DEFINE PROPERTY statement

Defines a property of a class, declares a property prototype in an ABL interface, or overrides an abstract property inherited from an ABL or .NET abstract super class. A property is a named member of a class that allows you to read or write a value using the same syntax as a data member. However, a property can define special methods (accessors) that specify if and how the property can be read or written. The following description begins with the syntax for defining a property that you can both read and write.

Note:

This statement is applicable only when used in a class or interface definition (.cls) file.

Syntax 

 

DEFINE [ PRIVATE | PROTECTED | PUBLIC ] [ STATIC | ABSTRACT ] [ OVERRIDE ]   PROPERTY property-name   { {         AS primitive-type-name |         AS [ CLASS ] object-type-name      } [ EXTENT [ constant ] ] }   [ INITIAL { constant | { [ constant [ , constant ] ... ] } } ]   [ NO-UNDO ]   {         [ accessor-access-mode ] GET [ implementation ] .         SET [ implementation ] .      |         GET [ implementation ] .         [ accessor-access-mode ] SET [ implementation ] .   }

Use the following syntax to define a property that you can read only:

 

DEFINE [ PRIVATE | PROTECTED | PUBLIC ] [ STATIC | ABSTRACT ] [ OVERRIDE ]   PROPERTY property-name   { {         AS primitive-type-name |         AS [ CLASS ] object-type-name      } [ EXTENT [ constant ] ] }   [ INITIAL { constant | { [ constant [ , constant ] ... ] } } ]   [ NO-UNDO ]     GET [ implementation ] .

Use the following syntax to define a property that you can write only:

 

DEFINE [ PRIVATE | PROTECTED | PUBLIC ] [ STATIC | ABSTRACT ] [ OVERRIDE ]   PROPERTY property-name   { {         AS primitive-type-name |         AS [ CLASS ] object-type-name      } [ EXTENT [ constant ] ] }   [ INITIAL { constant | { [ constant [ , constant ] ... ] } } ]   [ NO-UNDO ]     SET [ implementation ] .

Use the following syntax to declare an interface property prototype:

 

DEFINE [ PUBLIC ] PROPERTY property-name   { { AS primitive-type-name | AS [ CLASS ] object-type-name }      [ EXTENT [ constant ] ] [ NO-UNDO ] }   { GET.      SET.     |     GET.     |     SET. }

Use the following syntax to declare an abstract property prototype:

 

DEFINE [ PROTECTED | PUBLIC ] [ OVERRIDE ] ABSTRACT PROPERTY property-name   { { AS primitive-type-name | AS [ CLASS ] object-type-name }      [ EXTENT [ constant ] ] [ NO-UNDO ] }   { GET.      SET.     |     GET.     |     SET. }

[ PRIVATE | PROTECTED | PUBLIC ]

Specifies the access mode for the property. A PRIVATE property can be accessed by the defining class. An instance can access a private property of another instance if both instances are of the same class. A PROTECTED property can be accessed by the defining class and any of its derived classes. An instance can access a protected property of a second instance that is at the same level or higher in the class hierarchy. A PUBLIC property can be accessed by:

The defining class

Any of its derived classes

Any class or procedure that has access to a class instance that defines or inherits the property

Any piece of code can access a PUBLIC static property. The default access mode is PUBLIC.

This access mode applies to both reading and writing the property value by default. However, you can set a different access mode for reading or writing the property value (but not both) using an appropriate accessor-access-mode option to define the corresponding accessor (GET or SET).

When declaring an interface property prototype, the access mode for the property must be PUBLIC (the default).

When defining an abstract property, the access mode for the property cannot be PRIVATE.

[ STATIC ]

Defines a property that is a static member of the class type for which it is defined and that is scoped to the ABL session where it is referenced. ABL creates one copy of the specified class static property on first reference to the class type, and ABL creates only one such copy for any number of instances of the class that you create. You can reference an accessible static property in any piece of code.

Without this option, ABL defines an instance property that is scoped to a single instance of the class where it is defined. ABL creates one copy of the specified instance property for each such class instance that you create. You can reference any public instance property (abstract or non-abstract) in any procedure, or in any instance or static method defined inside or outside of the class where the instance property is defined. Any static method can reference the public instance property only using an object reference to a class instance that defines the property as a member. If the referencing static method is defined in the same class as the public instance property, the class must instantiate itself in order to have access to an instance reference.

