What is the yield keyword used for in C#?

Answer 1

This link has a simple example

Even simpler examples are here

public static IEnumerable testYieldb()
{
    for(int i=0;i<3;i++) yield return 4;
}

Notice that yield return won't return from the method. You can even put a WriteLine after the yield return

The above produces an IEnumerable of 4 ints 4,4,4,4

Here with a WriteLine. Will add 4 to the list, print abc, then add 4 to the list, then complete the method and so really return from the method(once the method has completed, as would happen with a procedure without a return). But this would have a value, an IEnumerable list of ints, that it returns on completion.

public static IEnumerable testYieldb()
{
    yield return 4;
    console.WriteLine("abc");
    yield return 4;
}

Notice also that when you use yield, what you are returning is not of the same type as the function. It's of the type of an element within the IEnumerable list.

You use yield with the method's return type as IEnumerable. If the method's return type is int or List and you use yield, then it won't compile. You can use IEnumerable method return type without yield but it seems maybe you can't use yield without IEnumerable method return type.

And to get it to execute you have to call it in a special way.

static void Main(string[] args)
{
    testA();
    Console.Write("try again. the above won't execute any of the function!\n");

    foreach (var x in testA()) { }


    Console.ReadLine();
}



// static List testA()
static IEnumerable testA()
{
    Console.WriteLine("asdfa");
    yield return 1;
    Console.WriteLine("asdf");
}