PROPERTY

PROPERTY declares a named value on a function block type that is read and written like a variable, but runs code when it is: a GET accessor computes the value on a read, and a SET accessor handles a write. A property can have either accessor or both, so it can be read-only, write-only, or read-write. A property declaration is terminated by END_PROPERTY.

Note

PROPERTY is a keyword only when the --allow-fb-inheritance flag is enabled. When the flag is not set, PROPERTY is an ordinary identifier and may be used as a variable or type name, exactly as in standard IEC 61131-3. Pass --allow-fb-inheritance to enable the object-oriented syntax, or select a dialect that includes it. See Enabling Dialects and Features for the dialects and flags reference.

Note

END_PROPERTY, END_GET and END_SET are gated by the same flag. GET and SET are keywords only where an accessor starts, directly inside a property. Everywhere else they stay ordinary identifiers, even with the flag enabled, so a variable or an input named SET keeps working.

IEC 61131-3	Object-oriented extension, as used by CODESYS and TwinCAT
Support	Parsed and analyzed: a function block that declares properties compiles, and the accessor bodies are checked. Reading or writing a property is not supported yet (see Current limitations). Enable with --allow-fb-inheritance; see Enabling Dialects and Features.

Syntax

A property is declared inside the function block it belongs to, between the function block’s body and END_FUNCTION_BLOCK, in any order with its methods:

PROPERTY property_name : type
    [GET
        variable_declarations
        statement_list
    END_GET]
    [SET
        variable_declarations
        statement_list
    END_SET]
END_PROPERTY

Inside GET, the property name is the result: assign it, as a method with a return type assigns its own name. Inside SET, the property name holds the value being written. Each accessor can declare its own local variables with VAR.

In a TwinCAT .TcPOU file, a property is a <Property> element with <Get> and <Set> children. IronPLC reads both forms.

Example

FUNCTION_BLOCK FB_Motor
    VAR
        _speed : REAL;
    END_VAR

    PROPERTY Speed : REAL
        GET
            Speed := _speed;
        END_GET
        SET
            _speed := Speed;
        END_SET
    END_PROPERTY
END_FUNCTION_BLOCK

Current limitations

A property can be declared, but not used yet. Reading it (x := motor.Speed) or writing it (motor.Speed := 2.0), from outside the function block or by its bare name inside it, reports P9999. Until that is supported, call a method or access a field of the function block instead.

Access modifiers such as PROPERTY PUBLIC are not parsed yet, and neither are properties declared in an INTERFACE.

See Also

• METHOD: declare an operation on a function block type

• INTERFACE: declare a set of method signatures

• Object Oriented Programming: inheritance, interfaces, and abstract types explained

• FUNCTION_BLOCK: the FUNCTION_BLOCK unit