How to fire multiple exceptions?

Question

foreach (Foo el in arr) {
    // ...
    Validate(el);
    // ...
}

In the example code, when the foreach is executed, an exception can be triggered from the function Validate which will be treated on who called the method containing the loop.

The exceptions triggered by Validate has to be handled by the user. The most convenient would be for the user to know all the errors that were played to correct them before re-running the function that contains the loop. For that, I’d have to do something like:

List<Exception> errors = new List<Exception>();

foreach (Foo el in arr) {
    // ...

    try {
        Validate(el);
    } catch(Exception e) {
        errors.Add(e);
    }

    // ...
}

if(erros.Any()) {
    throw errors; // somente objetos do tipo Exception podem ser disparados com throw.
}

How I fire multiple exceptions?

Answer 1

This seems to me a conceptual error in the code.

Apparently you are using exceptions to treat validations and this is not only wrong as it is very bad, it is an extreme misuse of the exception mechanism.

Leaving this aside the short answer to your question is: Is not possible.

Exceptions are for, as the name says, exceptional cases and not for data validation or similar tasks.

Assuming you don’t want to fix the code and stop using these exceptions, the most you can do is pass this list of exceptions to the GUI layer and there do something to show the desired messages (or do what you like).

But in this way, it is not possible to capture them using a block catch.

It is still possible to fire a AggregateException based on the list of exceptions and capture it. Something like:

To fire:

if(erros.Any()) {
    throw new AggegateException("Mensagem", errors);
}

To capture:

catch (AggregateException ae) 
{
    ae.Handle((x) =>
    {
        // Todas as exceções dentro de 'ae' passarão por este bloco
        return true; // Isso impede a parada do código
    });
}

It might be interesting to read these publications:

• Best way to deal with Exceptions

• I should use a "Try-catch" to identify if a password is wrong?

• Exceptions should be used for flow control?

• Exceptions consume a lot of processing. Truth or legend? (this is not related directly, but I think it’s good to see)

---

Functional code of the example with AggregateException. See working on . NET Fiddle.

using System;
using static System.Console;
using System.Collections.Generic;

public class Program
{
    public static void Main()
    {
        try
        {
            FazerAlgo();
        }
        catch (AggregateException ae)
        {
            ae.Handle((ex) =>
                      {
                          WriteLine(ex.Message);
                          return true;
                      });
        }
    }

    static void FazerAlgo()
    {
        throw new AggregateException("", new List<Exception>
                                     {
                                         new Exception("Erro 1"),
                                         new Exception("Erro 2")
                                     });
    }
}

Answer 2

If you really want to throw several exceptions at once for the code caller, use the class AggregateException, she gets a list of Exception.

I adapted your code as example:

List<Exception> errors = new List<Exception>();

foreach (Foo el in arr) {
    // ...

    try {
        Validate(el);
    } catch(Exception e) {
        errors.Add(e);
    }

    // ...
}

if(erros.Any()) {
    throw new AggregateException(
        "Vários erros ocorreram durante a execução do método.",
        errors)
}