Linq, streaming & collection

Question

I have this method which return an array of string :

 private static string[] ReturnsStringArray() { return new string[] { "The path of the ", "righteous man is beset on all sides", "by the inequities of the selfish and", "the tyranny of evil men. Blessed is", "he who, in the name of charity and", "good will, shepherds the weak through", "the valley of darkness, for he is", "truly his brother's keeper and the", "finder of lost children. And I will ", "strike down upon thee with great", "vengeance and furious anger those", "who attempt to poison and destroy my", "brothers. And you will know my name", "is the Lord when I lay my vengeance", "upon you" }; } }

I want to write a method which use this method. As this method returns an array and not a IEnumerable, is there the same result to write this :

 private static IEnumerable<string> ParseStringArray() { return ReturnsStringArray() .Where(s => s.StartsWith("r")) .Select(c => c.ToUpper()); }

and this :

 private static List<string> ParseStringArray() { return ReturnsStringArray() .Where(s => s.StartsWith("r")) .Select(c => c.ToUpper()) .ToList(); // return a list instead of an IEnumerable. }

Thank you.

EDIT

My question is : Is there any interest or benefits that the method ParseStringArray() returns an IEnumerable instead of a List because of this method calls ReturnsStringArray that returns an array of string and not a IEnumerable

What is the question? Both of the functions you supplied returns the same result. "righteous man is beset on all sides", but with uppercase. — NoLifeKing
– NoLifeKing, Commented Dec 23, 2011 at 9:21

Scott Rippey · Accepted Answer · 2011-12-23 09:47:45Z

When you return a List, you are saying that "all processing has been done, and the List contains the results".

However, when you return an IEnumerable, you are saying that "processing might still need to be done".

In your example, when you return an IEnumerable, the .Where and .Select have NOT bee processed yet. This is known as "deferred execution".
If the user uses the result 3 times, then the .Where and .Select will execute 3 times. There are a good number of tricky issues that can come from this.

I recommend using a List as often as possible when returning values from a method. In addition to the additional functionality you'll get from a List, .NET has many optimizations that require a List, debugging support is better, and there's a reduced chance of unintended side effects!

Suriya · Accepted Answer · 2011-12-23 09:43:31Z

A List is a concrete implementation of IEnumerable. The difference is that

1) IEnumerable is merely a sequence of string but a List is indexable by an int index, can be added to and removed from and have items inserted at a particular index.

2) An item can be iterated by a sequence, but doesn't allow random access. A List is a specific random-access variable-size collection.

Lukazoid · Accepted Answer · 2011-12-23 09:57:33Z

I would personally recommend returning a string[], as you will unlikely wish to add to the results (dismissing List<string>), but it looks like you may want sequential access (which IEnumerable<string> is not intended for). Whether you defer execution or not is up to you and the situation; if the result is used many times, it may be wise to call ToArray() before returning the results.

private static string[] ParseStringArray() { return ReturnsStringArray() .Where(s => s.StartsWith("r")) .Select(c => c.ToUpper()) .ToArray(); }

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