Talk about how to capture local variables in C# loop in detail

question:

I've had an interesting problem, and its code is probably like this.

List<Func<int>> actions = new List<Func<int>>();
 
int variable = 0;
while (variable < 5)
{
    (() => variable * 2);
    ++ variable;
}
 
foreach (var act in actions)
{
    (());
}

My expected output is 0,2,4,6,8, but it ends up with five 10s, which look like these action contexts capture the same variable.

Is there a workaround to achieve the expected results I want?

Solution:

The workaround is to use an intermediate variable in your loop body and feed it into the lambda body, refer to the following code:

List<Func<int>> actions = new List<Func<int>>();
 
int variable = 0;
while (variable < 5)
{
    int variable1 = variable;
    (() => variable1 * 2);
    ++variable;
}
 
foreach (var act in actions)
{
    (());
}
 
();
 

In fact, this situation will also happen under multi-threading, such as the following code:

for (int counter = 1; counter <= 5; counter++)
{
    new Thread (() =>  (counter)).Start();
}

You thought you would print out 1,2,3,4,5. The final result is very interesting. The output result here is: 2,1,3,3,4. The result on your side must be different again.

The only solution is to use local variables, and the modified code is as follows:

for (int counter = 1; counter <= 5; counter++)
{
    int localVar= counter;
    new Thread (() =>  (localVar)).Start();
}

Summarize

Many friends may not know why the code is correct after adding a variable1 variable. If you want to analyze it, you can look at the IL code generated in C#.

private static void Main(string[] args)
{
 List<Func<int>> actions = new List<Func<int>>();
 for (int variable = 0; variable < 5; variable++)
 {
  <>c__DisplayClass0_0 <>c__DisplayClass0_ = new <>c__DisplayClass0_0();
  <>c__DisplayClass0_.variable1 = variable;
  (new Func<int>(<>c__DisplayClass0_.<Main>b__0));
 }
 foreach (Func<int> act in actions)
 {
  (act());
 }
 ();
}

It can be clearly seen that the so-called variable1 has become a field under the anonymous class c__DisplayClass0_0, and the foreach loop is new every time, so this field must be different, which ensures the correct result.

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