Unit testing exception property

Question

I have exception

class SyntaxError : Exception {
    public SyntaxError(int l) {
        line = l;
    }
    public int line;
}

I'm using unit tests to test class Parser which on specific input should throw exception above. I'm using code like this:

    [TestMethod]
    [ExpectedException(typeof(Parser.SyntaxError))]
    public void eolSyntaxError()
    {
        parser.reader = new StringReader("; alfa\n; beta\n\n\n\na");
        parser.eol();
    }

Is there any smart simple way to check if SyntaxError.line == 1?

Best I come up with is:

    [TestMethod]
    public void eolSyntaxError()
    {
        try {
            parser.reader = new StringReader("; alfa\n; beta\n\n\n\na");
            parser.eol();
            Assert.Fail();
        } catch (SyntaxError e) {
            Assert.AreEqual(1, e.line);
        }
    }

I don't like it very much, is there better way?

Answer 1

Consider using FluentAssertions. Your test will then look like this:

[TestMethod]
public void eolSyntaxError()
{
    parser.reader = new StringReader("; alfa\n; beta\n\n\n\na");

    Action parseEol = () => parser.eol();

    parseEol
        .ShouldThrow<SyntaxError>()
        .And.line.Should().Be(1);
}

Otherwise, your approach is pretty much as good as it gets.

Answer 2

You could write a method similar to the one in NUnit

public T Throws<T>(Action code) where T : Exception
{
    Exception coughtException = null;
    try
    {
        code();
    }
    catch (Exception ex)
    {
        coughtException = ex;
    }

    Assert.IsNotNull(coughtException, "Test code didn't throw exception");
    Assert.AreEqual(coughtException.GetType(), typeof(T), "Test code didn't throw same type exception");

    return (T)coughtException;
}

And then you can use it in your test method

Parser.SyntaxError exception = Throws<Parser.SyntaxError>(() => parser.eol());
Assert.AreEqual(1, exception.line);

Answer 3

As per my comment, if the line at which you encounter the syntax error is relevant, then include it in your custom exception class, like so.

public class SyntaxError : Exception
{
     public SyntaxError(int atLine)
     {
         AtLine = atLine;
     }

     public int AtLine { get; private set; }
}

Then it's easy to test.

EDIT - After having read the question (!) here's a simple additional Assert method which will tidy up your exception assertions.

public static class xAssert
{
    public static TException Throws<TException>(Action a) where TException : Exception
    {
        try
        {
            a();
        }
        catch (Exception ex)
        {
            var throws = ex as TException;
            if (throws != null)
                return throws;
        }
        Assert.Fail();
        return default(TException);
    }
}

Usage as follows...

public class Subject
{
    public void ThrowMyException(int someState)
    {
        throw new MyException(someState);
    }

    public void ThrowSomeOtherException()
    {
        throw new InvalidOperationException();
    }
}

public class MyException : Exception
{
    public int SomeState { get; private set; }

    public MyException(int someState)
    {
        SomeState = someState;
    }
}

[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void TestMethod1()
    {
        var subject = new Subject();
        var exceptionThrown = xAssert.Throws<MyException>(() => { subject.ThrowMyException(123); });

        Assert.AreEqual(123, exceptionThrown.SomeState);
    }
}