Why does a try/finalize block does not throw CS0165: Use of unassigned local variable compilation error?

Question

Why is it that variant_1 does compile while variant_2 fails with CS0165: Use of unassigned local variable , as expected? I imagined maybe finally initializes unassigned variables to their default, but I do not see that happening.

I ran this code with .NET Core 3.1

static string variant_1()
{
    string str;

    try
    {
        str = "grrr";
    }
    finally
    {

    }

    return str; //no problem ?
}

static string variant_2()
{
    string str;

    try
    {
        str = "grrr";
    }
    catch (Exception ex)
    {

    }

    return str; //does not compile: `CS0165: Use of unassigned local variable`
}

Answer 1

Let's imagine that something bad can happen here:

static string variant_2()
{
    string str;

    try
    {
        //Something bad happens
        str = "grrr";
    }
    catch (Exception ex)
    {

    }

    return str; //does not compile: `CS0165: Use of unassigned local variable`
}

Maybe it's a ThreadAbortException . Maybe it's something else. What happens? Control flow enters the catch and then the remainder of the function executes . str was never assigned.

For the finally case though, if any exception occurs, control flow may enter the finally but then it exits the method . return is never going to be encountered. The only path through that try / finally block that continues to the return is one where no exception occurs in the try part and so the assignment that occurs inside it must have been made.