Unassigned local variable mystery

问题

    int x;
    if (Q())
        x = 123;
    if (R())
        Console.WriteLine(x); // illegal

    int x;
    if (Q())
        x = 123;
    if (false)
        Console.WriteLine(x); // legal!!

May I know why second one is legal while former one is throwing 'using unassigned local variable' compile time exception?

回答1:

This tells compiler that the condition under if in the statement given below will never execute, so the constraint of unused variable does not apply to it.

if (false)
        Console.WriteLine(x); // legal!!

回答2:

Because the second one is eliminated by compiler as never executed.

回答3:

If Q() == FALSE and R() == true, x is not set and it will try to use it.

来源：https://stackoverflow.com/questions/11071682/unassigned-local-variable-mystery