Exception e , means e is a reference variable to the Exception class but not object. Then how can we access the properties like e.message;

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C#

using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.IO
    namespace JUSTFORPRACTICE
     {
       public class a
        {
         public static void Main()
         {
            try
            {
                StreamReader str = new StreamReader(@"C:\Users\IBM_ADMIN\Downloads\IBM\metal.txt");
                Console.WriteLine(str.ReadToEnd());//output of str.ReadToEnd() is a string
                str.Close();
            }
            catch(Exception e)
            {
                Console.WriteLine(e.Message);
                Console.WriteLine("\n\n");
                Console.WriteLine(e.StackTrace);//It gave information
            }
         }
       }
     }

Code is working fine but my doubt is in catch() we declared a reference variable e for exception class but 'e' is not an instance of the class exception since lacks complete definition as

Exception e = new exception();

So how can we access the Exception class properties as e.Message or e.StackTrace
This silly doubt eating my brain.

Posted 23-Jun-15 22:43pm

Annihilated

Updated 23-Jun-15 22:59pm

v3

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Comments

F-ES Sitecore 24-Jun-15 4:48am

I don't see any problem with the code, you might to explain a bit further

Annihilated 24-Jun-15 4:55am

code is working very fine....but my doubt is in the catch() where just Exception e is written over there......does it mean e is an object??

F-ES Sitecore 24-Jun-15 4:58am

Yes, e is an object of type Exception.

Annihilated 24-Jun-15 5:01am

but we didnt give full definition as
exception e = new exception() but how by only writing exception e creates object to class exception?

F-ES Sitecore 24-Jun-15 5:19am

.net passes the exception to your code block in the "e" variable. A function is defined like this

void MyFunc(string blah)
{
Debug.WriteLine(blah.ToUpper());
}

when I call it;

string a = "Hello";
MyFunc(a);

then inside MyFunc the blah variable is "Hello" which is basically the contents of the "a" variable I called it with. So when you define a catch

catch(Exception e)
{
}

then .net "calls" your catch and passes in the exception which becomes the "e" param. You could also do this if you wanted;

catch(Exception xxyyz)
{
Debug.WriteLine (xxyyz.Message);
}

it doesn't have to be "e", it can be anything like a normal function param

Annihilated 24-Jun-15 5:49am

Got it......thanks man

Sinisa Hajnal 24-Jun-15 4:54am

The framework creates the exception at the moment something goes wrong. catch (Exception e) is NOT a declaration of the exception.

You can do it in your own code:
Exception e = new Exception
e.Message = "My own message";
Throw e;

but try..catch depends on the framework to catch exception OBJECT instatiated "behind the scenes" with all the data you need to handle it.

I hope this clears your doubts and saves your brain ;)

Annihilated 24-Jun-15 4:57am

does it mean that by just writing - exception e creates an object to the class Exception

CPallini 24-Jun-15 5:07am

Nope, the exception is created anyway (if needed). Writing catch (Exception e) roughly means 'I want the exception thrown bounded to the variable e'.

Annihilated 24-Jun-15 4:54am

code is working very fine....but my doubt is in the catch() where just Exception e is written over there......does it mean e is an object??

Sinisa Hajnal 24-Jun-15 5:50am

Yes, e is an object of type Exception. No, writing Exception e will NOT create the object (see my comment above).

exception in a catch block is a variable, treat catch like a method, maybe it would be easier to understand. Catch is a method with parameter of type Exception which is called by the framework and not your code.

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OriginalGriff · Accepted Answer · 2015-06-23T23:04:00

When you set up a catch block:

C#

try
    {
    ...
    }
catch(Exception e)
    {
    Console.WriteLine(e.Message);
    Console.WriteLine("\n\n");
    Console.WriteLine(e.StackTrace);//It gave information
    }

Or indeed, declare any variable:

C#

string s;
Button b;
StreamReader str;

You aren't creating an instance of the class, you are declaring a place to store a reference to an instance of the class - a container if you like.

Just as a cup can contain coffee, the cup itself is not the wonderful liquid - indeed, when you finish drinking the Coffee instance, you can reuse the same cup to contain a new instance of Coffee, and start drinking that.

As you have noticed, in order to use a class, you need (at some point) to create a new instance of it, and load that into a variable of some form - but you don't always want to create a new one every time. If you write a method that returns a Streamreader:

C#

private StreamReader GetStream(path)
   {
   return new StreamReader(path);
   }

You can create a variable and fill it with an instance without explicitly using the new keyword:

C#

StreamReader str = GetStream(@"C:\Users\IBM_ADMIN\Downloads\IBM\metal.txt");

And that is what effectively happens when you write a catch block: the Exception has already created the new Exception class instance, and you are declarign a variable to put it into. Using new here would imply that it was created at that point, and was unrelated to the error which created and threw the actual exception you are trying to recover information for.