Use NUnit Assert.Throws method or ExpectedException attribute?

The main difference is:

ExpectedException() attribute makes test passed if exception occurs in any place in the test method.
The usage of Assert.Throws() allows to specify exact place of the code where exception is expected.

NUnit 3.0 drops official support for ExpectedException altogether.

So, I definitely prefer to use Assert.Throws() method rather than ExpectedException() attribute.

I prefer assert.throws since it allows me to verify and assert other conditions after the exception is thrown.

    [Test]
    [Category("Slow")]
    public void IsValidLogFileName_nullFileName_ThrowsExcpetion()
    {
        var a = new MyTestObject();

        // the exception we expect thrown from the IsValidFileName method
        var ex = Assert.Throws<ArgumentNullException>(() => a.IsValidLogFileName(""));

        // now we can test the exception itself
        Assert.That(ex.Message == "Blah");

    }

The first allows you to test for more than one exception, with multiple calls:

Assert.Throws(()=>MethodThatThrows());
Assert.Throws(()=>Method2ThatThrows());

The second only allows you to test for one exception per test function.