How to create an IEnumerable to get all X lazily in a single statement

Question

C# doesn't allow lambda functions to represent iterator blocks (e.g., no "yield return" allowed inside a lambda function). If I wanted to create a lazy enumerable that yielded all the drives at the time of enumeration for example, I'd like to do something like

IEnumerable<DriveInfo> drives = {foreach (var drive in DriveInfo.GetDrives()) 
                                     yield return drive;};

It took a while, but I figured out this as a way to get that functionality:

var drives = Enumerable.Range(0, 1).SelectMany(_ => DriveInfo.GetDrives());

Is there a more idiomatic way?

score 1 · Answer 1 · edited May 23 '17 at 12:28

1

 var drives = new Func<IEnumerable<DriveInfo>>(DriveInfo.GetDrives);

 foreach(var drive in drives())
     Console.WriteLine(drive);

This solution has an important feature; It recognizes that multiple enumeration can be potentially bad. For instance, the underlying enumerator might make snap-shot of the list and then only ever enumerate over than list. There have been other discussions on multiple-enumerators See Answers here and here

edited May 23 '17 at 12:28

Community

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answered Sep 22 '13 at 15:03

Meirion Hughes

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Instead of `drives.Invoke()`, just say `drives()`. – Jeppe Stig Nielsen Sep 22 '13 at 15:07
Oh didn't know you could do that. I've been calling invoke all this time. :D – Meirion Hughes Sep 22 '13 at 15:11
Can you qualify why multiple enumeration is bad? I don't think it would be an issue here since the behavior would be the *intended* behavior. – Dax Fohl Sep 22 '13 at 15:13
Maybe your `drives` variable should be called `getDrives`? It does not have the type `IEnumerable<>` though, so it can't be passed to a method that wants an `IEnumerable<>`. – Jeppe Stig Nielsen Sep 22 '13 at 15:13

score 1 · Accepted Answer · answered Sep 22 '13 at 15:33

Why not write your own method, something like:

public static IEnumerable<T> GetLazily<T>(Func<IEnumerable<T>> getSource)
{
  foreach (var t in getSource())
    yield return t;
}

or:

public static IEnumerable<T> GetLazily<T>(Func<IEnumerable<T>> getSource)
{
  return (new int[1]).SelectMany(_ => getSource());
}

It ought to allow usage like this:

var drives = GetLazily(DriveInfo.GetDrives);

How to create an IEnumerable to get all X lazily in a single statement

2 Answers2