Note:

In ABL, all PRIVATE and PROTECTED access modes are class based. An instance can access the PRVIATE member of another instance if both instances are from the same class. Likewise, a PROTECTED member can be accessed from any instance of the class or sublcass that defines it. In other words, an instance can access a protected member of a second instance that is from the same level or higher in the class hierarchy.

For more information on the mechanism for accessing properties of different access modes and scopes, see the reference entry for Class-based property access.

The STATIC option is not valid when you:

Define or implement an abstract property

Declare an interface property prototype

Implement an interface property

[ ABSTRACT ]

Defines the property as an abstract member of the class type for which it is defined. The defining class type must also be abstract. If you define an abstract property, it has the following requirements:

You must specify the OVERRIDE option if an inherited abstract property has the same name and data type.

You can specify either a PROTECTED or a PUBLIC access mode, depending on any inherited abstract property you might be overriding.

The abstract property must be overridden and implemented in a derived class.

Any class definition for an instance property that includes the OVERRIDE option and does not include the ABSTRACT option defines a property implementation. If it also includes the ABSTRACT option, the inherited abstract property remains abstract.

This option is not valid either when you define a static property or when you declare an interface property prototype.

[ OVERRIDE ]

Specifies that this instance property overrides an abstract property inherited from an ABL or .NET abstract class.

Note:

A .NET abstract property or class is defined in C# with the abstract keyword.

When you specify OVERRIDE, the property definition must match the inherited abstract property with respect to name, data type (including any EXTENT), specified accessors (GET or SET), and NO-UNDO setting. In addition, the access mode must not be more restrictive than the access mode defined for the overridden property. When overriding a .NET abstract property, the data type must also map appropriately to the inherited .NET property data type. For more information, see the description of the primitive-type-name and object-type-name options.

If you specify the ABSTRACT option, your overriding property is also defined as abstract, and it must be implemented in a class derived from the defining class. Note that you do not have to override an inherited abstract property that you want to remain abstract as long as the inheriting class is also abstract. However, doing so allows you to specify a less restrictive access mode for the abstract property.

If you do not specify the ABSTRACT option, your property definition implements the inherited abstract property.

This option is not valid:

When you define a static property

When you declare an interface property prototype

To override an inherited property that is implemented (not abstract)

property-name

Specifies the name of the property. The property-name must be unique among the names of all properties, events, and variable data members that are defined in the class and its inherited class hierarchy, and that are accessible to the defining class (not defined as PRIVATE in a super class).

Note:

Members of a class are grouped into six namespaces, including buffers/temp-tables, methods, variables/properties/events, ProDataSets, queries, and data-sources. Variables, properties, and events defined as members of a class share the same namespace. There can be only one class member in this namespace with a given name (not counting abstract member overrides).

You can reference this name from outside the property definition in order to read a value from or write a value to the property. You can reference this name from inside the property definition (in a GET or SET accessor) in order to read or write the value of the default memory for the property, according to its data type.

Note:

If the property is defined in a class that is derived from a .NET class, other restrictions on property-name apply. For more information, see the CLASS statement reference entry.

AS primitive-type-name

Specifies a built-in primitive type (primitive-type-name) and an optional initial value (constant) for the default memory of the property. The built-in data type can be one of the following:

 

Syntax 

CHARACTER | COM-HANDLE | DATE | DATETIME | DATETIME-TZ | DECIMAL | HANDLE | INT64 | INTEGER| LOGICAL | LONGCHAR | MEMPTR | RAW | RECID | ROWID | AS-data-type

If you are overriding a .NET abstract property or implementing a property defined in a .NET interface, primitive-type-name must specify the exact same .NET mapped data type that is defined for the corresponding .NET property. For a .NET mapped data type that is a default match for a given ABL primitive type, you must use the default matching ABL data type, as shown in Table 24. (For example, INTEGER indicates a .NET System.Int32.) For a .NET mapped data type that is not a default match for one of the ABL primitive types, ABL provides a data type keyword (AS-data-type) that you must use to explicitly indicate the required .NET data type, as shown in Table 25. (For example, the AS data type, UNSIGNED-BYTE, indicates a .NET System.Byte.)

Note:

At run time, a property defined using an AS-data-type keyword behaves in ABL like the corresponding ABL primitive type shown in Table 25. (For example, an UNSIGNED-BYTE behaves like an INTEGER.)

