Why local variable in C# must be initialized after initializing and cast inside If operator condition?

Question

public void A()
{
    if (!(_s is string v))
        return;

    Console.WriteLine(v);
}

This code compiles good
But

public void A()
{
    if (false == _s is string v)
        return;

    Console.WriteLine(v); <-- 'Local variable 'v' might not be initialized before accessing'
}

But this returns compiler error
What's the reason?

Answer 1

The flow analysis isn't perfect.

if (true == _s is string v)
  Console.WriteLine(v); 

also fails.

Definite assignment flow analysis is conservative - its purpose is to make sure you can't read an unassigned local. It's desirable for it to also be able to figure out branches that can never be reached given a set of conditions, but that's a bonus.

In the end, you would need to do a specific analysis to handle the case of false == _s is string v. But why, when there's no reason not to use !(_s is string), which is handled just fine? You can always do the analyzer more complicated, but there's also a value to keeping it simple.