The goal of this document is to explain how to work with -or avoid- null values in GeneXus-generated applications.
An attribute can have one of the following values:
- An actual value.
This is a special value, i.e. zero for numeric data, empty string for character data, etc.
- Null, if it is allowed to
Null means that the value is not set, unspecified, does not apply or unknown.
It is important to understand the difference between these last two special values: Null is not the same as Empty.
To be able to accept null values, an Attribute must be declared as nullable by setting the Nullable property - Attribute to Yes.
Nullable attributes can be set as null in different ways.
- Variables cannot be null
- Nullity is not copied on assignments
If X is an attribute and its value is null, the statement "Y = X" does _not_ set the value of Y to null. It's set to empty. You can change this behavior by writing the following code:
X = Y
- Nullity is not supported in parameter passing
Parameter passing protocol does not support nulls. The empty value is always received by the referenced object, even in the following situations:
- In a transaction there is a object.call(attribute) where the attribute is on the form and the control has Empty As Null = Yes.
- object.call(nullvalue(attribute)) and Generate null for nullvalue = Yes.
In this case a specification error is displayed.
The best way to avoid null values in an attribute is to set it's Nullable property - Attribute to No.
In backward compatibility situations or, if you cannot reorganize the database (to avoid nulls), there are several programming considerations you may take into account:
This minimizes the risk of setting a value to null. Nullvalue function behavior may be affected by the value of Generate null for nullvalue() property.
Empty value for each DBMS/Data type