Question

I’d like to enforce a struct to always be valid regarding a certain contract, enforced by the constructor. However the contract is violated by the default operator.

Consider the following, for example:

struct NonNullInteger
{
    private readonly int _value;

    public int Value
    {
        get { return _value; }
    }

    public NonNullInteger(int value)
    {
        if (value == 0)
        {
            throw new ArgumentOutOfRangeException("value");
        }

        _value = value;
    }
}

// Somewhere else:
var i = new NonNullInteger(0); // Will throw, contract respected
var j = default(NonNullInteger); // Will not throw, contract broken

As a workaround I changed my struct to a class so I can ensure the constructor is always called when initializing a new instance. But I wonder, is there absolutely no way to obtain the same behavior with a struct?

Answer 1

One approach is to arrange things so that the default value satisfies the contract:

struct NonNullInteger
{
    private readonly int _valueMinusOne;

    public int Value
    {
        get { return _valueMinusOne + 1; }
    }

    public NonNullInteger(int value)
    {
        if (value == 0)
        {
            throw new ArgumentOutOfRangeException("value");
        }

        _valueMinusOne = value - 1;
    }
}