How the Call Stack Works

How the Call Stack Works

Examine the output of Figure carefully. You see the code enter Main( ), Func1( ), Func2( ), and the TRy block. You never see it exit the try block, though it does exit Func2( ), Func1( ), and Main( ). What happened?

When the exception is thrown, execution halts immediately and is handed to the catch block. It never returns to the original code path. It never gets to the line that prints the exit statement for the try block. The catch block handles the error, and then execution falls through to the code following the catch block.

If there is no exception handler at all, the stack is unwound, returning to the calling method in search of an exception handler. This unwinding continues until the Main( ) method is reached, and if no exception handler is found, the default (ugly) exception handler is invoked and the program terminates.

In this example, because there is a catch block, the stack does not need to unwind. The exception is handled, and the program can continue execution. Unwinding the stack becomes a bit more clear if you move the TRy/catch blocks up to Func1( ), as Figure shows.

Unwinding the stack by one level

using System;

namespace UnwindingTheStackByOneLevel
   class Tester

      static void Main(  )
         Console.WriteLine( "Enter Main..." );
         Tester t = new Tester(  );
         t.Run(  );
         Console.WriteLine( "Exit Main..." );
      public void Run(  )
         Console.WriteLine( "Enter Run..." );
         Func1(  );
         Console.WriteLine( "Exit Run..." );

      public void Func1(  )
         Console.WriteLine( "Enter Func1..." );
            Console.WriteLine( "Entering try block..." );
            Func2(  );
            Console.WriteLine( "Exiting try block..." );
            Console.WriteLine( "Exception caught and handled!" );
         Console.WriteLine( "Exit Func1..." );

      public void Func2(  )
         Console.WriteLine( "Enter Func2..." );
         throw new ApplicationException(  );
         Console.WriteLine( "Exit Func2..." );

Now the output looks like this:

    Enter Main...
    Enter Run...
    Enter Func1...
    Entering try block...
    Enter Func2...
    Exception caught and handled!
    Exit Func1...
    Exit Run...
    Exit Main...

This time the exception is not handled in Func2( ); it is handled in Func1( ). When Func2( ) is called, it uses Console.WriteLine( ) to display its first milestone:

    Enter Func2...

Then Func2( ) throws an exception and execution halts. The runtime looks for a handler in Func2( ), but there isn't one. Then the stack begins to unwind, and the runtime looks for a handler in the calling function: Func1( ). There is a catch block in Func1( ), so its code is executed. Execution then resumes immediately following the catch statement, printing the exit statement for Func1( ) and then for Main( ).

Notice that even though the exception is handled, you are now in Func1, and there is no automatic way to return to where you were in Func2.

If you're not entirely sure why the "Exiting Try Block" statement and the "Exit Func2" statement are not printed, try putting the code into a debugger and then stepping through it.

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