問題描述
我可以在 VB.NET 中為 IEnumerable 函數實現收益返回嗎？ (Can I implement yield return for IEnumerable functions in VB.NET?)
Possible Duplicate: Yield In VB.NET
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.
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參考解法
方法 1:
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
方法 2:
The new Async CTP includes support for Yield in VB.NET.
See Iterators in Visual Basic for information on usage.
方法 3:
See my answers here:
- Yield in VB.NET
- Iterator pattern in 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.
方法 4:
No yield return in VB.NET :( Just create a list and return it.
方法 5:
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)
(by CoderDennis、JaredPar、CoderDennis、Joel Coehoorn、bbmud、Thomas Levesque)