Variable declaration in loop doesn't get reset to default

Question

Consider following code:

for (int i = 0; i < 10; i++)
{
    bool b;          /* #1 */
    if (i == 0)
    {
        b = true;    /* #2 */
    }
}

I have set breakpoints at #1 and #2.

The first time (i = 0), b is set to false at #1 and set to true at #2.

The second time (i = 1), b is true at #1.

This doesn't make sense to me, because I assumed that when starting in the second loop (i = 1), b should be false at declaration again.

I assumed that b = false at #1 in the second loop.

Anyone care to explain?

You can't use b before it's initialised to a known value anyway, so why does it matter? I would guess it's because b is actually declared local to the method above your loop, so it'll hold its value — James
– James, Commented Jun 17, 2014 at 13:08
@JamesBarrass really? I thought all structs were supposed to be self initializing to their default value? — Ian
– Ian, Commented Jun 17, 2014 at 13:09
@JamesBarrass mostly out of curiousity. I want to know why the compiler works this way, so why not ask? — Recipe
– Recipe, Commented Jun 17, 2014 at 13:10
I don't think it's an exact duplicate of this question, although the subject is definitely the same. I doubt that the linked question would provide the answer the OP seeks. I was unable to find a better-fitting duplicate, so I voted to re-open the question. — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Jun 17, 2014 at 13:17

Sergey Kalinichenko · Accepted Answer · 2014-06-17 13:25:02Z

7

The value of b that you see in the debugger is invalid. C# requires all local variables to be initialized or assigned before you can read from them. Therefore, the value that you see in the debugger in the Visual Studio is useless, because it cannot be read. There is no way to write a piece of C# code that "sees" that value being set to true on the second iteration, because the compiler would flag such use as invalid:

for (int i = 0; i < 10; i++)
{
    bool b;          /* #1 */
    Console.WriteLine(b); // <<== INVALID!!! This will not compile.
    // error CS0165: Use of unassigned local variable `b'
    if (i == 0)
    {
        b = true;    /* #2 */
    }
}

If you add an explicit initialization to false, the code above would compile. The debugger would show false at the breakpoint, too.

edited Jun 17, 2014 at 13:25

answered Jun 17, 2014 at 13:13

Sergey Kalinichenko

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2 Comments

Sergey Kalinichenko Over a year ago

@JamesBarrass You are right, "wrong" is not the right word. I think "useless" would be better. Thanks!

James Over a year ago

Agreed, it is useless!

James · Accepted Answer · 2014-06-17 13:21:12Z

2

Note that if you try

for (int i = 0; i < 10; i++)
{
    bool b;          /* #1 */

    if (!b)
    {
        i = 100000;
    }

    if (i == 0)
    {
        b = true;    /* #2 */
    }
}

You'll get a compile error, since variables must be initialised before use

but for your curiosity....

If you look at the IL you'll note that b is declare as local.

.maxstack 2
.locals init (
    [0] int32 i,
    [1] bool b,
    [2] bool CS$4$0000
)

That means it's allocated stack space at when the method is loaded on to the stack. The space it uses won't change during the methods execution so it won't reset unless you tell it to with something like b = default(typeof(bool));

answered Jun 17, 2014 at 13:21

James

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Comments

James · Accepted Answer · 2014-06-17 13:32:26Z

1

The C# spec dictates that local variables are scoped to within the for statement, however, what you will probably find is the compiler is re-using the same variable across all iterations as an optimization rather than creating/throwing away the same var each time.

With that being said, given you don't explicitly set a default value then it makes sense that b doesn't actually reset back to false on the next iteration. If you set a default value for b then you should find your code works as expected, and in fact you should give it a default value anyway; it's not a good idea to rely on defaults because these can change.

Explicit declarations give much more clarity and improve readability.

edited Jun 17, 2014 at 13:32

answered Jun 17, 2014 at 13:24

James

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Comments

Thirusanguraja Venkatesan · Accepted Answer · 2014-06-17 13:32:59Z

0

Note that in your code you declared

bool b;

and you not assigned default value to that declared variable and then you trying to assign value to bool b; in the if condition block only it will make some confusion.

So avoid those kind of confusion first you must assign the value to declared variable bool b; after declaring it will give you clear idea about default value and future assigned value by you in the loop

Like bool b = True; or bool b = False;

Modified Code

for (int i = 0; i < 10; i++)
{
    bool b = False;          /* #1 */
    if (i == 0)
    {
        b = true;    /* #2 */
    }
}

answered Jun 17, 2014 at 13:32

Thirusanguraja Venkatesan

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2 Comments

Recipe Over a year ago

I know this, but the real question was why the compiler acted the way it does. Thanks for taking the effort to answer though! Greatly appreciated.

Thirusanguraja Venkatesan Over a year ago

The C# compiler does not allow the use of uninitialized variables.

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