如何在C＃中为变量创建闭包？

Question

The following example prints the number "5" five times after 1 second.

for (int i = 0; i < 5; ++i)
{
    EasyTimer.SetTimeout(() => Console.WriteLine(i), 1000);
}

(Assume EasyTime.SetTimeout behaves like JavaScript's setTimeout )

How can we create a closure around i such that it keeps it remembers its value for the callback?

I know how to do it in JavaScript but I'm not sure if we can do it the same way in C#.

Answer 1

Inside the block, assign i to a new variable j, and use j in the lambda, as follows:

for (int i = 0; i < 5; ++i)
{
    int j = i;
    EasyTimer.SetTimeout(() => Console.WriteLine(j), 1000);
}

Notably, this issue only crops up because the lambda is a closure over i , so it gets the value when the lambda executes, not when it's created.

Answer 2

If you are using C#5.0 you can write the following:

foreach (int i in Enumerable.Range(0, 5))
{
    EasyTimer.SetTimeout(() => Console.WriteLine(i), 1000);
}

It will automatically generate local variable. This will only work in foreach loop, not for.