Friday, July 26, 2019

c# - Access to Modified Closure

I know this is an old question, but I've recently been studying closures and thought a code sample might be useful. Behind the scenes, the compiler is generating a class that represents a lexical closure for your function call. It probably looks something like:


private sealed class Closure
{
public string[] files;
public int i;
public bool YourAnonymousMethod(string name)
{
return name.Equals(this.files[this.i]);
}
}

As mentioned above, your function works because the predicates are invoked immediately after creation. The compiler will generate something like:


private string Works()
{
var closure = new Closure();
closure.files = new string[3];
closure.files[0] = "notfoo";
closure.files[1] = "bar";
closure.files[2] = "notbaz";
var arrayToSearch = new string[] { "foo", "bar", "baz" };
//this works, because the predicates are being executed during the loop
for (closure.i = 0; closure.i < closure.files.Length; closure.i++)
{
if (Array.Exists(arrayToSearch, closure.YourAnonymousMethod))
return closure.files[closure.i];
}
return null;
}

On the other hand, if you were to store and then later invoke the predicates, you would see that every single call to the predicates would really be calling the same method on the same instance of the closure class and therefore would use the same value for i.

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