OneOf

ShouldNotBeOneOf is the inverse of ShouldBeOneOf.

ShouldBeOneOf

var apu = new Person { Name = "Apu" };
var homer = new Person { Name = "Homer" };
var skinner = new Person { Name = "Skinner" };
var barney = new Person { Name = "Barney" };
var theBeSharps = new List<Person> { homer, skinner, barney };
apu.ShouldBeOneOf(theBeSharps.ToArray());

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Exception

apu
should be one of
[Homer, Skinner, Barney]
but was
Apu

ShouldNotBeOneOf

var apu = new Person { Name = "Apu" };
var homer = new Person { Name = "Homer" };
var skinner = new Person { Name = "Skinner" };
var barney = new Person { Name = "Barney" };
var wiggum = new Person { Name = "Wiggum" };
var theBeSharps = new List<Person> { apu, homer, skinner, barney, wiggum };
wiggum.ShouldNotBeOneOf(theBeSharps.ToArray());

snippet source | anchor

Exception

wiggum
should not be one of
[Apu, Homer, Skinner, Barney, Wiggum]
but was
Wiggum