Why can't I declare an external variable in C#?

Question

I need to declare a variable inside an if statement. Then I will use it on the outside. But as far as I know, there are no external variables in C#. But I need to do this.

I have two classes both derived from one class.

Base class: Operand Derived classes: NormalOperand SpecialOperand

bool normal

Declared somewhere

if(normal)
    NormalOperand o = stack.Pop() as NormalOperand;
else
    SpecialOperand o = stack.Pop() as SpecialOperand;

I don't want to deal this differences below. Is there any hack to do that? Or do I have to deal with it everywhere I do something related to this?

Answer 1

I don't see the issue, just declare o as the base class Operand.

Operand o = stack.Pop(); // add as Operand if needed

Later if you need to know the type of o (using virtual/abstract methods on base class should avoid this) then use:

if (o is NormalOperand)
{
    // TODO: maybe check for null
    (o as NormalOperand).NormalSpecificMethod();
}