"use of unassigned variable" compile error that will never happen

Question

I have the following code, which results in a compile error, despite seemingly being correct:

bool aBool = SomePredicate();
if (!aBool && !OutFunction(out int value))
{
    return;
}
int result = -1; //other code here...
if (!aBool && SomeCondition(result, value)) // CS0165: Use of unassigned variable 'value'.
{

}

The variable aBool is not modified between the two conditions, meaning that if aBool was false during the first check it'll always be assigned. Otherwise, if aBool was true, then the variable would never be used, as the expression should end once the first part is evaluated.

Despite this, why does this result in a compile error? (C# 7.3) Additionally, what is the best way to get around it?

Main focus is on the "Why?" part

Answer 1

&& is a short-circuiting AND so if !aBool is evaluated to false, !OutFunction(out int value) is never evaluated and the method continues, so at SomeCondition(result, value) value can be not initiated.

You can try using Logical AND operator &:

bool aBool = SomePredicate();
if (!aBool & !OutFunction(out int value))
{
    return;
}
int result = -1; //other code here...
if (!aBool && SomeCondition(result, value))
{

}

bool SomePredicate() => false;
bool OutFunction(out int i) => false;
bool SomeCondition(int i, int j) => false;

So it always executes (Demo @sharplab.io). Or rewrite code in some other way (for example by declaring and initializing the variable before the OutFunction call).

Compiler has some limitations of conditional flow and definite assignment analysis when it comes to such checks and it seems that this is one of the corner cases. Some of them were recently fixed for later compilers with C# 10 feature Improved Definite Assignment Analysis.

UPD

From the discussion @github.com/dotnet/csharplang several useful links were referenced, one of them stating the following:

To provide a little background, the compiler does what is usually referred to as a "path-independent flow analysis" for these situations. What this means practically is that conditional states are intersected at the end of the condition.

and

One way to learn about the states would be to do what's called a "path-dependent" flow analysis, but that can be significantly more expensive and is not in the design of C# 8.

So the simpler [mre] would be just:

using System;

bool aBool = SomePredicate();
int value;

if (!aBool)
{
    value = 1;
}

if (!aBool) 
{
   Console.WriteLine(SomeCondition(value));// CS0165: Use of unassigned variable 'value'.
}

bool SomePredicate() => false;
bool SomeCondition(int i) => false;

@sharplab.io