How can an object not be compared to null?

KeyValuePair<K,V> is a struct, not a class. It's like doing:

int i = 10;
if (i != null) ...

(Although that is actually legal, with a warning, due to odd nullable conversion rules. The important bit is that the if condition will never be true.)

To make it "optional", you can use the nullable form:

static void Foo(KeyValuePair<object,string>? pair)
{
    if (pair != null)
    {
    }
    // Other code
}

Note the ? in KeyValuePair<object,string>?

I'm answering this despite its age because it is the 1st Google result for "test keyvaluepair to null"

The answer specified is correct, but it doesn't completely answer the problem, at least the one I was having, where I need to test the existence of the KeyValuePair and move on to another check of the Dictionary if it doesn't exist the way I'm expecting.

Using the method above didn't work for me because the compiler chokes on getting KeyValuePair.Value of KeyValuePair<>?, so it is better to utilize default(KeyValuePair<>) as seen in this question + answers. The default for KeyValuePair