How to create a custom C# exception?

Question

I'm currently learning C#, but I am stuck on something and I can't find a solution for it.
I am trying to create my own Exception class.

The exception is called "InvalidNumberException", it checks if a number is equal to 5. I know it may seem kinda stupid, but I just need to get the idea about creating Custom Exceptions.

So far I found on MSDN that for creating the Exception I need these four constructors:

public class InvalidNumberException : System.Exception {     public InvalidNumbertException() : base() { }     public InvalidNumberException(string message) : base(message) { }     public InvalidNumberException(string message, System.Exception inner) : base(message, inner) { }      // A constructor is needed for serialization when an     // exception propagates from a remoting server to the client.      protected InvalidNumberException(System.Runtime.Serialization.SerializationInfo info,         System.Runtime.Serialization.StreamingContext context) { } } 

but I don't know how to implement the method or constructor in this class that a number entered from the console is equal to 5, and if it is not, it throws an exception.

I'll appreciate if someone helps me with this.

Answer 1

The exception itself shouldn't do the checking. Instead, whatever code you have that works with the number should do this. Try something like this:

if (number == 5)     throw new InvalidNumberException(); 

Answer 2

You don't need all those constructors. Think about the exception you are creating - to ensure an int is != to 5.

So, I would have a single constructor like this:

public InvalidNumberException(int value)     : base(String.Format("Some custom error message. Value: {0}", value)) { } 

And use it like this:

if (number == 5)     throw new InvalidNumberException(number); 

You shouldn't be wrapping that in a try block. It's the job of the code that executes the above code block to watch for exceptions, e.g:

try {     CallSomeMethodWhichChecksTheNumberAndThrowsIfNecessary(); } catch (InvalidNumberException inex) {     // do something, print to console, log, etc } 

I'm hoping this is just for learning because there is the Int32.Parse and Int32.TryParse methods for this purpose - a custom exception class is not required.