How to update a global variable inside `where` clause in LINQ?

问题

I want to filter a list using LINQ with Where extension method. But apart from filtering I also want to update a global variable inside Where. However I cannot do it. Consider this example:

var list = new List<string> { "1", "2", "3", "4", "5" };

bool flag = false;
var newList = list.Where(item =>
{
    flag = true;
    return item == "2";
});

// Here I expect flag = true, but in fact it's false
Console.Write(flag);

As you can see I set flag = true, still the value flag == false after execution. It does not make sense to me. Can you explain what is going on under the hood and why flag is not changed. Also is there a way to change global variables inside LINQ at all?

回答1:

Linq queries are lazy, so until you enumerate newList you will not see a change, because your where has not been executed.

var list = new List<string> { "1", "2", "3", "4", "5" };

bool flag = false;
var newList = list.Where(item =>
{
    flag = true;
    return item == "2";
});

Console.WriteLine(flag); // Flag is still false.

foreach (var item in newList) {
  // It doesn't matter what we do here, just that we enumerate the list.
}

Console.Write(flag); // Flag is now true.

The foreach causes the where to execute and sets your flag.

I would really advise against using the where predicate to create a side effect, by the way, but this is how you'd do it.

回答2:

Just call ToArray() or ToList() to actually execute the code that sets the flag:

var newList = list.Where(item =>
{
    flag = true;
    return item == "2";
}).ToArray();

The predicate that you pass to the Where method is not evaluated until the list is actually enumerated.

来源：https://stackoverflow.com/questions/55868254/how-to-update-a-global-variable-inside-where-clause-in-linq