modify a variable in a anonymous method

Question

I want to modify a local variable in a function of extension method. See

int myvar=0;
MyList.Where(
    x =>
        {
            if (condition)
                myvar += 1;
            return false;
        });
return myvar;

Why that is not working?

Answer 1

You really don't want to modify a local variable in the body of a Where predicate. Functions with side-effects like this are bad news; try to imagine what would happen if this comes (for example) from a parallel enumerable generated by AsParallel() - you'll have a race condition.

If you explain what you're trying to accomplish, I'm sure that one of us could provide a better means to that end. My guess is that it would look something like this:

int count = myList.Count(x => condition(x));

Answer 2

Where方法返回一个IEnumerable<T>但实际上并没有进行枚举（使用foreach或通过手动迭代生成的IEnumerator<T> ）。