c# - 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 exception.

Inner exception will occur first and then the current exception will occur (if there is an exception) that is the reason why InnerException is checked against null. In order to retain inner exception we have to pass it as a parameter.

Am I right with this?

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($"Inner Exception:
{inMes}");
            Console.WriteLine($"Outter Exception:
{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.