The Throws and ThrowsAsync helpers in the NSubstitute.ExceptionExtensions namespace can be used to throw exceptions when a member is called.

//For non-voids:
calculator.Add(-1, -1).Throws(new Exception()); // Or .Throws<Exception>()

//For voids and non-voids:
calculator
    .When(x => x.Add(-2, -2))
    .Throw(x => new Exception()); // Or .Throw<Exception>() -  - don't use .Throw*s* in this case

//Both calls will now throw.
Assert.Throws<Exception>(() => calculator.Add(-1, -1));
Assert.Throws<Exception>(() => calculator.Add(-2, -2));

Returns

Another way is to use the underlying method, .Returns. See also Callbacks.

//For non-voids:
calculator.Add(-1, -1).Returns(x => { throw new Exception(); });

//For voids and non-voids:
calculator
    .When(x => x.Add(-2, -2))
    .Do(x => { throw new Exception(); });

//Both calls will now throw.
Assert.Throws<Exception>(() => calculator.Add(-1, -1));
Assert.Throws<Exception>(() => calculator.Add(-2, -2));