Question

I want to create my own EMailAddress class that acts like a string class.

So I like to do this

private EMailAddress _emailAddress = "[e-mail]";

instead of

private EMailAddress _emailAddress = new EMailAddress("[e-mail]");

Is there any way to accomplish what I want, or do I need to use the second alternative. Since string is sealed I can’t use that, and the = operator can’t be overloaded so I am out of ideas how to fix this….

Answer 1

You can, with an implicit conversion:

public class EMailAddress
{
    private string _address;

    public EMailAddress(string address)
    {
        _address = address;
    }

    public static implicit operator EMailAddress(string address)
    {
        // While not technically a requirement; see below why this is done.
        if (address == null)
            return null;

        return new EMailAddress(address);
    }
}

Implicit conversions should only be used if no data is lost in the conversion. Even then, I recommend you use this feature sparingly, because it can make your code more difficult to read.

In this example, the implicit operator returns null when a null string is passed. As Jon Hanna correctly commented, it is undesirable to have these two snippets of code behave differently:

// Will assign null to the reference
EMailAddress x = null;

// Would create an EMailAddress object containing a null string
string s = null;
EMailAddress y = s;