How to parse xpath if statement with C#

I have an xpath which contains an if statement. I want to evaluate it with c#. If i dont have any functions in xpath I use

node.SelectSingleNode(xpath);

When I try to do this with if statement I get an invalid token error.

This is my xpath:

if (substring-before(substring-after(//IzdaniRacunEnostavni/@xsi:noNamespaceSchemaLocation,'http://ift.tt/1vLROET'),'_EnostavniRacun.xsd')='eSLOG_1-6') then //Postavka/StevilkaVrstice[text()!='']/../../ZneskiPostavke/VrstaZneskaPostavke[text() ='203']/../ZnesekPostavke else     //Postavka/StevilkaVrstice[text() !='']/../../ZneskiPostavke/VrstaZneskaPostavke[text() ='66']/../ZnesekPostavke

If I test this xpath with http://ift.tt/1n7QXVm it shows as valid.