Also note that when implementing a .NET array property, you must specify the .NET array object type (for example, "System.Int32[]" or "System.Byte[]"); you cannot use an ABL array equivalent (such as INTEGER EXTENT or UNSIGNED-BYTE EXTENT).

These are the same set of primitive types that can be specified for the return type of a method. Thus, for example, a property cannot have a complex type such as a temp-table, ProDataSet, or any other data type that is invalid as a method return type. For more information on these primitive types, see the Data types reference entry.

Note:

To manage public access for a complex data type that cannot be specified for a property, you can define both a data member of that complex type and a separate public method that passes the complex type as a parameter.

AS [ CLASS ] object-type-name

Specifies an object reference with the data type of a class or interface for the default memory of the property. The default value of the property is the Unknown value (?). You cannot assign an initial value using the INITIAL option.

object-type-name

Specifies the type name of an ABL or .NET class or interface. Specify an object type name using the syntax described in the Type-name syntax reference entry. With an appropriate USING statement, you can also specify a class or interface name alone, without the qualifying package or namespace.

You cannot directly specify the type name of a .NET mapped object type (such as System.Int32). To define a property that matches a .NET mapped type, you must define it as the corresponding ABL primitive type (primitive-type-name).

If you are overriding a .NET abstract property or implementing a property defined in a .NET interface, object-type-name must specify the exact same .NET object type as the corresponding interface property. However, for .NET inner (nested) type, note the difference in the ABL syntax, which replaces the corresponding period (.) in the .NET object type with a plus (+) (see the Type-name syntax reference entry).

Also note that when implementing a .NET array property, you must specify the exact .NET array object type (for example, "System.Drawing.Point[]"); you cannot use an ABL array equivalent (such as System.Drawing.Point EXTENT).

CLASS

If the specified class or interface type name conflicts with an abbreviation of a built-in primitive type name, such as INT for INTEGER, you must specify the CLASS keyword.

For more information on object references, see the Class-based object reference reference entry.

[ EXTENT [ constant ] ]

Defines the property as an array of data elements, where the element data type is specified by either the AS primitive-type-name option or the AS object-type-name option. This option can specify an array property as either determinate (has a defined number of elements) or indeterminate (has an undefined number of elements). To define a determinate array property, specify the EXTENT option with the constant argument. This optional argument is an integer value that represents the number of elements in the property array. To define an indeterminate array property, specify the EXTENT option without the constant argument.

The EXTENT is part of the property data type. For more information, see the Type-name syntax reference entry.

An indeterminate array property can be in one of two states: fixed or unfixed, meaning it either has a fixed dimension or it does not. An indeterminate array property has an unfixed dimension when first defined. You can fix the dimension of an indeterminate array property by:

Initializing the array values when you define the property, using the INITIAL option

Setting the number of elements in the array property using the EXTENT statement

Assigning a determinate array to the indeterminate array, fixing it to the dimension of the determinate array

Passing array parameters to a procedure, user-defined function, or class-based method, so that the indeterminate array property is the target for the passing of a determinate array, fixing the indeterminate array to the dimension of the determinate array

Once fixed, ABL treats a fixed indeterminate array as a determinate array.

If you do not use the EXTENT option (or you specify constant as 0), the property is not an array property.

[ INITIAL { constant | [ constant [ , constant ] . . . ] } ]

The initial value of the property you want to define. If you use the AS primitive-type-name option and you do not use the INITIAL constant option, the initial value is the default initial value for the data type of the variable or array element.

When you define an array property, you can supply initial values for each element in the array. For example:

 

DEFINE VARIABLE array-var AS CHARACTER NO-UNDO EXTENT 3   INITIAL ["Add","Delete","Update"].

If you do not supply enough values to fill up the elements of the array, the AVM puts the last value you named into the remaining elements of the array. If you supply too many values, the AVM raises an error.

If you define a property as an indeterminate array, and you supply initial values for elements in the array, the AVM fixes the number of elements in the array and treats the fixed indeterminate array as a determinate array. For example, the arrays defined by the following statements are equivalent:

 

DEFINE VARIABLE x  AS INTEGER NO-UNDO EXTENT   INITIAL [1,2,3]. DEFINE VARIABLE x1 AS INTEGER NO-UNDO EXTENT 3 INITIAL [1,2,3].

You can also use the EXTENT statement to fix the number of elements in an unfixed indeterminate array variable. For more information, see the EXTENT statement reference entry.

The INITIAL option is not valid either when you define an abstract property or when you declare an interface property prototype.

Table 36 lists the default initial values for the various property data types.

