C# Exception Handling

1. Normal Case

    1.1 try-catch

    static void Main(string[] args)
    ...{
       try
        ...{
            //Code which can cause problems(exceptions)
        }
        catch( Exception_type e)
        ...{
            //Code for handling the problems(exceptions)
        }
    }

   A. If exception occurs inside the try block, the control transfers to the catch block, then to the line of code immediately following the catch() block.

// A.
    static void Main(string[] args)
    ...{
        int x = 0;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: Exception occured!");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

/* Out put: 
Catch Block: Exception occured!
Ending Block: It's the end!

*/

 

   B. Otherwise, it will go to the line of code following the catch() block directly.    

// B.
    static void Main(string[] args)
    ...{
        int x = 2;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: Exception occured!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

/*  Output:
Try Block: Not executed line!
Ending Block: It's the end!

   */

 

      1.2 try-finally

    static void Main(string[] args)
    ...{
        try
        ...{
            //Code which can cause problems(exceptions)
        }
        finally
        ...{
            //Code for handling the problems(exceptions)
            //Code for cleanup.
            //Code will always execute regardless of whether an an exception has occured or not
        }
    }

   

   A. If exception occurs inside the try block, because there is no exception handling catch block, the execution will get terminated. But before the termination of the program, statements inside the finally block will get executed. That's why we only can see "Finally BLock...", but no "Ending block..."

// A.
    static void Main(string[] args)
    ...{
        int x = 0;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }


/**//* Output
Unhandled Exception: System.DivideByZeroException: Attempted to divide by zero.
   at Program.Main(String[] args) in ....................*.cs:line **
Finally Block: We always come here!
*/

 

    B. Although there is no exception occurs, the code inside the Finally() blocks will still be execute. We can find this from the output.

// B.    
    static void Main(string[] args)
    ...{
        int x = 2;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

/**//* Output:
Try Block: Not executed line!
Finally Block: We always come here!
Ending Block: It's the end!
*/

 

    1.3 try-catch-finally

    static void Main(string[] args)
    ...{
        try
        ...{
            //Code which can cause problems(exceptions)
        }
        catch (Exception_type e)
        ...{
            //Code for handling the problems(exceptions)
        }
        finally
        ...{
            //Code for cleanup.
            //Code will always execute regardless of whether an an exception has occured or not
        }
    }

 

    A. If exception occurs inside the try block, the control transfers to the catch block, then to the finally block.

// A.
    public static void Main()
    ...{
        int x = 0;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: Exception occured!");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

/**//* Output
Catch Block: Exception occured!
Finally Block: We always come here!
Ending Block: It's the end!
*/

 

   B. If there is exception occurs inside the try block, the control will ignore the catch block, go to finally() block directly. Anyway, the code inside finally() block will be execute.

// B.
    public static void Main()
    ...{
        int x = 2;
        int div = 0;
        try
        ...{
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: Exception occured!");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

/**//*
Try Block: Not executed line!
Finally Block: We always come here!
Ending Block: It's the end!
*/

 

2. Multiple Catch()

     try-catch1-catch2-catch3

    static void Main(string[] args)
    ...{
        try
        ...{
            //Code which can cause problems(exceptions)
        }
        catch (Exception_type e1)
        ...{
            //Code for handling the first exception
        }
        catch (Exception_type e2)
        ...{
            //Code for handling the second exception
        }
        catch (Exception_type e3)
        ...{
            //Code for handling the third exception
        }
        finally
        ...{
            //Code for cleanup.
            //Code will always execute regardless of whether an an exception has occured or not
        }
    }

 The catch() blocks are examined in order from top to bottom. The runtime compares the type of exception that has occured with the exception type declared in each catch() block to find a match - either an exact match or a supertype match, then executes the code inside the first such match.

