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Ok, as I was poking around with building a custom enumerator, I had noticed this behavior that concerns the yield
Say you have something like this:
public class EnumeratorExample
{
public static IEnumerable<int> GetSource(int startPoint)
{
int[] values = new int[]{1,2,3,4,5,6,7};
Contract.Invariant(startPoint < values.Length);
bool keepSearching = true;
int index = startPoint;
while(keepSearching)
{
yield return values[index];
//The mind reels here
index ++
keepSearching = index < values.Length;
}
}
}
What makes it possible underneath the compiler's hood to execute the index ++ and the rest of the code in the while loop after you technically do a return from the function?
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The yield keyword pauses generator function execution and the value of the expression following the yield keyword is returned to the generator's caller. It can be thought of as a generator-based version of the return keyword. yield can only be called directly from the generator function that contains it.
The Yield keyword in Python is similar to a return statement used for returning values or objects in Python. However, there is a slight difference. The yield statement returns a generator object to the one who calls the function which contains yield, instead of simply returning a value.
In its simplest form, a yield statement looks much like a return statement, except that instead of stopping execution of the function and returning, yield instead provides a value to the code looping over the generator and pauses execution of the generator function.
You use a yield return statement to return each element one at a time. The sequence returned from an iterator method can be consumed by using a foreach statement or LINQ query. Each iteration of the foreach loop calls the iterator method.
The compiler rewrites the code into a state machine. The single method you wrote is split up into different parts. Each time you call MoveNext (either implicity or explicitly) the state is advanced and the correct block of code is executed.
Suggested reading if you want to know more details:
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