How to check if an object is NULL in Inno Setup Pascal Script?

Question

I am writing a pascal function using Inno setup studio which checks if an object is null and does something

so far I have:

XMLDocument.setProperty('SelectionLanguage', 'XPath');
XMLNode := XMLDocument.selectSingleNode(APath);
if (XMLNode=Null) then
begin
//do stuff
End
Else
//do other stuff
End

but I keep getting invalid variant operation error.

How do I check if an object is null in Inno Setup Pascal Script code?

Answer 1

For checking if a Variant is NULL use the VarIsNull function:

if VarIsNull(XMLNode) then

However, in your case, the problem is little more complicated. The selectSingleNode method returns always a variant of type varDispatch whose actual data pointer points to a found XML DOM node, or to nil in case no such node is found. In Delphi (language in which Inno Setup Pascal Script is written) there is the VarIsClear function which covers also such situation. Unfortunately, it is not published in Inno Setup. You can however test this case with a statement like this:

if (IDispatch(XMLNode) = nil) then

That will get the data from the returned varDispatch variant and those data test for nil.

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Martijn Laan added the VarIsClear function to Unicode version of Inno Setup in this commit, so since Inno Setup 5.5.6 you can use VarIsClear instead of the above hack.