 

Table 33:

Default variable initial values

Data type	Default initial value
CHARACTER	"" (an empty string)
CLASS1,2	Unknown value (?)
COM-HANDLE2	Unknown value (?)
DATE	Unknown value (?) (displays as blanks)
DATETIME	Unknown value (?)
DATETIME-TZ	Unknown value (?)
DECIMAL	0
HANDLE2	Unknown value (?)
INT64	0
INTEGER	0
LOGICAL	no
LONGCHAR	Unknown value (?)
MEMPTR2	A zero-length sequence of bytes
RAW2	A zero-length sequence of bytes
RECID	Unknown value (?)
ROWID2	Unknown value (?)

---

1

If you display a class instance using the MESSAGE statement, ABL automatically invokes the ToString( ) method (provided by the Progress.Lang.Object class) on the object reference. To display a class instance in a frame (for example, using the DISPLAY statement), you must first explicitly convert the object reference to a displayable type using the INT64 function, the INTEGER function, the STRING function, or the ToString( ) method.

2

You cannot use the INITIAL option to specify an initial value for this data type as part of the property definition.

[ NO-UNDO ]

When the value of a property is changed during a transaction and the transaction is undone, the AVM restores the value of the property to its prior value. If you do not want, or if you do not need, the value of such a property to be undone even when it has been changed during a transaction, use the NO-UNDO option. NO-UNDO properties are more efficient; use this option whenever possible.

[ accessor-access-mode ]

Specifies the access mode for the immediately following accessor (GET or SET). The possible access modes include:

 

Syntax 

PROTECTED | PRIVATE

To use an accessor-access-mode with accessors:

You must define at least one accessor (GET or SET) that defaults to the access mode of the property. Therefore, if you define only one accessor, you cannot specify a separate accessor-access-mode for it.

If you define two accessors, you can specify an accessor-access-mode for one of the accessors, but only if this accessor-access-mode is more restrictive than the access mode of the property.

If you are defining an abstract property, you cannot specify the PRIVATE access mode for an accessor.

So, if the property access mode is PUBLIC, any accessor-access-mode that you specify must be either PROTECTED or PRIVATE; if the property access mode is PROTECTED, any accessor-access-mode that you specify must be PRIVATE, unless the property is abstract.

GET [ implementation ] .

An accessor (GET) that defines the property as readable. If you do not define a GET accessor for a class property, the property cannot be read, not even from within the defining class.

It is valid to omit the GET accessor when defining a property for an interface. It is then up to the class implementing the interface to decide whether to define the GET accessor for the property or not. However, as stated above, if you do not define a GET accessor for a class property, the property cannot be read. If you do not define a GET accessor, then you must define a SET accessor.

You can define a GET accessor using two basic forms: with or without an implementation. However, if you are defining an abstract property or an interface property prototype, you cannot define the GET accessor with an implementation. This is the syntax for a GET accessor with an implementation:

 

Syntax 

GET ( [ array-index-parameter ] ) : get-logic END [ GET ] .

array-index-parameter

Specifies an INPUT parameter that provides access to the index value of the current element of an array property from within the get-logic. When a property is defined as an array using the EXTENT option, the GET accessor references the single element of the property array. The array-index-parameter specifies the subscript value for the element being accessed. This is the syntax:

 

Syntax 

INPUT array-index-name AS { INTEGER | INT64 }

array-index-name

Specifies the name of the INPUT parameter that provides the subscript value (index) for this array element. You must use this value in a subscript on the property name (property-name[ array-index-name ]) to access the default memory for the current array element.

INTEGER | INT64

Specifies the data type of the index. Use the data type that supports the largest dimension that is defined for or that you expect to be fixed for the property array at run time.

Where supported, an unsubscripted array reference is a reference to the whole array. For a property array, an unsubscripted read reference invokes the GET accessor for each array element in ascending order by subscript value.

get-logic

Can contain ABL code that executes for any operation that reads the property from outside its own GET accessor definition. This code can include any ABL statements that are valid in a method of a class, including CATCH and FINALLY blocks. However, if you define the property itself as static, you cannot access any instance members of classes (including the defining class) or use the SUPER and THIS-OBJECT system references; you can only access static class members and the local data elements of the GET accessor. The actual value that you return from the property is entirely dependent on your get-logic code. Within get-logic, any operation that reads from property-name (such as the right-hand side of an assignment) directly reads the value of the property’s default memory. However, any operation that writes to property-name within get-logic invokes the property’s own SET accessor to assign the value, exactly like writing the property from outside of its own definition. If the SET accessor is defined with an implementation, that implementation determines how the value is written to the property.

