var websters = new Dictionary<string, string> { { "Cromulent", "I never heard the word before moving to Springfield." } };
websters.ShouldContainKeyAndValue("Cromulent", "Fine, acceptable.");

websters
    should contain key
"Cromulent"
    with value
"Fine, acceptable."
    but value was
"I never heard the word before moving to Springfield."

var websters = new Dictionary<string, string> { { "Chazzwazzers", "What Australians would have called a bull frog." } };
websters.ShouldNotContainValueForKey("Chazzwazzers",  "What Australians would have called a bull frog.");

websters
    should not contain key
"Chazzwazzers"
    with value
"What Australians would have called a bull frog."
    but does