Is it possible to throw an exception if the input isn't valid?

antlr4, c#, exception

Solution

@CoronA was right. The error happens in the lexer.. So, while I still think that creating an ErrorStrategy would be better, this is what actually worked for me and my goal of throwing an exception for undefined input.

First, I created a derived class that inherits from `BaseErrorListener` and implements `IAntlrErrorListener<T>`. The second part was my problem all along it seems. Because my visitor inherited from `FooBarBaseVistor<int>`, my error listener also needed to be of type to register it with my lexer.

class ThrowExceptionErrorListener : BaseErrorListener, IAntlrErrorListener<int>
{
    //BaseErrorListener implementation; not called in my test, but left it just in case

    public override void SyntaxError(IRecognizer recognizer, IToken offendingSymbol, int line, int charPositionInLine, string msg, RecognitionException e)
    {
        throw new ArgumentException("Invalid Expression: {0}", msg, e);
    }

    //IAntlrErrorListener<int> implementation; this one actually gets called.

    public void SyntaxError(IRecognizer recognizer, int offendingSymbol, int line, int charPositionInLine, string msg, RecognitionException e)
    {
        throw new ArgumentException("Invalid Expression: {0}", msg, e);
    }
}

And changed my `Calculator` class to attach my custom error listener to the lexer. Note that you don't have to remove the `ConsoleListener` like I did for the error to actually be thrown. Since I'm not really using it, I figured it best to go ahead and do so.

public static class Calculator
{
    public static int Evaluate(string expression)
    {
        var lexer = new BasicMathLexer(new AntlrInputStream(expression));
        lexer.RemoveErrorListeners(); //removes the default console listener
        lexer.AddErrorListener(new ThrowExceptionErrorListener());

        var tokens = new CommonTokenStream(lexer);
        var parser = new BasicMathParser(tokens);

        var tree = parser.compileUnit();

        var visitor = new IntegerMathVisitor();

        return visitor.Visit(tree);
    }
}

And that's it. An argument exception is thrown and this test now passes.

    [TestMethod]
    [ExpectedException(typeof(ArgumentException))]
    public void BadInput()
    {
        var expr = "1 + 5 + 2(3)";
        int value = Calculator.Evaluate(expr);
    }

One last note. If you throw a `RecognitionException` here, it will just get swallowed up again. `ParseCancelationException` is recommended, because it does not derive from `RecognitionException`, but I choose an `ArgumentException` because I felt that made the most sense to the client C# code.

Problem

Original source

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