UP | HOME

Store validation state in types

Navigation   notoc

Blog   notoc nav

Introduction   notoc

Store validation state in types to avoid use of invalid data and unnecessary revalidation.

Static set of values

You have a static set of airports. Don't store and pass them as strings, use an Airport enum. Convert to and from whatever external representation there is (might be a string) at the boundaries of your application and perform validation there.

Airport ParseAirport(string s); // or throw

string PrintAirport(Airport a);

Externally managed set of values

Your set of airports is provided externally on application init. Use an Airport newtype with a validating smart constructor.

public struct Airport
{
    private Airport(string name)
    {   Name = name;
    }

    public string Name { get; }

    public Airport Mk(string name)
    {   if (name is invalid airport designation)
        {   throw new Exception($"Invalid Airport {name}");
        }
        return new Airport(name);
    }
}

Bounded set of values

A bounded integer range. Use a newtype with a smart constructor that enforces the bounds.

public struct Int100
{
    private Int100(int value)
    {   Value = value;
    }

    public int Value { get; }

    public Int100 Mk(int x)
    {   if (x > 100 || x < -100)
        { throw new Exception($"Out of bounds {x}");
        }
        return new Int100(x);
    }
}