You do not have to use the property’s default memory to provide the value read from a property. You can also use any other accessible data, such as a data member, as storage to access property values. To return a value from get-logic to any outside reader of the property, you must return a value with the correct data type using the RETURN statement (similar to any method that returns a value). If you do not invoke the RETURN statement in get-logic, the property returns the Unknown value (?).

This is the syntax for a GET accessor without an implementation:

 

Syntax 

GET.

Without an implementation, an operation that reads the property directly accesses the current value in the property’s default memory.

If the property is an array, the default accessor implementation handles the subscript references automatically. Providing an array-index-parameter in this case is a compiler error.

SET [ implementation ] .

An accessor (SET) that defines the property as writable. If you do not define a SET accessor, the property cannot be written, not even from within the defining class. You can define a SET accessor using two basic forms: with or without an implementation.

It is valid to omit the SET accessor when defining a property for an interface. It is then up to the class implementing the interface to decide whether to define the SET accessor for the property or not. However, as stated above, if you do not define a SET accessor for a class property, the property cannot be written. If you do not define a SET accessor, then you must define a GET accessor.

You can define a SET accessor using two basic forms: with or without an implementation. However, if you are defining an abstract property or an interface property prototype, you cannot define the SET accessor with an implementation. This is the syntax for a SET accessor with an implementation:

 

Syntax 

SET ( parameter-definition [ , array-index-parameter ] ) :   set-logic END [ SET ] .

parameter-definition

Specifies an INPUT parameter that provides access to the value written to the property from within the set-logic. The set-logic can contain ABL code that executes for any operation that writes to the property from outside its own SET accessor definition. This code can include any ABL statements that are valid in a method of a class, including CATCH and FINALLY blocks. However, if you define the property itself as static, you cannot access any instance members of classes (including the defining class) or use the SUPER and THIS-OBJECT system references; you can only access static class members and the local data elements of the SET accessor. To access the value being written to the property, specify parameter-definition using the following syntax:

 

Syntax 

INPUT parameter-name AS   { primitive-type-name | [ CLASS ] object-type-name }

INPUT parameter-name

Specifies the name of the INPUT parameter that you reference in set-logic to obtain the value written to the property.

AS { primitive-type-name | [ CLASS ] object-type-name }

Specifies the data type of the parameter as a built-in primitive type (primitive-type-name), or as a built-in or user-defined object type (object-type-name)). You must specify exactly the same data type as you have defined for the property.

array-index-parameter

Specifies an INPUT parameter that provides access to the index value of the current element of an array property from within the set-logic. When a property is defined as an array using the EXTENT option, the SET accessor references a single element of the property array. The array-index-parameter specifies the subscript value for the element being accessed. This is the syntax:

 

Syntax 

INPUT array-index-name AS { INTEGER | INT64 }

array-index-name

Specifies the name of the INPUT parameter that provides the subscript value (index) for this array element. You must use this value in a subscript on the property name (property-name[ array-index-name ]) to access the default memory for the current array element.

INTEGER | INT64

Specifies the data type of the index. Use the data type that supports the largest dimension that is defined for or that you expect to be fixed for the property array at run time.

Where supported, an unsubscripted array reference is a reference to the whole array. For a property array, an unsubscripted write reference invokes the SET accessor for each array element in ascending order by subscript value.

set-logic

Can contain ABL code that accesses the value written to the property as parameter-name, then uses parameter-name (if you choose) to set the new value for the property. The actual value that you use to set the property is entirely dependent on your set-logic code. Within set-logic, any operation that writes to property-name (such as the left-hand side of an assignment) directly writes the specified value to the property’s default memory. However, any operation that reads from property-name within set-logic invokes the property’s own GET accessor to read the value, exactly like reading the property from outside of its own definition. If the GET accessor is defined with an implementation, that implementation determines the value that is read from the property.

You do not have to use the property’s default memory to store the property value. You can use any other accessible data, such as a data member, as storage to store property values. If you do not write a value to property-name within set-logic, the property’s default memory retains whatever value it had prior to any operation that writes to the property.

This is the syntax for a SET accessor without an implementation:

 

Syntax 

SET.

Without an implementation, any operation that writes the property writes the value directly to the property’s default memory without passing through parameter-name.

