Unit test exception messages with xUnit

Question

I'm currently converting my MsTest unit tests to xUnit. With xUnit, is there a way to test exception messages? Is it correct to test exception messages as opposed just the exception type?

Answer 1

I think it is correct to test for both Exception type and message. And both are easy in xUnit:

var exception = Assert.Throws<AuthenticationException>(() => DoSomething());
Assert.Equal(message, exception.Message);

Answer 2

Better to use the Record.Exception method as it matches the AAA pattern:

    [Fact]
    public void Divide_TwoNumbers_ExpectException()
    {
        var sut = new Calculator();
        var exception = Record.Exception(() => sut.Divide(10, 0));
        Assert.IsType(typeof(DivideByZeroException), exception);
    }

Hope this helps ...

Answer 3

Something like this

 var ex = Record.Exception(() => DoSomeThing());
 Assert.IsType(typeof(ArgumentNullException), ex);
 Assert.True(ex.Message.Contains("Your exception message"));

Answer 4

xUnit uses Assert.Throws to test for exception types. You could catch the exception and Assert against the message if you needed. I think in general you want to test that the expected exception is thrown, and the exact message is really not necessary.

Assert.Throws<ArgumentNullException>()

The exception might be if you have a custom exception you are unit testing and you want to make sure the message generated is what you expect. Or if there are two ways that the same exception type can be thrown but with different messages, then asserting against the message would be valuable

Answer 5

BTW, Resharper prefers not to use typeof and suggests Assert.IsType instead, e.g.

var ex = Record.Exception(() => new FooController(null, null));
Assert.IsType<ArgumentNullException>(ex);