ExpectedException Assert

Question

I need to write a unit test for the next function and I saw I can use [ExpectedException]

this is the function to be tested.

public static T FailIfEnumIs         


        

Answer 1

While ExpectedException cannot be used as-is to verify the exception's message, you could implement your own exception validation logic by inheriting from ExpectedExceptionBaseAttribute:

By implementing your own expected exception verification. you can specify additional information and requirements that the built-in methods of the ExpectedExceptionAttribute class cannot handle, such as the following:

• Verifying the state of the exception.
• Expecting more than one type of exception.
• Displaying a custom message when a wrong type of exception is thrown.
• Controlling the outcome of a negative test.

In your case, it could look something like this:

public sealed class ExpectedExceptionMessageAttribute : ExpectedExceptionBaseAttribute
{
    readonly string _expectedMessage;
    public ExpectedExceptionMessageAttribute(string expectedMessage)
    {
        _expectedMessage = expectedMessage;
    }

    protected override void Verify(System.Exception exception)
    {
        // Handle assertion exceptions from assertion failures in the test method
        base.RethrowIfAssertException(exception);

        Assert.IsInstanceOfType(exception, typeof(T), "wrong exception type");
        Assert.AreEqual(_expectedMessage, exception.Message, "wrong exception message");
    }
}

HAving said that, I would still be inclined to use the direct try-catch approach though as it is more specific in where exactly the exception is expected to be thrown:

public static void Throws(Action action, Predicate predicate = null) 
                    where T : Exception
{
    try
    {
        action();
    }
    catch (T e)
    {
        if (predicate == null || predicate(e))
        {
            return;
        }

        Assert.Fail($"Exception of type {typeof(T)} thrown as expected, but the provided predicate rejected it: {e}");
    }
    catch (Exception e)
    {
        Assert.Fail($"Expected exception of type {typeof(T)} but a different exception was thrown: {e}");
    }

    Assert.Fail($"No exception thrown, expected {typeof(T)}");
}