If the property is an array, the default accessor implementation handles the subscript references automatically. Providing an array-index-parameter in this case is a compiler error.

Note:

This is equivalent to, but more efficient than, using an implementation to explicitly assign property-name the value of parameter-name.

Examples 

The examples that follow show two different ways to access the same private data of a class using properties.

The first example, shows a class (r-DefineProperties1) that defines a PUBLIC property (cCurrentSalesRepName) followed by a procedure that accesses this property. The property is defined with two accessors without implementations, providing direct access to the property value. The GET accessor is PUBLIC, but the SET accessor is PRIVATE, allowing only the class or an instance of the class to set the property value. In this case, the class sets the property from data (RepName field) in the sports2000 database provided by a buffer (bSalesRep), which is PRIVATE.

The class also provides a PUBLIC method (getNextSalesRep( )) to read the SalesRep table one record at a time and set the cCurrentSalesRepName property to the value of the RepName field for each record. This PUBLIC method also uses a PRIVATE method (restartSalesRep( )) to reset the record position to the beginning of the table, based on an INPUT parameter. The class constructor also uses this PRIVATE method to initialize the record buffer to the first record in the table. The class sets the cCurrentSalesRepName property to the Unknown value (?) if the SalesRep table is empty or getNextSalesRep( ) reaches the end of the SalesRep table.

 

r-DefineProperties1.cls

CLASS r-DefineProperties1:     DEFINE PUBLIC PROPERTY cCurrentSalesRepName AS CHARACTER NO-UNDO     GET.     PRIVATE SET.   DEFINE PRIVATE BUFFER bSalesRep FOR SalesRep.     CONSTRUCTOR PUBLIC r-DefineProperties1 ():     restartSalesRep() NO-ERROR.     IF ERROR-STATUS:ERROR THEN RETURN ERROR ERROR-STATUS:GET-MESSAGE(1).   END CONSTRUCTOR.     METHOD PRIVATE VOID restartSalesRep ():     FIND FIRST bSalesRep NO-ERROR.     IF NOT AVAILABLE bSalesRep THEN DO:       cCurrentSalesRepName = ?.       RETURN ERROR "SalesRep table empty".     END.     ELSE       cCurrentSalesRepName = bSalesRep.RepName.   END METHOD.     METHOD PUBLIC VOID getNextSalesRep (INPUT lRestart AS LOGICAL):     IF lRestart THEN DO:       restartSalesRep() NO-ERROR.       IF NOT AVAILABLE bSalesRep THEN         RETURN ERROR ERROR-STATUS:GET-MESSAGE (1).     END.     ELSE DO:       FIND NEXT bSalesRep NO-ERROR.       IF NOT AVAILABLE bSalesRep THEN         cCurrentSalesRepName = ?.       ELSE         cCurrentSalesRepName = bSalesRep.RepName.     END.   END METHOD.   END CLASS.

The following procedure (r-runDefineProperties1.p) instantiates the r-DefineProperties1 class, referenced by clProps, and reads and displays the value of the clProps:cCurrentSalesRepName property in a message, starting with the first SalesRep record found as part of class instantiation. The procedure then displays the value of clProps:cCurrentSalesRepName in a message for each record found by the clProps:getNextSalesRep( ) method, restarting from the beginning of the SalesRep table at the direction of the user.

 

r-runDefineProperties1.p

DEFINE VARIABLE clProps AS CLASS r-DefineProperties1 NO-UNDO.   clProps = NEW r-DefineProperties1() NO-ERROR. IF ERROR-STATUS:ERROR THEN DO:   MESSAGE ERROR-STATUS:GET-MESSAGE(1) VIEW-AS ALERT-BOX.   QUIT. END.   DO WHILE NOT clProps:cCurrentSalesRepName = ?:   MESSAGE "The current sales rep is "     clProps:cCurrentSalesRepName + ".  Continue?"     VIEW-AS ALERT-BOX QUESTION BUTTONS YES-NO UPDATE lContinue AS LOGICAL.   IF NOT lContinue THEN LEAVE.   ELSE DO:     clProps:getNextSalesRep(FALSE).     IF clProps:cCurrentSalesRepName = ? THEN DO:       MESSAGE "End of sales rep list.  Restart?"         VIEW-AS ALERT-BOX QUESTION BUTTONS YES-NO         UPDATE lRestart AS LOGICAL.       IF lRestart THEN         clProps:getNextSalesRep(TRUE).     END.   END. END.

