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    <p>There is nothing wrong with designing a class so that an ID of 0 indicates that the entity has not yet been serialized. I have built systems in the past that successfully used this approach. Just make sure that these semantics are well defined in your API, and that this meaning is respected in all of the code.</p> <p>One trap to watch out for is using the ID to define an equality relationship (such as for generating hash codes for a dictionary). This must only be done for non-zero IDs. For testing equality with two IDs of zero, reference equality may be used.</p> <p>However, since unstored entities may have their ID change at some point in the future, it is very important that such objects are never stored in a Dictionary. Or, at the very least, such items must be removed from any dictionaries before saving and then restored afterwards using the new ID.</p> <p>With that one reservation, this design should work fine.</p> <pre><code>public class Test : IEquatable&lt;Test&gt; { /// &lt;summary&gt; /// The unique identifier for this Test entity, or zero if this /// Test entity has not yet been serialized to the database. /// &lt;/summary&gt; public int Id { get; private set; } public string Something { get; set; } public override bool Equals(object obj) { return Equals(obj as Test); } public bool Equals(Test other) { if (other == null) return false; // Distinct entities may exist with the Id value of zero. if (Id == 0) return object.ReferenceEquals(this, other); return Id == other.Id; } /// &lt;summary&gt; /// Gets a hash code for this Test entity. Warning: an instance with /// an Id of zero will change its identity when saved to the DB. Use with care. /// &lt;/summary&gt; /// &lt;returns&gt;The hash code.&lt;/returns&gt; public override int GetHashCode() { return Id; } } </code></pre>
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    1. PODealing with id's in entity object design
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    1. USJeffrey L Whitledge
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    1. COI realize that an id of 0 indicating that an object has no id is certainly a valid working solution; I was just wondering whether I would be feasible to do something better. I believe it is always better to not allow "incorrect" data and documenting your way out of it, as opposed to not have it be possible at all.
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    2. CO@DEHAAS - Yes, that was one of the options in your original question. I was just saying that in my opinion it is fine. I see no distinction between defining 0 as unsaved vs. defining null that way, except that the first is slightly less trouble than the second. In my opinion, properly documenting and implementing a special value is a good solution.
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    3. CO@DEHAAS - Think of this way: in a `double` the first bit is interpreted to indicate whether the numeric value is positive or negative. It is, however, just a bit like any other, and can be interpreted other ways in other contexts. Defining certain combinations of bits as meaning one thing as opposed to another thing is a basic part of programming, and does not indicate bad design.
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