Hi, what is your output from this in Dynamo, does it return DB.Solids?
PS: Why don’t you try the JoinGeometry class as mentioned in this topic: Join solids in Revit API
What is your end goal actually? Do you need to get the exact geometry of the difference between both sets of columns or only check if there is a difference? If it is the latter, you could try getting the sums of volumes for both sets of columns, using the Solid.Volume property. If both sums are equal in the end, there is no difference 