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I find myself foreach-ing over IEnumerables a lot just so that I can return each result. Is there a way to compress something like this
foreach (var subSelector in subSelectors) { foreach (var node in FindSingle(context, subSelector)) yield return node; } To remove the inner foreach?
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The yield is the income the investment returns over time, typically expressed as a percentage, while the return is the amount that was gained or lost on an investment over time, usually expressed as a dollar value.
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.
It specifies that an iterator has come to an end. You can think of yield break as a return statement which does not return a value. For example, if you define a function as an iterator, the body of the function may look like this: for (int i = 0; i < 5; i++) { yield return i; } Console.
In the iterator block, the yield keyword is used together with the return keyword to provide a value to the enumerator object. This is the value that is returned, for example, in each loop of a foreach statement. The yield keyword is also used with break to signal the end of iteration."
This is a somewhat frequently requested feature that C# does not support. See this Connect item for details:
http://connect.microsoft.com/VisualStudio/feedback/details/256934/yield-return-to-also-yield-collections
The proposed syntax is usually something like:
public static IEnumerable<T> PreorderTraversal<T>(this BinaryTree<T> root) { if (root == null) yield break; yield return root.Item; yield foreach root.Left.PreorderTraversal(); yield foreach root.Right.PreorderTraversal(); } If you are interested in playing with a C#-like language that supports this feature, take a look at Cω:
http://research.microsoft.com/en-us/um/cambridge/projects/comega/
You might also want to read this paper on the feature by the implementors of Cω:
http://research.microsoft.com/en-us/projects/specsharp/iterators.pdf
If you're interested in a non-C#-like language that supports this feature, take a look at the "yield!" feature of F#. (I just love that the name of the feature is "yield!")
Even if you are not interested in the theoretical stuff, it sounds like you face this situation as a practical problem. You should also read Wes Dyer's article on techniques for efficiently doing this sort of nested iteration without "yield foreach":
http://blogs.msdn.com/b/wesdyer/archive/2007/03/23/all-about-iterators.aspx
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No, there isn't, unless you completely replace every yield return with a single return statement using LINQ.
For example:
return someSet .Concat(someOtherSet.SelectMany(s => FindSingle(context, s)); If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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