You can use <Type>.nullValue in OCL and Executable Action Language (EAL) expressions when you need to assign a typed null value to a nullable attribute or variable, or return null from a method.
Syntax
<Type>.nullValue
Replace <Type> with the type of the value you are assigning. Use the typed null value on the right-hand side of an assignment.
someObject.someAttribute := <Type>.nullValue
Clear an attribute
For example, if a Car object has an attribute named Name, assign the null value for the attribute's type to remove its current value:
vCar.Name := <NameType>.nullValue
Here, <NameType> stands for the type of Name. After the assignment, Name has no value.
For clearing a nullable attribute or a single-link association (0..1), you can also use setToNull. For example:
Order.allinstances->first.OrderDate.setToNull
Use setToNull when the intention is specifically to clear the target property. Use <Type>.nullValue when you need a null value in an assignment, variable expression, or return expression.
Assign null to a variable
You can assign a typed null value to a variable when the variable needs to hold no object or value:
vCar := Car.nullValue
In this example, vCar is assigned a null value of type Car.
Return null from a method
A method can return a typed null value when it has no result to return:
return Car.nullValue
The returned null value must have the method's return type.
Do not use nullValue for comparisons
Do not compare a value with <Type>.nullValue.
vCurrent_UppdragTyp = Uppdragstyp.nullValue -- WRONG
Use .isNull to test whether a value is null, and .notNull to test whether it has a value:
vCurrent_UppdragTyp.isNull
vCurrent_UppdragTyp.notNull
These expressions return Boolean results and can be used where a Boolean condition is required. See OCL Boolean Operators for Boolean expressions and OCL General Operators for finding available operators in the OCL Editor.