C# Closures, why is the loopvariable captured by reference?

Question

In this example, I\'m attempting to pass by value, but the reference is passed instead.

for (int i = 0; i < 10; i++)
{
    Thread t = new Thread(() =>         


        

Answer 1

This is easier to understand if you look at what happens, in terms of scope:

for (int i = 0; i < 10; i++)
{
    Thread t = new Thread(() => new PhoneJobTest(i);    
    t.Start();
}

Basically translates to something very close to this:

int i = 0;
while (i < 10)
{
    Thread t = new Thread(() => new PhoneJobTest(i);    
    t.Start();
    i++;
}

When you use a lambda expression, and it uses a variable declared outside of the lambda (in your case, i), the compiler creates something called a closure - a temporary class that "wraps" the i variable up and provides it to the delegate generated by the lambda.

The closure is constructed at the same level as the variable (i), so in your case:

int i = 0;
ClosureClass = new ClosureClass(ref i); // Defined here! (of course, not called this)
while (i < 10)
{
    Thread t = new Thread(() => new PhoneJobTest(i);    
    t.Start();
    i++;
}

Because of this, each Thread gets the same closure defined.

When you rework your loop to use a temporary, the closure is generated at that level instead:

for (int i = 0; i < 10; i++)
{
    int jobNum = i;
    ClosureClass = new ClosureClass(ref jobNum); // Defined here!
    Thread t = new Thread(() => new PhoneJobTest(jobNum);    
    t.Start();
}

Now, each Thread gets its own instance, and everything works properly.