Chapter 6: Calling Functions and Events

Errors when calling functions and events dynamically

If you call a function or event dynamically, different conditions create different results, from no effect to an execution error. The tables in this section illustrate this.

Functions

The rules for functions are similar to those for events, except functions must exist: if a function is not found, an error always occurs. Although events can exist without a script, if a function is defined it has to have code. Consider the following statements:

This statement calls a function without looking for a return value:
```
object.DYNAMIC funcname( )
```

This statement looks for an integer return value:

int li_int

li_int = object.DYNAMIC funcname( )

This statement looks for an Any return value:

any la_any

la_any = object.DYNAMIC funcname( )

The following table uses these statements as examples.

**Table 6-3: Dynamic function calling errors**
Condition 1	Condition 2	Result	Example
The function does not exist.	None.	Execution error 65: Dynamic function not found.	All the statements cause error 65.
The function is found and executed but is not defined with a return value.	The code is looking for a return value.	Execution error 63: Function/event with no return value used in expression.	Statements 2 and 3 cause error 63.

Events

Consider these statements:

This statement calls an event without looking for a return value:
```
object.EVENT DYNAMIC eventname( )
```

This example looks for an integer return value:

int li_int

li_int = object.EVENT DYNAMIC eventname( )

This example looks for an Any return value:

any la_any

la_any = object.EVENT DYNAMIC eventname( )

The following table uses these statements as examples.

**Table 6-4: Dynamic event calling errors**
Condition 1	Condition 2	Result	Example
The event does not exist.	The code is not looking for a return value.	Nothing; the call fails silently.	Statement 1 fails but does not cause an error.
	The code is looking for a return value.	A null of the Any datatype is returned.	La_any is set to null in statement 3.
		If the expected datatype is not Any, execution error 19 occurs: Cannot convert Any in Any variable to datatype.	The assignment to li_int causes execution error 19 in statement 2.
The event is found but is not implemented (there is no script).	The event has a defined return value.	A null of the defined datatype is returned.	If eventname is defined to return integer, li_int is set to null in statement 2.
	The event does not have a defined return value.	A null of the Any datatype is returned.	La_any is set to null in statement 3.
		If the expected datatype is not Any, execution error 19 occurs: Cannot convert Any in Any variable to datatype.	The assignment to li_int causes execution error 19 in statement 2.
The event is found and executed but is not defined with a return value.	The code is looking for a return value.	Execution error 63: Function/event with no return value used in expression.	Statements 2 and 3 cause error 63.

When an error occurs

You can surround a dynamic function call in a try-catch block to prevent the application from terminating when an execution error occurs. Although you can also handle the error in the SystemError event, you should not allow the application to continue once the SystemError event is invoked—the SystemError event should only clean up and halt the application.

For information on using try-catch blocks, see the section on exception handling in the Resource Guide.

If the arguments do not match

Function arguments are part of the function’s definition. Therefore, if the arguments do not match (a compatible match, not an exact match), it is essentially a different function. The result is the same as if the function did not exist.

If you call an event dynamically and the arguments do not match, the call fails and control returns to the calling script. There is no error.

Error-proofing your code

Calling functions and events dynamically opens up your application to potential errors. The surest way to avoid these errors is to always make static calls to functions and events. When that is not possible, your design and testing can ensure that there is always an appropriate function or event with the correct return datatype.

One type of error you can check for and avoid is data conversion errors.

The preceding tables illustrated that a function or event can return a null value either as an Any variable or as a variable of the expected datatype when a function or event definition exists but is not implemented.

If you always assign return values to Any variables for dynamic calls, you can test for null (which indicates failure) before using the value in code.

This example illustrates the technique of checking for null before using the return value.

any la_any
integer li_gotvalue
la_any = object.DYNAMIC uf_getaninteger( )
IF IsNull(la_any) THEN
   ... // Error handling
ELSE
   li_gotvalue = la_any
END IF

Overloading, overriding, and extending functions and events

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