Iterator Pattern VB.net (C# would use yield!)

Question

Hello,

I want to implement the iterator pattern in VB.net, which does not have the yield keyword.

Any ideas or links please?

Answer 1

Hmm, looks like you might be out of luck:

I was struggling with an issue today when converting some C# to VB.NET. C# has a really cool "yield return" statement that is used in an iterator block to provide a value to the enumerator object. VB.NET does not have the "yield" keyword. So, there are a few solutions (none of which are really clean) to get around this. You could use a return statement to return the value if you are looping through and would like to break an enumerator and return a single value. However, if you'd like to return the entire enumeration, create a List() of the child type and return the list. Since you are usually using this with an IEnumerable, the List() will work nice.

That was written a year ago, not sure if anyone has come up with anything else better since then..

Answer 2

VB.net does not support the creation of custom iterators and thus has no equivalent to the C# yield keyword. However you might want to look at this KB article for more information.

Answer 3

C#'s yield keyword forces the compiler to create a state machine in the background to support it. VB.Net does not have the yield keyword. But it does have a construct that would allow you to create a state machine within a function: Static function members.

It should be possible to mimic the effects of a yield return function by creating a generic class that implements IEnumerable as well as the needed state machine and placing an instance as a static member inside your function.

This would, of course, require implementing the class outside of the function. But if done properly the class should be re-usable in the general case. I haven't played with the idea enough to provide any implementation details, though.

Answer 4

Still getting my head around this concept also.

The article 'Use Iterators in VB Now' was recently published in Visual Studio Magazine.

http://visualstudiomagazine.com/columns/article.aspx?editorialsid=2972

Answer 5

Just implement the IEnumerator and IEnumerable interfaces. This is perfectly explained here

Answer 6

I think is better explained here