Pass second parameter if first is not set (C#)

I have below switch:

int op = step.BasicOP;

string myOp = "someOperation";
OperationType x;

switch(myOp)
{
 case "test":
   x = OperationType.OP1;
   //some code
   break;
 case = "someOperation";
   x = OperationType.OP2;
   //some code
 case "other example":
   //some code
   break;
 default:
   throw new ArgumentException("not covered");
}

addRound.addProps("Operation", x);

but for case "other example" I did not set x. Instead I need to pass there op. So I'm doing like:

//if x is not set, pass/take op
addRound.addProps("Operation", x : op);

But this is wrong. How to correct write?