The catch() block including derived exception types should always precede the catch() block wich including the base exception types. Otherwise, the catch() with derived types will never get executed.

    static void Main(string[] args)
    ...{
        try
        ...{
            //Code which can cause problems(exceptions)
        }
        catch (Exception e1)
        ...{
            Console.WriteLine("Catch Block: Exception occured!");
            // This block will catch all types of Exceptions, INCLUDING IOException specifically.
        }
        catch (IOException e2)
        ...{
            Console.WriteLine("Catch Block: IOException occured!");
            // This block will never be executed because IOException is a derived class of Exception.
        }
        finally
        ...{
            //.....
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

 

3. Catch all Exceptions

We can catch all exceptions occured inside the try() block by using a catch() block without arguments, like this: The output will be the same as 1.3A:

    static void Main(string[] args)
    ...{
        int x = 0;
        int div = 0;
        try
        ...{
            //Code which can cause problems(exceptions)
            div = 100 / x;
            Console.WriteLine("Try Block: Not executed line!");
        }
        catch
        ...{
            //Code for handling the problems(exceptions)
            Console.WriteLine("Catch Block: Exception occured!");
        }
        finally
        ...{
            //Code for cleanup. 
            //Code will always execute regardless of whether an an exception has occured or not
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!", div);
    }

 

/* Output
Catch Block: Exception occured!
Finally Block: We always come here!
Ending Block: It's the end!
*/

4. Throw Exception

    4.1   throw exception

    A. Inside the try() block, we throw out a general Exception, which will be catched by the second catch() block.

    public static void Main()
    ...{
        try
        ...{
            throw new Exception();
        }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: DivideByZeroException occured! No.1");
        }
        catch (Exception e2)
        ...{
            Console.WriteLine("Catch Block: General Exception occured! No.2");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!");
    }

/**//* Output
Catch Block: General Exception occured! No.2
Finally Block: We always come here!
Ending Block: It's the end!
*/

    B. Inside the try() block, we throw out a special Exception - DivideByZeroException(), which will be catched by the first catch() block.

    public static void Main()
    ...{
        try
        ...{
            throw new DivideByZeroException();
         }
        catch (DivideByZeroException ex)
        ...{
            Console.WriteLine("Catch Block: DivideByZeroException occured! No.1");
        }
        catch (Exception e2)
        ...{
            Console.WriteLine("Catch Block: General Exception occured! No.2");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!");
    }

/**//* Output
Catch Block: DivideByZeroException occured! No.1
Finally Block: We always come here!
Ending Block: It's the end!
*/

 

    4.2   throw user-defined exception

     In addition to the exception classes provided by the FCL, it's also possible in C# to declare yo own, application-specific exception exception types by extending the System.Exception or any of its derived classes.

using System;

public class vvException : Exception
...{
    int divNum; 

    public vvException(int divs)
    ...{
        this.divNum = divs;
    }

    public int Div
    ...{
        get
        ...{
            return divNum;
        }
    }

    public override string Message
    ...{
        get
        ...{
            return "Error: The dividend can't be "+ Div;
        }
    }
}

public class vvCalculation
...{
    public vvCalculation(int div)
    ...{
        if (div == 10)
        ...{
            throw new vvException(div);
        }
    }
}

class Program
...{
    public static void Main()
    ...{
        Console.WriteLine("Please input an integer as dividend..");
        int dividend = Int32.Parse(Console.ReadLine());
        vvCalculation doCalculation;

        try
        ...{
            doCalculation = new vvCalculation(dividend);
        }
        catch (vvException)
        ...{
            Console.WriteLine("Catch Block: vvException occured! Sorry we don't take 10 as the dividend!");
        }
        catch (Exception)
        ...{
            Console.WriteLine("Catch Block: General Exception occured! No.2");
        }
        finally
        ...{
            Console.WriteLine("Finally Block: We always come here!");
        }
        Console.WriteLine("Ending Block: It's the end!");
    }
}

/**//* Output
Please input an integer as dividend..
10
Catch Block: vvException occured! Sorry we don't take 10 as the dividend!
Finally Block: We always come here!
Ending Block: It's the end!
*/

 

    4.3   re-throw exception

    For the exceptions we catched in the catch() block, we can re-throw them to the higher context. We can consider to wrap that exception with some additional information before re-throwing it.

class vvClass
...{
    public void Calculation()
    ...{
        try
        ...{
            int x = 0;
            int div = 100 / x;
        }
        catch (DivideByZeroException e)
        ...{
            Console.WriteLine("Catch Block: Catched exception in lower level!");
            throw;
        }
    }
}

class Program
...{
    public static void Main()
    ...{
        vvClass vc = new vvClass();
        try
        ...{
            vc.Calculation();
        }
        catch(Exception e)
        ...{
            Console.WriteLine("Catch Block: Catched exception in higher level!");
        }
        Console.WriteLine("Ending Block: It's the end!");
    }
}

/**//* Output
Catch Block: Catched exception in lower level!
Catch Block: Catched exception in higher level!
Ending Block: It's the end!
*/