Can I implement yield return for IEnumerable functions in VB.NET?

Question

In C#, when writing a function that returns an IEnumerble<>, you can use yield return to return a single item of the enumeration and yield break; to signify no remaining items. What is the VB.NET syntax for doing the same thing?

An example from the NerdDinner code:

public IEnumerable<RuleViolation> GetRuleViolations() {

   if (String.IsNullOrEmpty(Title))
       yield return new RuleViolation("Title required","Title");

   if (String.IsNullOrEmpty(Description))
       yield return new RuleViolation("Description required","Description");

   if (String.IsNullOrEmpty(HostedBy))
       yield return new RuleViolation("HostedBy required", "HostedBy");

   if (String.IsNullOrEmpty(Address))
       yield return new RuleViolation("Address required", "Address");

   if (String.IsNullOrEmpty(Country))
       yield return new RuleViolation("Country required", "Country");

   if (String.IsNullOrEmpty(ContactPhone))
       yield return new RuleViolation("Phone# required", "ContactPhone");

   if (!PhoneValidator.IsValidNumber(ContactPhone, Country))
       yield return new RuleViolation("Phone# does not match country", "ContactPhone");

   yield break;
}

This convert C# to VB.NET tool gives a "YieldStatement is unsupported" error.

Answer 1

No yield return in VB.NET :( Just create a list and return it.

Answer 2

There is currently no equivalent to C#'s yield return in VB.Net from a language syntax level.

However there was a recent write up in MSDN magazine by Bill McCarthy on how to implement a similar pattern in VB.Net 9.0

• http://visualstudiomagazine.com/articles/2009/02/01/use-iterators-in-vb-now.aspx

Answer 3

See my answers here:

• http://stackoverflow.com/questions/97381/yield-in-vb-net
• http://stackoverflow.com/questions/250874/iterator-pattern-vb-net-c-would-use-yield

To summarize:
VB.Net does not have yield, but C# implements yield by converting your code to a state machine behind that scenes. VB.Net's Static keyword also allows you to store state within a function, so in theory you should be able to implement a class that allows you to write similar code when used as a Static member of a method.

Answer 4

Behind the scenes, the compiler creates an enumerator class to do the work. Since VB.NET does not implement this pattern, you have to create your own implementation of IEnumerator(Of T)