In the next example, the r-DefineProperties2 class defines a PUBLIC property (cNextSalesRepName) that provides the same data as the cCurrentSalesRepName property defined by the r-DefineProperties1 class. However, the GET accessor of the cNextSalesRepName property is also used to provide the same access to the SalesRep table that the getNextSalesRep( ) method provides for the r-DefineProperties1 class. So, the value of cNextSalesRepName changes with each access.

 

r-DefineProperties2.cls

CLASS r-DefineProperties2:     DEFINE PUBLIC PROPERTY lSalesRepRestart AS LOGICAL NO-UNDO     PRIVATE GET.     SET.   DEFINE PRIVATE BUFFER bSalesRep FOR SalesRep.     DEFINE PUBLIC PROPERTY cNextSalesRepName AS CHARACTER     GET ():       IF lSalesRepRestart THEN DO:         restartSalesRep() NO-ERROR.         IF NOT AVAILABLE(bSalesRep) THEN           RETURN ERROR ERROR-STATUS:GET-MESSAGE(1).       END.       ELSE DO:         FIND NEXT bSalesRep NO-ERROR.         IF NOT AVAILABLE(bSalesRep) THEN           cNextSalesRepName = ?.         ELSE           cNextSalesRepName = bSalesRep.RepName.       END.       RETURN cNextSalesRepName.     END GET.     PRIVATE SET.     CONSTRUCTOR PUBLIC r-DefineProperties2     (OUTPUT cFirstSalesRepName AS CHARACTER):     ASSIGN       lSalesRepRestart   = TRUE       cFirstSalesRepName = cNextSalesRepName.   END CONSTRUCTOR.     METHOD PRIVATE VOID restartSalesRep ():     lSalesRepRestart = FALSE.     FIND FIRST bSalesRep NO-ERROR.     IF NOT AVAILABLE bSalesRep THEN DO:       cNextSalesRepName = ?.       RETURN ERROR "SalesRep table is empty".     END.     ELSE       cNextSalesRepName = bSalesRep.RepName.   END METHOD.   END CLASS.

Because this cNextSalesRepName property incorporates the record access provided by the getNextSalesRep( ) method, the following procedure that accesses the cNextSalesRepName property must also use the property in a manner similar to how the r-runDefineProperties1.p procedure uses the getNextSalesRep( ) method. As a result, the class also provides a second, publicly writable property (lSalesRepRestart) to indicate when the reading of SalesRep records must restart from the beginning of the table. (Note that the getNextSalesRep( ) method provides its own INPUT parameter to indicate whether to restart record reading.)

Finally, the r-DefineProperties2 class constructor sets lSalesRepRestart together with an initial read of the cNextSalesRepName property in order to initialize the record buffer to the first record of the table and pass the associated RepName field value to the instantiating procedure as an OUTPUT parameter.

Thus, the following procedure (r-runDefineProperties2.p) instantiates the r-DefineProperties2 class, referenced by clProps, and reads and displays the data from the clProps:cNextSalesRepName property in a manner similar to how the r-runDefineProperties1.p procedure reads and displays the same data using the getNextSalesRep( ) method. However, because the clProps:cNextSalesRepName property always returns the RepName field for the next SalesRep record in the table, the r-runDefineProperties2.p procedure must provide a separate variable (cCurrentSalesRepName) of its own, which provides the same function that the cCurrentSalesRepName property provides for the r-DefineProperties1 class, which is to maintain a current value read from the RepName field.

 

r-runDefineProperties2.p

DEFINE VARIABLE clProps              AS CLASS r-DefineProperties2 NO-UNDO. DEFINE VARIABLE cCurrentSalesRepName AS CHARACTER NO-UNDO.   clProps = NEW r-DefineProperties2(OUTPUT cCurrentSalesRepName) NO-ERROR.   DO WHILE NOT cCurrentSalesRepName = ?:   MESSAGE "The current sales rep is " cCurrentSalesRepName + ".  Continue?"     VIEW-AS ALERT-BOX QUESTION BUTTONS YES-NO     UPDATE lContinue AS LOGICAL.   IF NOT lContinue THEN LEAVE.   ELSE DO:     cCurrentSalesRepName = clProps:cNextSalesRepName.     IF cCurrentSalesRepName = ? THEN DO:       MESSAGE "End of sales rep list.  Restart?"         VIEW-AS ALERT-BOX QUESTION BUTTONS YES-NO         UPDATE lRestart AS LOGICAL.       IF lRestart THEN         ASSIGN           clProps:lSalesRepRestart = TRUE           cCurrentSalesRepName     = clProps:cNextSalesRepName.     END.   END. END.

