Testing for existence of a property

[Marc Lang]

I have some recursive code that parses a file and sets values of properties in a class based on those values.

It's fairly flexible and dynamic though, so the class it's writing to may not always have properties to hold all the values.

How do I test for the existence of a property in a class before setting it?

If I try set a property that doesn't exist I get <PROPERTY DOES NOT EXIST>

I have set up a Catch to catch that error and swallow it, but hoping for a more elegant solution?

Try {

// First check the property exists
set help = $property(pObject,propname) // THROWS ERROR HERE IF PROPERTY DOESN'T EXIST

// doing stuff
Set $property(pObject,propname) = value


}
Catch ex {

If (ex.Name = "<PROPERTY DOES NOT EXIST>")
{
// Do nothing
}

}

[Andrew Makinson]

Does $Data command work on properties? Not near a cache server so can't test.

I.e 

For a variable we can use 

IF $D(x) write "exists"

Sent from my iPhone

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[Lexi Hayden]

You could probably use %Dictionary.CompiledClass to see if the class definition includes the property.

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[Dale du Preez]

In the same vein, you could use the %Dictionary.CompiledProperty class as follows to do the exact lookup you want:

    If ##class(%Dictionary.CompiledProperty).IDKEYExists($classname(pObject),propname) {
        Set $property(pObject,propname) = value
    }

If you are on a much older version of Ensemble/Cache where $classname() is not available, you can use pObject.%ClassName(1), but this not as fast as $classname(pObject).

Note that the approach above may have more overhead than simply using a try/catch block as you need to do a database lookup.

Dale

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[Ted Peck]

Another good trick is to put a try/catch around the property access. If it doesn't exist you will catch a <PROPERTY DOES NOT EXIST> exception. This is actually more efficient in most cases than querying the class dictionary.

On 10/3/2012 10:46 AM, Lexi Hayden wrote:

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[Ted Peck]

Whoops - I just noticed that's where this thread started. Still, it's not as ugly as it seems.

[Marc Lang]

Thanks folks.

I'll create a utility function based on the try/catch method, which accepts an object and property name., and returns true/false.

That hides the guts of it behind a one-line function call, and should be pretty fast.

Thanks