nunit Write a custom constraint for the constraint model Match an integer approximatively

Example

Suppose we want to write a constraint which matches a number, but approximatively. Say, you are supposed to have 95 people in a survey, but 93 or 96 will do as well. We can write a custom constraint of the form:

public class AlmostEqualToConstraint : Constraint
{
    readonly int _expected;
    readonly double _expectedMin;
    readonly double _expectedMax;
    readonly int _percentageTolerance;


    public AlmostEqualToConstraint(int expected, int percentageTolerance)
    {
        _expected = expected;
        _expectedMin = expected * (1 - (double)percentageTolerance / 100);
        _expectedMax = expected * (1 + (double)percentageTolerance / 100);                    
        _percentageTolerance = percentageTolerance;
        Description = $"AlmostEqualTo {expected} with a tolerance of {percentageTolerance}%";
    }


    public override ConstraintResult ApplyTo<TActual>(TActual actual)
    {
        if (typeof(TActual) != typeof(int))
            return new ConstraintResult(this, actual, ConstraintStatus.Error);

        var actualInt = Convert.ToInt32(actual);

        if (_expectedMin <= actualInt && actualInt <= _expectedMax)
            return new ConstraintResult(this, actual, ConstraintStatus.Success);
        else
            return new ConstraintResult(this, actual, ConstraintStatus.Failure);
    }
}