Thus, the logic of the r-runDefineProperties2.p procedure is almost identical to the r-runDefineProperties1.p procedure, reading a clProps:cNextSalesRepName property instead of invoking a clProps:getNextSalesRep( ) method in order to read an appropriate value from the SalesRep table.

For more examples of property definitions, including static and abstract properties, see the descriptions of r-CustObj.cls, r-CustObjStatic.cls, and r-CustObjAbstract.cls in the CLASS statement reference entry.

Notes 

You cannot define a property within a procedure (internal or external) or within a method of a class (constructor or destructor). It can only be defined as a member of a class or as a prototype in an interface. For more information about declaring property prototypes in an interface, see the INTERFACE statement reference entry.

PUBLIC and PROTECTED properties defined within a class definition (.cls) file maintain their characteristics throughout the inherited class hierarchy. Thus, you cannot shadow (override) properties in a subclass that are defined in a super class.

You must define at least one GET or SET accessor, or define both accessors, for the property to be valid.

You never invoke the GET and SET accessors defined for a property directly. These methods are only invoked implicitly when you read (GET) or write (SET) the value of the property. For example, you can read or write the property by using the property in an Assignment (=) statement or by passing the property as a parameter to a method, procedure, or user-defined function. For information on reading and writing property values, see the Class-based property access reference entry. As an alternative, you can define a method in a class to read or write the value of a data member that you separately define in the same class.

If the property name is an ABL reserved keyword, you might need to use the THIS-OBJECT system reference or Type-name syntax to reference the property. For more information on referencing properties, see the reference entry for Class-based property access.

To avoid any unexpected performance impact when using the property, ensure that the body of any GET or SET accessor minimizes processing that might incur delay when referencing the property.

You cannot specify formatting information in a property definition. You can control property formatting using the same default rules for formatting a variable or data member:

a)	Rely on the default formatting for the data type of the property. For more information, see the Data types reference entry.

b)	Add formatting information when you access the property in a statement (for example, using a DISPLAY statement).

You can handle application errors in an a property accessor as in any ABL block. By executing a RETURN ERROR action at the block level or a THROW action at the block level with the presence of a ROUTINE-LEVEL ON ERROR UNDO, THROW statement, the AVM returns the ERROR condition to the statement that references the property and works much like an error raised by a method. If a RETURN ERROR also includes the option to return a character string value, or you set the ReturnValue property of a Progess.Lang.AppError object that you THROW, you can get this value using the RETURN-VALUE function following the statement that references the property or in a CATCH block that catches the Progress.Lang.AppError object. If the body of an accessor contains an UNDO block, any unhandled ERROR condition in that block undoes only the data within that block, according to the NO-UNDO setting of the data. The property value, itself, is not undone unless the property is defined without NO-UNDO. For more information, see OpenEdge Development: Object-oriented Programming.

If ERROR is raised during execution of an Assignment (=) statement, the value on the left-hand side usually remains unchanged from its value prior to the assignment. However, if the left-hand side of the assignment is a property and its SET accessor invokes the RETURN ERROR statement (raising ERROR on the assignment), the value of the property can be changed. This is because the SET accessor is a method, and like all methods that raise ERROR, any data elements that the SET accessor changes retain their most recent values after ERROR is raised. Thus, if the SET accessor changes the property value before invoking RETURN ERROR, the property retains its most recent change in value after ERROR is raised on the assignment.

If an ABL property that implements a property defined in a .NET interface is accessed from .NET and it raises ERROR out of the accessor block, ABL returns a .NET System.Exception to the caller. If the error is raised by executing a RETURN ERROR with the optional error string, the Message property of the System.Exception describes the operation where the error occurred, but the error string is available only to the ABL session, using the RETURN-VALUE function. If the error is raised by executing a RETURN ERROR with an optional ABL error object or by executing an UNDO, THROW, the System.Exception Message property includes both a description of the operation where the error occurred and any messages from the ABL error object. If the error is fatal, the AVM responds as for any ABL class, generating a protrace file and exiting the session.

See also 

Assignment (=) statement, Class-based property access, Data types, DEFINE VARIABLE statement, DISPLAY statement, METHOD statement, RETURN statement