How to make a 'struct' Nullable by definition?

Question

struct AccountInfo
{
   String Username;
   String Password;
}

now if I want to have a Nullable instance I should write:

Nullable<AccountInfo> myAccount = null;

But I want make the struct Nullable by nature and it can be used like this (without use of Nullable<T>):

AccountInfo myAccount = null;

I was thinking with just two members, it would be a funny class I was ashamed someone look into my code :) — Xaqron, Dec 20 '10 at 17:03
A mutable structure is much more problematic than a class with only two members. — Dan Bryant, Dec 20 '10 at 17:39
Side comment: any reason on using `String` instead of `string`? — danielrozo, Feb 06 '14 at 13:02
The answers that say "make it a class," while fundamentally correct, don't account for the case where you are using a third party library that contains a struct type. In some cases, you may want a data member of that type that can be uninitialized or unset, and there's no obvious default value that works. Edit: Which ziplin's solution below DOES address. — Tim Keating, Apr 28 '15 at 20:34

score 74 · Accepted Answer · edited May 23 '17 at 11:47

You can't. Struct are considered value types, and by definition can't be null. The easiest way to make it nullable is to make it a reference type.

The answer you need to ask yourself is "Why is this a struct?" and unless you can think of a really solid reason, don't, and make it a class. The argument about a struct being "faster", is really overblown, as structs aren't necessarily created on the stack (you shouldn't rely on this), and speed "gained" varies on a case by case basis.

See the post by Eric Lippert on the class vs. struct debate and speed.

score 55 · Answer 2 · edited Jul 04 '18 at 14:56

55

When you declare it, declare it with a "?" if you prefer

AccountInfo? myAccount = null;

edited Jul 04 '18 at 14:56

answered Dec 20 '10 at 16:57

Dlongnecker

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That's not "defining" it. That's declaring it. – Chris Farmer Dec 20 '10 at 17:01

Greg Beech · Answer 3 · 2010-12-20T17:01:18.240

The short answer: Make it a class.

The long answer: This structure is mutable which structs should never be, doesn't represent a single value which structs always should, and has no sensible 'zero' value which structs also always should, so it probably shouldn't be a value type. Making it a class (reference type) means that it is always possible for it to be null.

Note: Use of words such as "never" and "always" should be taken with an implied "almost".

How to make a 'struct' Nullable by definition?

3 Answers3