using an IEnumerable list outside of foreach loop

Question

I am using IEnumerable in a for each loop as follows:

foreach (IListBlobItem blobItem in container.ListBlobs())
{        
    if (blobItem is CloudBlobDirectory)
    {
        CloudBlobDirectory directory = (CloudBlobDirectory)blobItem;
        IEnumerable<IListBlobItem> blobs = directory.ListBlobs(true);
    }                
}
await ProcessBlobs(blobs);

I would like to use blobsvariable outside of this foreach loop but I get this message: blobs doesnot exist in the current context

I decided to define blobs outside of the foreach loop:

IEnumerable<IListBlobItem> blobs = new IEnumerable<IListBlobItem>;

foreach (IListBlobItem blobItem in container.ListBlobs())
{           
    if (blobItem is CloudBlobDirectory)
    {
        //Console.WriteLine(blobItem.Uri);
        CloudBlobDirectory directory = (CloudBlobDirectory)blobItem;
        IEnumerable<IListBlobItem> blobs = directory.ListBlobs(true);                    
    }
}

but I get the error: can not create an instance of the abstract class or interface IEnumerable<IListBlobItem>

Do you have any idea how can I solve this problem?

Create a `List`... Also you cannot have declaration twice... — Johnny, Nov 05 '19 at 09:43
IEnumerable is an interface and you can't instantiate an interface. Instead, you can find a class that implements IEnumerable and use that. Maybe use `List`and do `IEnumerable blobs = new List();` — Hans Kilian, Nov 05 '19 at 09:56

Dmitry Bychenko · Accepted Answer · 2019-11-05T09:47:12.473

2

You can declare blobs being an empty collection, say, array:

   // Empty
   IEnumerable<IListBlobItem> blobs = new IListBlobItem[0];

   foreach (IListBlobItem blobItem in container.ListBlobs())
   {
      if (blobItem is CloudBlobDirectory)
      {
          CloudBlobDirectory directory = (CloudBlobDirectory)blobItem;
          blobs = directory.ListBlobs(true);                    
      }            
   }

   // process either blobs from foreach or an empty collection
   await ProcessBlobs(blobs);

edited Nov 05 '19 at 09:47

answered Nov 05 '19 at 09:44

Dmitry Bychenko

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IListBlobItem is interface. `new IListBlobItem` is wrong syntax – Roma Ruzich Nov 05 '19 at 09:46
1

@Roma Ruzich: yes, `new IListBlobItem` is wrong syntax, but `new IListBlobItem[0]` is *not*: it's an *array* of `Length = 0` of `IListBlobItem` items – Dmitry Bychenko Nov 05 '19 at 09:48
Ops, sorry. But, is it better to declare the variable with a default value or not? – Roma Ruzich Nov 05 '19 at 09:56
@Roma Ruzich: `default(IEnumerable)` will return `null` which is *not a good practice* in case of collections (empty collection is a better choice) – Dmitry Bychenko Nov 05 '19 at 10:39
Explain, please, why empty collection is better then `null`. – Roma Ruzich Nov 05 '19 at 10:57
@Roma Ruzich: https://stackoverflow.com/questions/1969993/is-it-better-to-return-null-or-empty-collection – Dmitry Bychenko Nov 05 '19 at 11:06
thank you. I read this post. But there is no explanation why empty collection is better. I think that `null` is better because: 1) no place on heap 2) when you try to do some manipulation with `null`-collection you take exception – Roma Ruzich Nov 05 '19 at 11:16
1

@Roma Ruzich: 1. when working with *collection* (often a lot of items), `4` or `8` *bytes* on a heap is not worth be optimized 2. usually, we don't want any `NullReferenceException` to be thrown, but just *return* - if we don't have any blob to process then `await ProcessBlobs(blobs);` shouldn't throw any exception but *do nothing*. `ProcessBlobs` can well be something like `foreach (var blob in blobs) {...}`, if `blobs` is `null` this (quite natural) implementation *crashes* but does as expected in case `blobs` is an empty collection – Dmitry Bychenko Nov 05 '19 at 11:26
1

@ Dmitry Bychenko, thanks thanks for the explanation – Roma Ruzich Nov 05 '19 at 11:35

score 2 · Answer 2 · answered Nov 05 '19 at 09:48

2

Try using Enumerable.Empty<TResult>, like so:

IEnumerable<IListBlobItem> blobs = Enumerable.Empty<IListBlobItem>();

This will return an empty, non-null enumerable.

See .NET API Documentation

answered Nov 05 '19 at 09:48

Jakob Möllås

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Farhad Rahmanifard · Answer 3 · 2019-11-05T09:55:01.540

You are trying to create an object of an interface which is impossible. Instead, declare blobs as object an then convert it to IEnumerable<IListBlobItem>.

object blobs = null;

foreach (IListBlobItem blobItem in container.ListBlobs())
{           
    if (blobItem is CloudBlobDirectory)
    {
        //Console.WriteLine(blobItem.Uri);
        CloudBlobDirectory directory = (CloudBlobDirectory)blobItem;
        blobs = directory.ListBlobs(true);                    
    }
}
///usage:
///(IEnumerable<IListBlobItem>)blobs

Also, you can declare blobs as IEnumerable<IListBlobItem> which other answers cover.

Ray Krungkaew · Answer 4 · 2019-11-05T09:52:33.393

1

Use default to get default value. This will return null for reference type

IEnumerable<IListBlobItem> blobs = default(IEnumerable<IListBlobItem>);

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/default

edited Nov 05 '19 at 09:52

answered Nov 05 '19 at 09:44

Ray Krungkaew

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`default` will return `null`, which is often not a good practice for collections (an empty collection usually is a better choice) – Dmitry Bychenko Nov 05 '19 at 09:50
Agreed. Thanks for the comment. – Ray Krungkaew Nov 05 '19 at 09:53

score 1 · Answer 5 · answered Nov 05 '19 at 09:48

1

Set IEnumerable blobs like property in this way:

IEnumerable<IListBlobItem> blobs{get;set;}

answered Nov 05 '19 at 09:48

BASKA

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using an IEnumerable list outside of foreach loop

5 Answers5