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>?