Allow null sets an attribute's nullability, i.e., whether this attribute should be able to hold "no value". It affects the in-memory value (it will show up with a question mark at the end of the type, like Boolean?).

In C# and OCL, it is a bit tricky to know whether to check for both null and '' (empty string) for strings. Hence, the ->isnullorempty operator on a string is good to use.

Nullability is at its best for numbers, helping the user to distinguish whether someone has written 0 or no one has written anything yet...

Read more about Initial values and Default Database values for a fuller explanation of the properties InitialValue, DefaultDBValue and Allow Null.

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[[Category:Attributes and data types [Beginner]]]