Pattern matching - variable in scope outside if-block

Question

I\'m trying to understand why y is in scope in the following example:

static void Main(string[] args)
{
    int x = 1;
    if (x is int y) { }

         


        

Answer 1

The problem is the output code created, which results in this strange behavior, which is a bug in my opinion. I guess this will be fixed eventually.

The thing is this: x is int y evaluates to true on compile time, resulting in the if rendered obsolete and the assignment done before the if. See the compiled code here:

int num = 1;
int num2 = num;
bool flag = true;
if (flag)
{
}
Console.WriteLine("This should NOT be in scope:" + num2);

There seems a bug in the compilation of the is object variant. It compiles to something like this (note the parenthesis I added):

int num = 1;
object arg;
bool flag = (arg = num as object) != null;

Console.WriteLine("This should NOT be in scope:" + arg);

Which is valid, even in C# 6. However, the compiler thinks arg isn't always set. Adding an else branch fixes this bug:

else { y = null; }

Results in:

int num = 1;
object arg;
bool flag = arg = num as object != null;
if (!flag)
{
    arg = null;
}
Console.WriteLine("This should NOT be in scope:" + arg);

Answer 2

That's is something I had found in VS15P4 (or P5) and it was being tracked on GitHub:

Definite assignment versus pattern-matching when pattern always matches #14651

Putting aside all the discussions, it is a scoping issue which occurs when the pattern always matches, as the title suggests.

I thought it was fixed before RC so I haven't actually tested afterwards but it seems the issue still persists.

Let's see if it will be fixed in the actual release, which is tomorrow :)