C#: Setting custom Messages in user defined Exceptions

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User defined exceptions are very common in programs and very helpful too. For instance, consider you have an Account class and on that class, you need to implement a custom exception. Consider the scenario where a user withdraws money and the amount he tries to withdraw is greater than his available balance. We can define a very simple exception inheriting the ApplicationException class

class InsufficientFundsException : ApplicationException
{
    public int  errorCode;
    public InsufficientFundsException(int x)
    {
        errorCode = x;
    }
}

  
This takes an error code and will return it when catching the exception, so in the main program:

if (moneyToWithdraw > accountBalance)
    {
        throw new  InsufficientFundsException(2001);
    }

and in try catch block,

try
{
    WithdrawMoney(moneyToWithdraw, accountBalance);
}
catch (InsufficientFundsException e)
{
    Console.WriteLine(e.errorCode);
    Console.WriteLine(e.Message);
}

Here, we print the errorCode and Message of the error.
https://programmium.files.wordpress.com/2015/03/blog1.png  

We see this error "Error in the application" because of the Message property of the exception is not set explicitly for a user defined exception, remember the C# compiler will assign Error in the application implicitly and its really not a good way to popup an error because the error message is so common.

https://programmium.files.wordpress.com/2015/03/blog2err.png
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How to set a custom Message

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There are two ways to set message, they are:

1. Override the Message property
2. Constructor Chaining

The first is rather lengthy and constructor chaining is a rather more elegant approach.

Override the Message property

class InsufficientFundsException : ApplicationException
{
    public int  errCode;
    private string  err_msg;
 
    public InsufficientFundsException(int err_no, string err)
    {
        this.errCode = err_no;
        err_msg = err;
    }
 
    public override  string Message
    {
        get
        {
            return err_msg;
        }
            
    }
}

Look at the above code, in line 12, the property Message is overridden, since the original implementation of Message property is read-only, only get is implemented and passes a string.

Constructor Chaining

Using constructor chaining we can achieve this with only three lines of code.  The code also Overrides the Message property because many devs do not care for Constructor chaining. 

class InsufficientFundsException : ApplicationException
{
    public int  errCode;
    public InsufficientFundsException(int err_no, string err)
        : base(err)
    {
        this.errCode = err_no;
    }
}

Here, in this approach, I just chain the current constructor of the exception InsufficientFundsException with the base class ApplicationException's constructor which accepts a string and set it as the Message. The final code is:

lusing System;
 
namespace Blog_Entry_app
{
    class Program
    {
        static void  Main(string[] args)
        {
            int moneyToWithdraw = 10;
            int accountBalance = 5;
 
            try
            {
                Console.WriteLine(WithdrawMoney(moneyToWithdraw, accountBalance));
            }
            catch (InsufficientFundsException e)
            {
                Console.WriteLine(e.errCode);
                Console.WriteLine(e.Message);
            }
            Console.ReadKey();
             
        }
 
        static int  WithdrawMoney(int  moneyToWithdraw, int  accountBalance)
        {
            if (moneyToWithdraw > accountBalance)
                throw new  InsufficientFundsException(200,"Custom Message");
            else
                return accountBalance = accountBalance - moneyToWithdraw;
        }
    }
 
    class InsufficientFundsException : ApplicationException
    {
        public int  errCode;
        public InsufficientFundsException(int err_no, string err)
            : base(err)
        {
            this.errCode = err_no;
        }
    }
 
}

 would print the custom error successfully.

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