For locating elements using the ClassName you are not allowed to use compund class names. For example, in your case the element has 3 CSS classes and when you enter all of them in the
driver.FindElement(By.ClassName(" it is considered as a compound class name. This means that, it is considered as single class with spaces, which is not allowed in locating element with ClassName.
Use a single class name to locate your element. If it doesn't have a unique class, then use CSSSelector (something like
driver.FindElement(By.CssSelector("input[name='CommandBarItem']"));) or an even better option would be to use XPath. You can get the XPath of the target element using the FirePath extension of Mozilla FireFox.
Addendum As per the updates in the Question,
You want to test whether clicking one of the 3 links (+, New and v) will open a drop down list or not.
With your test script you are trying to click the div that encloses the 3 links and not the links themselves.
You should test each of the three links separately. Try,
Clicking + icon:
Clicking NEW link:
Clicking v (down arrow):
You can try the same with CSSSelector or with XPath as well. Just remember, DON'T TRY TO CLICK THE DIV. CLICK THE LINKS THAT HAVE AN ACTION TO OPEN DROP DOWN LIST WRITTEN ON.