finally block still runs.


using System;

class A
{
public static void Main(String[] ar)
{
	try	
	{
		A ob = new A ();
	}	
	catch(Exception e)
	{
		Console.WriteLine( "Exception caught"+e);
	}
	finally
	{
		Console.WriteLine("A finally block is executed irrespective of an exception is thrown or not");
	}
}
}





Output -




A finally block is executed irrespective of an exception is thrown or not








If an exception thrown by try block is caught by catch block, finally block still runs.




//C# The finally block runs even when a thrown exception is caught by a catch block	

using System;

class Class1
{
static void Main(string[] args) 
{
	//Creating an String array
	String[] names = {"Tomato", "Apple", "Cranberries"};
	
	//Casting String array to an Object array
	Object[] obArr  = (Object[]) names;
		
	//Upcasting an int to Object 
	Object obj = (Object) 13;
	    
	try
	{
		//Trying to assign an int(casted to an Object)
		// to an String array(casted to an Object array)
		obArr[2] = obj;
	}
	catch(ArrayTypeMismatchException)
	{
		Console.WriteLine("An ArrayTypeMismatchException is caught");
	}
	finally
	{
		Console.WriteLine("A finally block is executed even after an exception is caught in the catch-block");
	}
}	
}





Output -




An ArrayTypeMismatchException is caught
A finally block is executed even after an exception is caught in the catch-block



An exception of type ArrayTypeMismatchException is thrown when a value of type int is attempted to be stored in a String array by casting. A catch block is declared with a matching ArrayTypeMismatchException class, therefore, the catch block catches the thrown exception, and the finally block runs despite the exception being caught in the catch block.




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If an exception thrown by try block is not caught by catch block, finally block still runs.




using System;

class A
{
public static void Main(String[] ar)
{
	try
	{
		//Assigning a null reference to a reference.
		A ob = null;

		//Calling ToString() on null reference throws NullPointerException
		Console.WriteLine(ob.ToString());
	}
	catch(ArithmeticException e)
	{
		Console.WriteLine("Exception caught - " + e);
	}
	finally
	{
		Console.WriteLine("A finally block is executed even after an exception is not caught in the catch-block");
	}
}
}





Output-




Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at A.Main(String[] ar)
A finally block is executed even after an exception is not caught in the catch-block



In the try block, calling toString() method on a null reference throws a NullReferenceException, the catch block is declared with a different exception of type ArithmeticException, hence the NullReferenceException is not caught in the catch block. Despite this, the finally block still runs and exception is thrown after executing the finally block.






If an exception is thrown by try block and a catch block is not defined but finally block is defined, finally block runs.




// C# The finally block runs even in the absence of a catch block

using System;

class A
{
public static void Main(String[] ar)
{
	try
	{
		//Assigning a null reference
		A ob = null;

		//Calling ToString() method on a null reference, which throws a NullReferenceException
		Console.WriteLine(ob.ToString());
	}
	finally
	{
		Console.WriteLine("A finally block is executed in the absence of catch-block, when exception is thrown.");
	}
}
}





Output-




Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object.
   at A.Main(String[] ar)
A finally block is executed in the absence of catch-block, when exception is thrown.








If an exception is not thrown by a try block and a catch block is not defined but a finally block is defined, the finally block runs.




//C# Example of finally block, which always runs	


using System;

class A
{
public static void main(String... ar)
{
	try
	{
		//Creating an object of class A	
		A  ob = new A();
	}
	finally
	{
		Console.WriteLine("A finally block is executed in the absence of catch-block, when exception is not thrown.");
	}	
}
}


Output-

A finally block is executed in the absence of catch-block, when exception is not thrown.

Therefore, a finally block(when defined) always runs, no matter if an exception is thrown or not.

Use of finally block

Objects may hold references to the system resources like memory, files, database which need to be released in a timely fashion when they are no longer needed. Hence, a finally block usually contains a clean-up code i.e. code to release any of such resources that were referenced or locked in the try block.

Example of a finally block

using System;
using System.IO;

class FinallyEx
{

public static void Main(String[] ar)
{
	FileStream f = null;
	try
	{
		//Creating a file stream  
		f = new FileStream("E:\\File.txt", FileMode.OpenOrCreate);

		//writing a byte to the stream  
        	f.WriteByte(65);

		//Flushing the data to a file
		f.Flush();				
	}
	catch(IOException e)
	{
		Console.WriteLine("Exception caught: " + e);
	}		
	finally
	{
		try
		{
			//Cleaning up the resources i.e. closing an opened file.
			f.Close();		
		}
		catch(IOException e)
		{
			Console.WriteLine(e);
		}
	}	
}		
}

This program creates a text file named File.txt in E drive and writes a byte to it. The finally block runs the clean-up code i.e. by calling the Close() method to close the opened stream, after the data is flushed to the file.

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