Sorry, that’s what I’m trying to tell you.
You NEED a map. This will not work unless you use one. 
Unless each item has its own struct, you can’t search through a bunch of structs for the one you need, you have to specify. The map would allow you to specify which one you need by passing in a string, name, or text variable (or other kinds but for this stick to one of those three).
Otherwise, you can just use “Set Members” and then “Set Struct” to save values,
and “Get” then break the output pin on the struct to Get a value- but you have to have already specified the struct, as I stated.
Also, casting doesn’t do what I think you think it does… It’s not a “Get” node, it’s a reshaping node. You cannot cast to variables (though you can cast a struct to another struct). What that means is you can make struct A BEHAVE AS struct B. Another word for cast is mold, if that helps?