What is inner Exception

Question

I have read the MSDN but, I could not understand this concept.

Correct me if I am wrong,

A innerexception will be used in hand with current ex

Answer 1

You can see the code below.

First step, I parse "abc" to integer. It will raise FormatException.

In the catch block, I try to open a text file to log the exception message. But this file didn't exist. FileNotFoundException will be raised.

I want to know what raised the second exception, so I add the first exception (or FormatException) to the constructor of the second exception.

Now the first exception is InnerException of the second exception.

In the catch block, I can access InnerException's properties to know what the first exception is.

Is it useful?

using System;
using System.IO;
public class Program
{
    public static void Main( )
    {
        try
        {
            try
            {
                var num = int.Parse("abc"); // Throws FormatException               
            }
            catch ( FormatException fe )
            {
                try
                {
                    var openLog = File.Open("DoesNotExist", FileMode.Open);
                }
                catch
                {
                    throw new FileNotFoundException("NestedExceptionMessage: File `DoesNotExist` not found.", fe );
                }                              
            }
        }
        // Consider what exception is thrown: FormatException or FileNotFoundException?
        catch ( FormatException fe)
        {
            // FormatException isn't caught because it's stored "inside" the FileNotFoundException
        }
        catch ( FileNotFoundException fnfe ) 
        {
            string inMes="", outMes;
            if (fnfe.InnerException != null)
                inMes = fnfe.InnerException.Message; // Inner exception (FormatException) message
            outMes = fnfe.Message;
            Console.WriteLine($"Inner Exception:\n\t{inMes}");
            Console.WriteLine($"Outter Exception:\n\t{outMes}");
        }        
    }
}

Console Output

Inner Exception:
    Input string was not in a correct format.
Outter Exception:
    NestedExceptionMessage: File `DoesNotExist` not found.

The Outter exception refers to the most deeply nested exception that is ultimately thrown. The Inner exception refers the most shallow (in scope) exception.